TY  - JOUR
A1  - Komarov, Maxim
A1  - Gupta, Shamik
A1  - Pikovskij, Arkadij
T1  - Synchronization transitions in globally coupled rotors in the presence of noise and inertia: Exact results
JF  - epl : a letters journal exploring the frontiers of physics
N2  - We study a generic model of globally coupled rotors that includes the effects of noise, phase shift in the coupling, and distributions of moments of inertia and natural frequencies of oscillation. As particular cases, the setup includes previously studied Sakaguchi-Kuramoto, Hamiltonian and Brownian mean-field, and Tanaka-Lichtenberg-Oishi and Acebron-Bonilla-Spigler models. We derive an exact solution of the self-consistent equations for the order parameter in the stationary state, valid for arbitrary parameters in the dynamics, and demonstrate nontrivial phase transitions to synchrony that include reentrant synchronous regimes. Copyright (C) EPLA, 2014
Y1  - 2014
U6  - https://doi.org/10.1209/0295-5075/106/40003
SN  - 0295-5075
SN  - 1286-4854
VL  - 106
IS  - 4
PB  - EDP Sciences
CY  - Mulhouse
ER  - 
TY  - JOUR
A1  - Zhang, Haocheng
A1  - Chen, Xuhui
A1  - Boettcher, Markus
T1  - Synchrotron polarization in blazars
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present a detailed analysis of time-and energy-dependent synchrotron polarization signatures in a shock-in-jet model for gamma-ray blazars. Our calculations employ a full three-dimensional radiation transfer code, assuming a helical magnetic field throughout the jet. The code considers synchrotron emission from an ordered magnetic field, and takes into account all light-travel-time and other relevant geometric effects, while the relevant synchrotron self-Compton and external Compton effects are handled with the two-dimensional Monte-Carlo/Fokker-Planck (MCFP) code. We consider several possible mechanisms through which a relativistic shock propagating through the jet may affect the jet plasma to produce a synchrotron and high-energy flare. Most plausibly, the shock is expected to lead to a compression of the magnetic field, increasing the toroidal field component and thereby changing the direction of the magnetic field in the region affected by the shock. We find that such a scenario leads to correlated synchrotron + synchrotron-self-Compton flaring, associated with substantial variability in the synchrotron polarization percentage and position angle. Most importantly, this scenario naturally explains large polarization angle rotations by greater than or similar to 180 degrees, as observed in connection with gamma-ray flares in several blazars, without the need for bent or helical jet trajectories or other nonaxisymmetric jet features.
KW  - galaxies: active
KW  - galaxies: jets
KW  - gamma rays: galaxies
KW  - radiation mechanisms: non-thermal
KW  - relativistic processes
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/789/1/66
SN  - 0004-637X
SN  - 1538-4357
VL  - 789
IS  - 1
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Meibohm, Jan
A1  - Schreck, Simon
A1  - Wernet, Philippe
T1  - Temperature dependent soft x-ray absorption spectroscopy of liquids
JF  - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques
N2  - A novel sample holder is introduced which allows for temperature dependent soft x-ray absorption spectroscopy of liquids in transmission mode. The setup is based on sample cells with x-ray transmissive silicon nitride windows. A cooling circuit allows for temperature regulation of the sample liquid between -10 degrees C and +50 degrees C. The setup enables to record soft x-ray absorption spectra of liquids in transmission mode with a temperature resolution of 0.5K and better. Reliability and reproducibility of the spectra are demonstrated by investigating the characteristic temperature-induced changes in the oxygen K-edge x-ray absorption spectrum of liquid water. These are compared to the corresponding changes in the oxygen K-edge spectra from x-ray Raman scattering. (C) 2014 AIP Publishing LLC.
Y1  - 2014
U6  - https://doi.org/10.1063/1.4896977
SN  - 0034-6748
SN  - 1089-7623
VL  - 85
IS  - 10
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Archambault, S.
A1  - Arlen, T.
A1  - Aune, T.
A1  - Beilicke, M.
A1  - Benbow, W.
A1  - Bird, R.
A1  - Boettcher, Markus
A1  - Bouvier, A.
A1  - Buckley, J. H.
A1  - Bugaev, V.
A1  - Ciupik, L.
A1  - Collins-Hughes, E.
A1  - Connolly, M. P.
A1  - Cui, W.
A1  - Dickherber, R.
A1  - Dumm, J.
A1  - Errando, M.
A1  - Falcone, A.
A1  - Federici, Simone
A1  - Feng, Q.
A1  - Finley, J. P.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Galante, N.
A1  - Gall, D.
A1  - Garson, A. III.
A1  - Gillanders, G. H.
A1  - Griffin, S.
A1  - Grube, J.
A1  - Gusbar, C.
A1  - Gyuk, G.
A1  - Hanna, D.
A1  - Holder, J.
A1  - Hughes, G.
A1  - Kaaret, P.
A1  - Kertzman, M.
A1  - Khassen, Y.
A1  - Kieda, D.
A1  - Krawczynski, H.
A1  - Lamerato, A.
A1  - Lang, M. J.
A1  - Li, K.
A1  - Madhavan, A. S.
A1  - Maier, G.
A1  - Majumdar, P.
A1  - McArthur, S.
A1  - McCann, A.
A1  - Millis, J.
A1  - Moriarty, P.
A1  - Mukherjee, R.
A1  - Nieto, D.
A1  - Ong, R. A.
A1  - Orr, M.
A1  - Otte, A. N.
A1  - Park, N.
A1  - Perkins, J. S.
A1  - Pohl, Martin
A1  - Popkow, A.
A1  - Prokoph, H.
A1  - Quinn, J.
A1  - Ragan, K.
A1  - Reynolds, P. T.
A1  - Richards, G. T.
A1  - Roache, E.
A1  - Roustazadeh, P.
A1  - Saxon, D. B.
A1  - Sembroski, G. H.
A1  - Senturk, G. D.
A1  - Skole, C.
A1  - Staszak, D.
A1  - Telezhinsky, Igor O.
A1  - Tesic, G.
A1  - Theiling, M.
A1  - Varlotta, A.
A1  - Vassiliev, V. V.
A1  - Vincent, S.
A1  - Wakely, S. P.
A1  - Weinstein, A.
A1  - Welsing, R.
A1  - Williams, D. A.
A1  - Zitzer, B.
T1  - Test of models of the cosmic infrared background with multiwavelength  observations of the blazar 1ES 1218+30.4 IN 2009
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - We present the results of a multi-wavelength campaign targeting the blazar 1ES 1218+30.4 with observations with the 1.3 m McGraw-Hill optical telescope, the Rossi X-ray Timing Explorer (RXTE), the Fermi Gamma-Ray Space Telescope, and the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The RXTE and VERITAS observations were spread over a 13 day period and revealed clear evidence for flux variability, and a strong X-ray and gamma-ray flare on 2009 February 26 (MJD 54888). The campaign delivered a well-sampled broadband energy spectrum with simultaneous RXTE and VERITAS very high energy (VHE, > 100 GeV) observations, as well as contemporaneous optical and Fermi observations. The 1ES 1218+30.4 broadband energy spectrum-the first with simultaneous X-ray and VHE gamma-ray energy spectra-is of particular interest as the source is located at a high cosmological redshift for a VHE source (z = 0.182), leading to strong absorption of VHE gamma rays by photons from the optical/infrared extragalactic background light (EBL) via gamma VHE +gamma EBL -> e(+) e(-)pair-creation processes. We model the data with a one-zone synchrotron self-Compton (SSC) emission model and with the extragalactic absorption predicted by several recent EBL models. We find that the observations are consistent with the SSC scenario and all the EBL models considered in this work. We discuss observational and theoretical avenues to improve on the EBL constraints.
KW  - BL Lacertae objects: general
KW  - BL Lacertae objects: individual (1ES1218+30.4)
KW  - cosmic background radiation
KW  - diffuse radiation
KW  - galaxies: jets
KW  - gamma rays: galaxies
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/788/2/158
SN  - 0004-637X
SN  - 1538-4357
VL  - 788
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Abramowski, Attila
A1  - Aharonian, Felix A.
A1  - Benkhali, Faical  Ait
A1  - Akhperjanian, A. G.
A1  - Angüner, Ekrem Oǧuzhan
A1  - Anton, Gisela
A1  - Balenderan, Shangkari
A1  - Balzer, Arnim
A1  - Barnacka, Anna
A1  - Becherini, Yvonne
A1  - Tjus, J. Becker
A1  - Bernlöhr, K.
A1  - Birsin, E.
A1  - Bissaldi, E.
A1  - Biteau, Jonathan
A1  - Boettcher, Markus
A1  - Boisson, Catherine
A1  - Bolmont, J.
A1  - Bordas, Pol
A1  - Brucker, J.
A1  - Brun, Francois
A1  - Brun, Pierre
A1  - Bulik, Tomasz
A1  - Carrigan, Svenja
A1  - Casanova, Sabrina
A1  - Cerruti, M.
A1  - Chadwick, Paula M.
A1  - Chalme-Calvet, R.
A1  - Chaves, Ryan C. G.
A1  - Cheesebrough, A.
A1  - Chretien, M.
A1  - Colafrancesco, Sergio
A1  - Cologna, Gabriele
A1  - Conrad, Jan
A1  - Couturier, C.
A1  - Cui, Y.
A1  - Dalton, M.
A1  - Daniel, M. K.
A1  - Davids, I. D.
A1  - Degrange, B.
A1  - Deil, C.
A1  - deWilt, P.
A1  - Dickinson, H. J.
A1  - Djannati-Ataï, A.
A1  - Domainko, W.
A1  - Dubus, G.
A1  - Dutson, K.
A1  - Dyks, J.
A1  - Dyrda, M.
A1  - Edwards, T.
A1  - Egberts, Kathrin
A1  - Eger, P.
A1  - Espigat, P.
A1  - Farnier, C.
A1  - Fegan, S.
A1  - Feinstein, F.
A1  - Fernandes, M. V.
A1  - Fernandez, D.
A1  - Fiasson, A.
A1  - Fontaine, G.
A1  - Foerster, A.
A1  - Fuessling, Matthias
A1  - Gajdus, M.
A1  - Gallant, Y. A.
A1  - Garrigoux, T.
A1  - Giavitto, G.
A1  - Giebels, B.
A1  - Glicenstein, J. F.
A1  - Grondin, M. -H.
A1  - Grudzinska, M.
A1  - Haeffner, S.
A1  - Hahn, J.
A1  - Harris, J.
A1  - Heinzelmann, G.
A1  - Henri, G.
A1  - Hermann, G.
A1  - Hervet, O.
A1  - Hillert, A.
A1  - Hinton, James Anthony
A1  - Hofmann, W.
A1  - Hofverberg, P.
A1  - Holler, M.
A1  - Horns, D.
A1  - Jacholkowska, A.
A1  - Jahn, C.
A1  - Jamrozy, M.
A1  - Janiak, M.
A1  - Jankowsky, F.
A1  - Jung, I.
A1  - Kastendieck, M. A.
A1  - Katarzynski, K.
A1  - Katz, U.
A1  - Kaufmann, S.
A1  - Khelifi, B.
A1  - Kieffer, M.
A1  - Klepser, S.
A1  - Klochkov, D.
A1  - Kluzniak, W.
A1  - Kneiske, T.
A1  - Kolitzus, D.
A1  - Komin, Nu
A1  - Kosack, K.
A1  - Krakau, S.
A1  - Krayzel, F.
A1  - Krueger, P. P.
A1  - Laffon, H.
A1  - Lamanna, G.
A1  - Lefaucheur, J.
A1  - Lemiere, A.
A1  - Lemoine-Goumard, M.
A1  - Lenain, J. -P.
A1  - Lennarz, D.
A1  - Lohse, T.
A1  - Lopatin, A.
A1  - Lu, C. -C.
A1  - Marandon, V.
A1  - Marcowith, Alexandre
A1  - Marx, R.
A1  - Maurin, G.
A1  - Maxted, N.
A1  - Mayer, M.
A1  - McComb, T. J. L.
A1  - Mehault, J.
A1  - Meintjes, P. J.
A1  - Menzler, U.
A1  - Meyer, M.
A1  - Moderski, R.
A1  - Mohamed, M.
A1  - Moulin, Emmanuel
A1  - Murach, T.
A1  - Naumann, C. L.
A1  - de Naurois, M.
A1  - Niemiec, J.
A1  - Nolan, S. J.
A1  - Oakes, L.
A1  - Ohm, S.
A1  - Wilhelmi, E. de Ona
A1  - Opitz, B.
A1  - Ostrowski, M.
A1  - Oya, I.
A1  - Panter, M.
A1  - Parsons, R. D.
A1  - Arribas, M. Paz
A1  - Pekeur, N. W.
A1  - Pelletier, G.
A1  - Perez, J.
A1  - Petrucci, P. -O.
A1  - Peyaud, B.
A1  - Pita, S.
A1  - Poon, H.
A1  - Puehlhofer, G.
A1  - Punch, M.
A1  - Quirrenbach, A.
A1  - Raab, S.
A1  - Raue, M.
A1  - Reimer, A.
A1  - Reimer, O.
A1  - Renaud, M.
A1  - de los Reyes, R.
A1  - Rieger, F.
A1  - Rob, L.
A1  - Romoli, C.
A1  - Rosier-Lees, S.
A1  - Rowell, G.
A1  - Rudak, B.
A1  - Rulten, C. B.
A1  - Sahakian, V.
A1  - Sanchez, David M.
A1  - Santangelo, Andrea
A1  - Schlickeiser, R.
A1  - Schuessler, F.
A1  - Schulz, A.
A1  - Schwanke, U.
A1  - Schwarzburg, S.
A1  - Schwemmer, S.
A1  - Sol, H.
A1  - Spengler, G.
A1  - Spies, F.
A1  - Stawarz, L.
A1  - Steenkamp, R.
A1  - Stegmann, Christian
A1  - Stinzing, F.
A1  - Stycz, K.
A1  - Sushch, Iurii
A1  - Szostek, A.
A1  - Tavernet, J. -P.
A1  - Tavernier, T.
A1  - Taylor, A. M.
A1  - Terrier, R.
A1  - Tluczykont, M.
A1  - Trichard, C.
A1  - Valerius, K.
A1  - van Eldik, C.
A1  - van Soelen, B.
A1  - Vasileiadis, G.
A1  - Venter, C.
A1  - Viana, A.
A1  - Vincent, P.
A1  - Voelk, H. J.
A1  - Volpe, F.
A1  - Vorster, M.
A1  - Vuillaume, T.
A1  - Wagner, S. J.
A1  - Wagner, P.
A1  - Ward, M.
A1  - Weidinger, M.
A1  - Weitzel, Q.
A1  - White, R.
A1  - Wierzcholska, A.
A1  - Willmann, P.
A1  - Woernlein, A.
A1  - Wouters, D.
A1  - Zabalza, V.
A1  - Zacharias, M.
A1  - Zajczyk, A.
A1  - Zdziarski, A. A.
A1  - Zech, Alraune
A1  - Zechlin, H. -S.
T1  - TeV gamma-ray observations of the young synchrotron-dominated SNRs G1.9+0.3 and G330.2+1.0 with HESS
JF  - Monthly notices of the Royal Astronomical Society
N2  - The non-thermal nature of the X-ray emission from the shell-type supernova remnants (SNRs) G1.9+0.3 and G330.2+1.0 is an indication of intense particle acceleration in the shock fronts of both objects. This suggests that the SNRs are prime candidates for very-high-energy (VHE; E > 0.1 TeV) gamma-ray observations. G1.9+0.3, recently established as the youngest known SNR in the Galaxy, also offers a unique opportunity to study the earliest stages of SNR evolution in the VHE domain. The purpose of this work is to probe the level of VHE gamma-ray emission from both SNRs and use this to constrain their physical properties. Observations were conducted with the H. E. S. S. (High Energy Stereoscopic System) Cherenkov Telescope Array over a more than six-year period spanning 2004-2010. The obtained data have effective livetimes of 67 h for G1.9+0.3 and 16 h for G330.2+1.0. The data are analysed in the context of the multiwavelength observations currently available and in the framework of both leptonic and hadronic particle acceleration scenarios. No significant gamma-ray signal from G1.9+0.3 or G330.2+1.0 was detected. Upper limits (99 per cent confidence level) to the TeV flux from G1.9+0.3 and G330.2+1.0 for the assumed spectral index Gamma = 2.5 were set at 5.6 x 10(-1)3 cm(-2) s(-1) above 0.26 TeV and 3.2 x 10(-12) cm(-2) s(-1) above 0.38 TeV, respectively. In a one-zone leptonic scenario, these upper limits imply lower limits on the interior magnetic field to B-G1.9 greater than or similar to 12 mu G for G1.9+0.3 and to B-G330 greater than or similar to 8 mu G for G330.2+1.0. In a hadronic scenario, the low ambient densities and the large distances to the SNRs result in very low predicted fluxes, for which the H.E.S.S. upper limits are not constraining.
KW  - radiation mechanisms: non-thermal
KW  - ISM: individual objects: SNR G1.9+0.3
KW  - ISM: individual objects: SNR G330.2+1.0
KW  - ISM: magnetic fields
KW  - ISM: supernova remnants
KW  - gamma-rays: ISM
Y1  - 2014
U6  - https://doi.org/10.1093/mnras/stu459
SN  - 0035-8711
SN  - 1365-2966
VL  - 441
IS  - 1
SP  - 790
EP  - 799
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Miedema, P. S.
A1  - Beye, Martin
A1  - Koennecke, R.
A1  - Schiwietz, G.
A1  - Föhlisch, Alexander
T1  - The angular- and crystal-momentum transfer through electron-phonon coupling in silicon and silicon-carbide: similarities and differences
JF  - New journal of physics : the open-access journal for physics
N2  - Electron-phonon scattering has been studied for silicon carbide (6H-SiC) with resonant inelastic x-ray scattering at the silicon 2p edge. The observed electron-phonon scattering yields a crystal momentum transfer rate per average phonon in 6H-SiC of 1.8 fs(-1) while it is 0.2 fs(-1) in crystalline silicon. The angular momentum transfer rate per average phonon for 6H-SiC is 0.1 fs(-1), which is much higher than 0.0035 fs(-1) obtained for crystalline silicon in a previous study. The higher electron-phonon scattering rates in 6H-SiC are a result of the larger electron localization at the silicon atoms in 6H-SiC as compared to crystalline silicon. While delocalized valence electrons can screen effectively (part of) the electron-phonon interaction, this effect is suppressed for 6H-SiC in comparison to crystalline silicon. Smaller contributions to the difference in electron-phonon scattering rates between 6H-SiC and silicon arise from the lower atomic mass of carbon versus silicon and the difference in local symmetry.
KW  - electron-phonon scattering
KW  - 6H-SiC
KW  - RIXS
Y1  - 2014
U6  - https://doi.org/10.1088/1367-2630/16/9/093056
SN  - 1367-2630
VL  - 16
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Zaritsky, Dennis
A1  - Courtois, Helene
A1  - Munoz-Mateos, Juan-Carlos
A1  - Sorce, Jenny
A1  - Erroz-Ferrer, S.
A1  - Comeron, S.
A1  - Gadotti, D. A.
A1  - Gil De Paz, A.
A1  - Hinz, J. L.
A1  - Laurikainen, E.
A1  - Kim, T.
A1  - Laine, J.
A1  - Menendez-Delmestre, K.
A1  - Mizusawa, T.
A1  - Regan, M. W.
A1  - Salo, H.
A1  - Seibert, M.
A1  - Sheth, K.
A1  - Athanassoula, E.
A1  - Bosma, A.
A1  - Cisternas, M.
A1  - Ho, Luis C.
A1  - Holwerda, B.
T1  - The baryonic Tully-Fisher relationship for S(4)G galaxies and the  "condensed" baryon fraction of galaxies
JF  - The astronomical journal
N2  - We combine data from the Spitzer Survey for Stellar Structure in Galaxies, a recently calibrated empirical stellar mass estimator from Eskew et al., and an extensive database of Hi spectral line profiles to examine the baryonic Tully-Fisher (BTF) relation. We find (1) that the BTF has lower scatter than the classic Tully-Fisher (TF) relation and is better described as a linear relationship, confirming similar previous results, (2) that the inclusion of a radial scale in the BTF decreases the scatter but only modestly, as seen previously for the TF relation, and (3) that the slope of the BTF, which we find to be 3.5 +/- 0.2 (Delta log M-baryon/Delta log v(c)), implies that on average a nearly constant fraction (similar to 0.4) of all baryons expected to be in a halo are "condensed" onto the central region of rotationally supported galaxies. The condensed baryon fraction, M-baryon/M-total, is, to our measurement precision, nearly independent of galaxy circular velocity (our sample spans circular velocities, vc, between 60 and 250 km s(-1), but is extended to v(c) similar to 10 km s(-1) using data from the literature). The observed galaxy-to-galaxy scatter in this fraction is generally <= a factor of 2 despite fairly liberal selection criteria. These results imply that cooling and heating processes, such as cold versus hot accretion, mass loss due to stellar winds, and active galactic nucleus driven feedback, to the degree that they affect the global galactic properties involved in the BTF, are independent of halo mass for galaxies with 10 < v(c) < 250 km s(-1) and typically introduce no more than a factor of two range in the resulting M-baryon/M-total. Recent simulations by Aumer et al. of a small sample of disk galaxies are in excellent agreement with our data, suggesting that current simulations are capable of reproducing the global properties of individual disk galaxies. More detailed comparison to models using the BTF holds great promise, but awaits improved determinations of the stellar masses.
KW  - galaxies: evolution
KW  - galaxies: formation
KW  - galaxies: fundamental parameters
KW  - galaxies: stellar content
KW  - galaxies: structure
Y1  - 2014
U6  - https://doi.org/10.1088/0004-6256/147/6/134
SN  - 0004-6256
SN  - 1538-3881
VL  - 147
IS  - 6
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Fox, Andrew J.
A1  - Wakker, Bart P.
A1  - Barger, Kathleen A.
A1  - Hernandez, Audra K.
A1  - Richter, Philipp
A1  - Lehner, Nicolas
A1  - Bland-Hawthorn, Joss
A1  - Charlton, Jane C.
A1  - Westmeier, Tobias
A1  - Thom, Christopher
A1  - Tumlinson, Jason
A1  - Misawa, Toru
A1  - Howk, J. Christopher
A1  - Haffner, L. Matthew
A1  - Ely, Justin
A1  - Rodriguez-Hidalgo, Paola
A1  - Kumari, Nimisha
T1  - The COS/UVES absorption survey of the magellanic stream. III. Ionization, total mass, and inflow rate onto the milky way
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - Dynamic interactions between the two Magellanic Clouds have flung large quantities of gas into the halo of the Milky Way. The result is a spectacular arrangement of gaseous structures, including the Magellanic Stream, the Magellanic Bridge, and the Leading Arm (collectively referred to as the Magellanic System). In this third paper of a series studying the Magellanic gas in absorption, we analyze the gas ionization level using a sample of 69 Hubble Space Telescope/Cosmic Origins Spectrograph sightlines that pass through or within 30 degrees of the 21 cm emitting regions. We find that 81% (56/69) of the sightlines show UV absorption at Magellanic velocities, indicating that the total cross-section of the Magellanic System is approximate to 11,000 deg(2), or around one-quarter of the entire sky. Using observations of the Si III/Si II ratio together with Cloudy photoionization modeling, we calculate the total gas mass (atomic plus ionized) of the Magellanic System to be approximate to 2.0 x 10(9) M-circle dot (d/55 kpc)(2), with the ionized gas contributing around three times as much mass as the atomic gas. This is larger than the current-day interstellar H I mass of both Magellanic Clouds combined, indicating that they have lost most of their initial gas mass. If the gas in the Magellanic System survives to reach the Galactic disk over its inflow time of similar to 0.5-1.0 Gyr, it will represent an average inflow rate of similar to 3.7-6.7 M-circle dot yr(-1), potentially raising the Galactic star formation rate. However, multiple signs of an evaporative interaction with the hot Galactic corona indicate that the Magellanic gas may not survive its journey to the disk fully intact and will instead add material to (and cool) the corona.
KW  - Galaxy: evolution
KW  - Galaxy: halo
KW  - ISM: abundances
KW  - Magellanic Clouds
KW  - quasars: absorption lines
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/787/2/147
SN  - 0004-637X
SN  - 1538-4357
VL  - 787
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Nuza, Sebastian E.
A1  - Parisi, Florencia
A1  - Scannapieco, Cecilia
A1  - Richter, Philipp
A1  - Gottloeber, Stefan
A1  - Steinmetz, Matthias
T1  - The distribution of gas in the Local Group from constrained cosmological simulations: the case for Andromeda and the Milky Way galaxies
JF  - Monthly notices of the Royal Astronomical Society
N2  - We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at z = 0 for three different phases: 'cold', 'hot' and H i, and compare our results with observations. The amount of material in the hot halo (M-hot a parts per thousand 4-5 x 10(10) M-aS (TM)), and the cold (M-cold(r a parts per thousand(2) 10 kpc) a parts per thousand 10(8) M-aS (TM)) and H i components displays reasonable agreement with observations. We also compute the accretion/ejection rates together with the H i (radial and all-sky) covering fractions. The integrated H i accretion rate within r = 50 kpc gives similar to 0.2-0.3 M-aS (TM) yr(-1), i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with T a parts per thousand(2) 10(5) K can contribute significantly too. The net accretion rates of all material at the virial radii are 6-8 M-aS (TM) yr(-1). At z = 0, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at z similar to 1, as a result of the kinematical evolution of the LG.
KW  - methods: numerical
KW  - Galaxy: halo
KW  - intergalactic medium
KW  - Local Group
KW  - large-scale structure of Universe
Y1  - 2014
U6  - https://doi.org/10.1093/mnras/stu643
SN  - 0035-8711
SN  - 1365-2966
VL  - 441
IS  - 3
SP  - 2593
EP  - 2612
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Toeroek, T.
A1  - Kliem, Bernhard
A1  - Berger, M. A.
A1  - Linton, M. G.
A1  - Demoulin, Pascal
A1  - van Driel-Gesztelyi, L.
T1  - The evolution of writhe in kink-unstable flux ropes and erupting filaments
JF  - Plasma physics and controlled fusion
N2  - The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-emptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold-Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of similar to 6 pi for the majority of filaments prior to their eruption.
KW  - magnetohydrodynamics (MHD)
KW  - Sun: corona
KW  - Sun: filaments
Y1  - 2014
U6  - https://doi.org/10.1088/0741-3335/56/6/064012
SN  - 0741-3335
SN  - 1361-6587
VL  - 56
IS  - 6
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Balzer, Arnim
A1  - Fuessling, M.
A1  - Gajdus, M.
A1  - Goering, D.
A1  - Lopatin, A.
A1  - de Naurois, M.
A1  - Schlenker, S.
A1  - Schwanke, U.
A1  - Stegmann, Christian
T1  - The HESS central data acquisition system
JF  - Astroparticle physics
KW  - DAQ
KW  - Data acquisition
KW  - VHE
KW  - Gamma ray astronomy
KW  - HESS
Y1  - 2014
U6  - https://doi.org/10.1016/j.astropartphys.2013.11.007
SN  - 0927-6505
SN  - 1873-2852
VL  - 54
SP  - 67
EP  - 80
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Shenar, Tomer
A1  - Hamann, Wolf-Rainer
A1  - Todt, Helge Tobias
T1  - The impact of rotation on the line profiles of Wolf-Rayet stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Massive Wolf-Rayet stars are recognized today to be in a very common, but short, evolutionary phase of massive stars. While our understanding of Wolf-Rayet stars has increased dramatically over the past decades, it remains unclear whether rapid rotators are among them. There are various indications that rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres, rotational velocities of Wolf-Rayet stars are very difficult to measure. However, recently observed spectra of several Wolf-Rayet stars reveal peculiarly broad and round emission lines. Could these spectra imply rapid rotation?
 Aims. In this work, we model the effects of rotation on the atmospheres of Wolf-Rayet stars. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may be explained with the help of stellar rotation, infer appropriate rotation parameters, and discuss the implications of our results.
 Methods. We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code. Since the observed spectra of WolfRayet stars are mainly formed in their expanding atmospheres, rotation must be accounted for with a 3D integration scheme of the formal integral. For this purpose, we assume a rotational velocity field consisting of an inner co-rotating domain and an outer domain, where the angular momentum is conserved.
 Results. We find that rotation can reproduce the unique spectra analyzed here. However, the inferred rotational velocities at the stellar surface are large (similar to 200 km s(-1)), and the inferred co-rotation radii (similar to 10R.) suggest the existence of very strong photospheric magnetic fields (similar to 20 kG).
KW  - stars: Wolf-Rayet
KW  - Magellanic Clouds
KW  - stars: magnetic field
KW  - stars: massive
KW  - gamma-ray burst: general
KW  - stars: rotation
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201322496
SN  - 0004-6361
SN  - 1432-0746
VL  - 562
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Komarov, Maxim
A1  - Pikovskij, Arkadij
T1  - The Kuramoto model of coupled oscillators with a bi-harmonic coupling function
JF  - Physica : D, Nonlinear phenomena
N2  - We study synchronization in a Kuramoto model of globally coupled phase oscillators with a bi-harmonic coupling function, in the thermodynamic limit of large populations. We develop a method for an analytic solution of self-consistent equations describing uniformly rotating complex order parameters, both for single-branch (one possible state of locked oscillators) and multi-branch (two possible values of locked phases) entrainment. We show that synchronous states coexist with the neutrally linearly stable asynchronous regime. The latter has a finite life time for finite ensembles, this time grows with the ensemble size as a power law. (C) 2014 Elsevier B.V. All rights reserved.
KW  - Kuramoto model
KW  - Bi-harmonic coupling function
KW  - Multi-branch entrainment
KW  - Synchronization
Y1  - 2014
U6  - https://doi.org/10.1016/j.physd.2014.09.002
SN  - 0167-2789
SN  - 1872-8022
VL  - 289
SP  - 18
EP  - 31
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - GEN
A1  - Liermann, Angelika
A1  - Hamann, Wolf-Rainer
A1  - Oskinova, Lida
T1  - The quintuplet cluster III. Hertzsprung-Russell diagram and cluster age (vol 540, pg A14, 2012)
T2  - Astronomy and astrophysics : an international weekly journal
KW  - open clusters and associations: individual: Quintuplet
KW  - infrared: stars
KW  - stars: early-type
KW  - stars: late-type
KW  - Hertzsprung-Russell and C-M diagrams
KW  - errata, addenda
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201117534e
SN  - 0004-6361
SN  - 1432-0746
VL  - 563
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Reindl, Nicole
A1  - Rauch, Thomas
A1  - Parthasarathy, M.
A1  - Werner, K.
A1  - Kruk, J. W.
A1  - Hamann, Wolf-Rainer
A1  - Sander, Andreas Alexander Christoph
A1  - Todt, Helge Tobias
T1  - The rapid evolution of the exciting star of the Stingray nebula
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. SAO 244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star.
 Aims. A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988-2006 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution.
 Methods. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere.
 Results. We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log(M/M-circle dot yr(-1)) = -9.0 to -11.6 and the terminal wind velocity increased from v(infinity) = 1800 km s(-1) to 2800 km s(-1). Since around 2002, the star stopped heating and has cooled down again to 55 kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. The results are discussed by considering different evolutionary scenarios.
 Conclusions. The position of SAO 244567 in the log T-eff-log g plane places the star in the region of sdO stars. By comparison with stellar-evolution calculations, we confirm that SAO 244567 must be a low-mass star (M < 0.55 M-circle dot). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAD 244567 would be a low-mass (0.354 M-circle dot) helium pre-white dwarf after the common-envelope phase, during which the planetary nebula was ejected.
KW  - stars: abundances
KW  - stars: evolution
KW  - stars: AGB and post-AGB
KW  - stars: individual: SAO 244567
KW  - stars: fundamental parameters
KW  - planetary nebulae: individual: Stingray nebula (Henize 3-1357)
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201323189
SN  - 0004-6361
SN  - 1432-0746
VL  - 565
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Steyrleuthner, Robert
A1  - Di Pietro, Riccardo
A1  - Collins, Brian A.
A1  - Polzer, Frank
A1  - Himmelberger, Scott
A1  - Schubert, Marcel
A1  - Chen, Zhihua
A1  - Zhang, Shiming
A1  - Salleo, Alberto
A1  - Ade, Harald W.
A1  - Facchetti, Antonio
A1  - Neher, Dieter
T1  - The Role of Regioregularity, Crystallinity, and Chain Orientation on Electron Transport in a High-Mobility n-Type Copolymer
JF  - Journal of the American Chemical Society
Y1  - 2014
U6  - https://doi.org/10.1021/ja4118736
SN  - 0002-7863
VL  - 136
IS  - 11
SP  - 4245
EP  - 4256
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Schleussner, Carl-Friedrich
A1  - Runge, Jakob
A1  - Lehmann, Jasvcha
A1  - Levermann, Anders
T1  - The role of the North Atlantic overturning and deep ocean for multi-decadal global-mean-temperature variability
JF  - Earth system dynamics
Y1  - 2014
U6  - https://doi.org/10.5194/esd-5-103-2014
SN  - 2190-4979
SN  - 2190-4987
VL  - 5
IS  - 1
SP  - 103
EP  - 115
PB  - Copernicus
CY  - Göttingen
ER  - 
TY  - JOUR
A1  - Arridge, Christopher S.
A1  - Achilleos, N.
A1  - Agarwal, Jessica
A1  - Agnor, C. B.
A1  - Ambrosi, R.
A1  - Andre, N.
A1  - Badman, S. V.
A1  - Baines, K.
A1  - Banfield, D.
A1  - Barthelemy, M.
A1  - Bisi, M. M.
A1  - Blum, J.
A1  - Bocanegra-Bahamon, T.
A1  - Bonfond, B.
A1  - Bracken, C.
A1  - Brandt, P.
A1  - Briand, C.
A1  - Briois, C.
A1  - Brooks, S.
A1  - Castillo-Rogez, J.
A1  - Cavalie, T.
A1  - Christophe, B.
A1  - Coates, Andrew J.
A1  - Collinson, G.
A1  - Cooper, J. F.
A1  - Costa-Sitja, M.
A1  - Courtin, R.
A1  - Daglis, I. A.
A1  - De Pater, Imke
A1  - Desai, M.
A1  - Dirkx, D.
A1  - Dougherty, M. K.
A1  - Ebert, R. W.
A1  - Filacchione, Gianrico
A1  - Fletcher, Leigh N.
A1  - Fortney, J.
A1  - Gerth, I.
A1  - Grassi, D.
A1  - Grodent, D.
A1  - Grün, Eberhard
A1  - Gustin, J.
A1  - Hedman, M.
A1  - Helled, R.
A1  - Henri, P.
A1  - Hess, Sebastien
A1  - Hillier, J. K.
A1  - Hofstadter, M. H.
A1  - Holme, R.
A1  - Horanyi, M.
A1  - Hospodarsky, George B.
A1  - Hsu, S.
A1  - Irwin, P.
A1  - Jackman, C. M.
A1  - Karatekin, O.
A1  - Kempf, Sascha
A1  - Khalisi, E.
A1  - Konstantinidis, K.
A1  - Kruger, H.
A1  - Kurth, William S.
A1  - Labrianidis, C.
A1  - Lainey, V.
A1  - Lamy, L. L.
A1  - Laneuville, Matthieu
A1  - Lucchesi, D.
A1  - Luntzer, A.
A1  - MacArthur, J.
A1  - Maier, A.
A1  - Masters, A.
A1  - McKenna-Lawlor, S.
A1  - Melin, H.
A1  - Milillo, A.
A1  - Moragas-Klostermeyer, Georg
A1  - Morschhauser, Achim
A1  - Moses, J. I.
A1  - Mousis, O.
A1  - Nettelmann, N.
A1  - Neubauer, F. M.
A1  - Nordheim, T.
A1  - Noyelles, B.
A1  - Orton, G. S.
A1  - Owens, Mathew
A1  - Peron, R.
A1  - Plainaki, C.
A1  - Postberg, F.
A1  - Rambaux, N.
A1  - Retherford, K.
A1  - Reynaud, Serge
A1  - Roussos, E.
A1  - Russell, C. T.
A1  - Rymer, Am.
A1  - Sallantin, R.
A1  - Sanchez-Lavega, A.
A1  - Santolik, O.
A1  - Saur, J.
A1  - Sayanagi, Km.
A1  - Schenk, P.
A1  - Schubert, J.
A1  - Sergis, N.
A1  - Sittler, E. C.
A1  - Smith, A.
A1  - Spahn, Frank
A1  - Srama, Ralf
A1  - Stallard, T.
A1  - Sterken, V.
A1  - Sternovsky, Zoltan
A1  - Tiscareno, M.
A1  - Tobie, G.
A1  - Tosi, F.
A1  - Trieloff, M.
A1  - Turrini, D.
A1  - Turtle, E. P.
A1  - Vinatier, S.
A1  - Wilson, R.
A1  - Zarkat, P.
T1  - The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant planets
JF  - Planetary and space science
N2  - Giant planets helped to shape the conditions we see in the Solar System today and they account for more than 99% of the mass of the Sun's planetary system. They can be subdivided into the Ice Giants (Uranus and Neptune) and the Gas Giants (Jupiter and Saturn), which differ from each other in a number of fundamental ways. Uranus, in particular is the most challenging to our understanding of planetary formation and evolution, with its large obliquity, low self-luminosity, highly asymmetrical internal field, and puzzling internal structure. Uranus also has a rich planetary system consisting of a system of inner natural satellites and complex ring system, five major natural icy satellites, a system of irregular moons with varied dynamical histories, and a highly asymmetrical magnetosphere. Voyager 2 is the only spacecraft to have explored Uranus, with a flyby in 1986, and no mission is currently planned to this enigmatic system. However, a mission to the uranian system would open a new window on the origin and evolution of the Solar System and would provide crucial information on a wide variety of physicochemical processes in our Solar System. These have clear implications for understanding exoplanetary systems. In this paper we describe the science case for an orbital mission to Uranus with an atmospheric entry probe to sample the composition and atmospheric physics in Uranus' atmosphere. The characteristics of such an orbiter and a strawman scientific payload are described and we discuss the technical challenges for such a mission. This paper is based on a white paper submitted to the European Space Agency's call for science themes for its large-class mission programme in 2013.
KW  - Uranus
KW  - Magnetosphere
KW  - Atmosphere
KW  - Natural satellites
KW  - Rings
KW  - Planetary interior
Y1  - 2014
U6  - https://doi.org/10.1016/j.pss.2014.08.009
SN  - 0032-0633
VL  - 104
SP  - 122
EP  - 140
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Evans, T. M.
A1  - Murphy, M. T.
A1  - Whitmore, J. B.
A1  - Misawa, T.
A1  - Centurion, Martin
A1  - Lopez, S.
A1  - Martins, C. J. A. P.
A1  - Molaro, P.
A1  - Petitjean, P.
A1  - Rahmani, H.
A1  - Srianand, R.
A1  - Wendt, Martin
T1  - The UVES Large Program for testing fundamental physics - III. Constraints on the fine-structure constant from three telescopes
JF  - Monthly notices of the Royal Astronomical Society
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - quasars: individual: HS 1549+1919
KW  - cosmology: miscellaneous
KW  - cosmology: observations
Y1  - 2014
U6  - https://doi.org/10.1093/mnras/stu1754
SN  - 0035-8711
SN  - 1365-2966
VL  - 445
IS  - 1
SP  - 128
EP  - 150
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
A1  - Hamann, Wolf-Rainer
T1  - The Wolf-Rayet stars in M31 I. Analysis of the late-type WN stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Comprehensive studies of Wolf-Rayet stars were performed in the past for the Galactic and the LMC population. The results revealed significant differences, but also unexpected similarities between the WR populations of these different galaxies. Analyzing the WR stars in M 31 will extend our understanding of these objects in different galactic environments.
 Aims. The present study aims at the late-type WN stars in M 31. The stellar and wind parameters will tell about the formation of WR stars in other galaxies with different metallicity and star formation histories. The obtained parameters will provide constraints to the evolution of massive stars in the environment of M 31.
 Methods. We used the latest version of the Potsdam Wolf-Rayet model atmosphere code to analyze the stars via fitting optical spectra and photometric data. To account for the relatively low temperatures of the late WN I 0 and WN I I subtypes, our WN models have been extended into this temperature regime.
 Results. Stellar and atmospheric parameters are derived for all known late-type WN stars in M 31 with available spectra. All of these stars still have hydrogen in their outer envelopes, some of them up to 50% by mass. The stars are located on the cool side of the zero age main sequence in the Hertzsprung-Russell diagram, while their luminosities range from 105 to 1064). It is remarkable that no star exceeds 106 L.
 Conclusions. If formed via single-star evolution, the late-type WN stars in M 31 stem from an initial mass range between 20 and 60 M-circle dot. From the very late-type WN9-11 stars, only one star is located in the S Doradus instability strip. We do not find any late-type WN stars with the high luminosities known in the Milky Way.
KW  - stars: massive
KW  - stars: evolution
KW  - stars: mass-loss
KW  - stars: Wolf-Rayet
KW  - stars: atmospheres
KW  - stars: winds
KW  - outflows
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201323240
SN  - 0004-6361
SN  - 1432-0746
VL  - 563
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Hainich, Rainer
A1  - Ruehling, Ute
A1  - Todt, Helge Tobias
A1  - Oskinova, Lida
A1  - Liermann, A.
A1  - Graefener, G.
A1  - Foellmi, C.
A1  - Schnurr, O.
A1  - Hamann, Wolf-Rainer
T1  - The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC.
 Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars.
 Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10(6) L-circle dot and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L-circle dot) = 5.3 ... 5.8.
 Conclusions. While the few extremely luminous stars (log (L/L-circle dot) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L-circle dot) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M-circle dot. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.
KW  - stars: Wolf-Rayet
KW  - Magellanic Clouds
KW  - stars: early-type
KW  - stars: atmospheres
KW  - stars: winds, outflows
KW  - stars: mass-loss
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201322696
SN  - 0004-6361
SN  - 1432-0746
VL  - 565
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Rumyantsev, Artem M.
A1  - Santer, Svetlana
A1  - Kramarenko, Elena Yu.
T1  - Theory of collapse and overcharging of a polyelectrolyte microgel induced by an oppositely charged surfactant
JF  - Macromolecules : a publication of the American Chemical Society
N2  - We report on the theoretical study of interaction of ionic surfactants with oppositely charged microgel particles in dilute solutions. Two approaches are proposed. Within the first approach, the micellization of the surfactants inside the microgel is taken into account while the second model focuses on the hydrophobic interactions of the surfactant tails with the hydrophobic parts of microgel subchains. It has been shown that microgels effectively absorb surfactant ions. At low surfactant concentration this absorption is realized due to an ion exchange between microgel counterions and surfactant ions. The ion exchange is significantly affected by the amount of the microgel counterions initially trapped within the microgel particles which depends on the size of the microgel, its ionization degree, cross-linking density as well as polymer concentration in the solution. Increase of the surfactant concentration causes contraction of the microgels, which can be realized as either a continuous shrinking or a jump-like collapse transition depending on the system parameters. In the collapsed state additional absorption of surfactants by microgels takes place due to an energy gain from micellization or hydrophobic interactions. This leads to microgel precipitation and successive microgel overcharging at an excess of the surfactant in the solution. The theoretical results are compared with the existing experimental data, in particular, on photosensitive surfactant/microgel complexes.
Y1  - 2014
U6  - https://doi.org/10.1021/ma500637d
SN  - 0024-9297
SN  - 1520-5835
VL  - 47
IS  - 15
SP  - 5388
EP  - 5399
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Miedema, Piter Sybren
A1  - Beye, Martin
A1  - Koennecke, R.
A1  - Schiwietz, Gregor
A1  - Föhlisch, Alexander
T1  - Thermal evolution of the band edges of 6H-SiC: X-ray methods compared to the optical band gap
JF  - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy
N2  - The band gap of semiconductors like silicon and silicon carbide (SIC) is the key for their device properties. In this research, the band gap of 6H-SiC and its temperature dependence were analyzed with silicon 2p X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) allowing for a separate analysis of the conduction-band minimum (CBM) and valence-band maximum (VBM) components of the band gap. The temperature-dependent asymmetric band gap shrinking of 6H-SiC was determined with a valence-band slope of +2.45 x 10(-4) eV/K and a conduction-band slope of -1.334 x 10(-4) eV/K. The apparent asymmetry, e.g., that two thirds of the band-gap shrinking with increasing temperature is due to the VBM evolution in 6H-SiC, is similar to the asymmetry obtained for pure silicon before. The overall band gap temperature-dependence determined with XAS and nonresonant XES is compared to temperature-dependent optical studies. The core-excitonic binding energy appearing in the Si 2p XAS is extracted as the main difference. In addition, the energy loss of the onset of the first band in RIXS yields to values similar to the optical band gap over the tested temperature range. (C) 2014 Elsevier B.V. All rights reserved.
KW  - RIXS
KW  - XAS
KW  - XES
KW  - Semiconductors
KW  - Silicon carbide
Y1  - 2014
U6  - https://doi.org/10.1016/j.elspec.2014.08.003
SN  - 0368-2048
SN  - 1873-2526
VL  - 197
SP  - 37
EP  - 42
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Navirian, Hengameh A.
A1  - Schick, Daniel
A1  - Gaal, Peter
A1  - Leitenberger, Wolfram
A1  - Shayduk, Roman
A1  - Bargheer, Matias
T1  - Thermoelastic study of nanolayered structures using time-resolved X-ray diffraction at high repetition rate
JF  - Applied physics letters
N2  - We investigate the thermoelastic response of a nanolayered sample composed of a metallic SrRuO3 electrode sandwiched between a ferroelectric Pb(Zr0.2Ti0.8)O-3 film with negative thermal expansion and a SrTiO3 substrate. SrRuO3 is rapidly heated by fs-laser pulses with 208 kHz repetition rate. Diffraction of X-ray pulses derived from a synchrotron measures the transient out-of-plane lattice constant c of all three materials simultaneously from 120 ps to 5 mu s with a relative accuracy up to Delta c/c = 10(-6). The in-plane propagation of sound is essential for understanding the delayed out-of-plane compression of Pb(Zr0.2Ti0.8)O-3.
Y1  - 2014
U6  - https://doi.org/10.1063/1.4861873
SN  - 0003-6951
SN  - 1077-3118
VL  - 104
IS  - 2
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Kurfuerst, P.
A1  - Feldmeier, Achim
A1  - Krticka, Jiri
T1  - Time-dependent modeling of extended thin decretion disks of critically rotating stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. During their evolution massive stars can reach the phase of critical rotation when a further increase in rotational speed is no longer possible. Direct centrifugal ejection from a critically or near-critically rotating surface forms a gaseous equatorial decretion disk. Anomalous viscosity provides the efficient mechanism for transporting the angular momentum outwards. The outer part of the disk can extend up to a very large distance from the parent star.
 Aims. We study the evolution of density, radial and azimuthal velocity, and angular momentum loss rate of equatorial decretion disks out to very distant regions. We investigate how the physical characteristics of the disk depend on the distribution of temperature and viscosity.
 Methods. We calculated stationary models using the Newton-Raphson method. For time-dependent hydrodynamic modeling we developed the numerical code based on an explicit finite difference scheme on an Eulerian grid including full Navier-Stokes shear viscosity.
 Results. The sonic point distance and the maximum angular momentum loss rate strongly depend on the temperature profile and are almost independent of viscosity. The rotational velocity at large radii rapidly drops accordingly to temperature and viscosity distribution. The total amount of disk mass and the disk angular momentum increase with decreasing temperature and viscosity.
 Conclusions. The time-dependent one-dimensional models basically confirm the results obtained in the stationary models as well as the assumptions of the analytical approximations. Including full Navier-Stokes viscosity we systematically avoid the rotational velocity sign change at large radii. The unphysical drop of the rotational velocity and angular momentum loss at large radii (present in some models) can be avoided in the models with decreasing temperature and viscosity.
KW  - stars: mass-loss
KW  - stars: evolution
KW  - stars: rotation
KW  - hydrodynamics
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201424272
SN  - 0004-6361
SN  - 1432-0746
VL  - 569
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Cardinaletti, Ilaria
A1  - Kesters, Jurgen
A1  - Bertho, Sabine
A1  - Conings, Bert
A1  - Piersimoni, Fortunato
A1  - Lutsen, Laurence
A1  - Nesladek, Milos
A1  - Van Mele, Bruno
A1  - Van Assche, Guy
A1  - Vandewal, Koen
A1  - Salleo, Alberto
A1  - Vanderzande, Dirk
A1  - Maes, Wouter
A1  - Manca, Jean V.
T1  - Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology
JF  - Journal of photonics for energy
N2  - When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.
KW  - organic photovoltaics
KW  - bulk heterojunction
KW  - thermal stability
KW  - phase separation
KW  - lifetime
Y1  - 2014
U6  - https://doi.org/10.1117/1.JPE.4.040997
SN  - 1947-7988
VL  - 4
PB  - SPIE
CY  - Bellingham
ER  - 
TY  - JOUR
A1  - Harris, Scott
A1  - Mellinger, Axel
T1  - Towards a better understanding of dielectric barrier discharges in ferroelectrets: Paschen breakdown fields in micrometer sized voids
JF  - Journal of applied physics
N2  - Charged cellular polypropylene foams (i.e., ferro-or piezoelectrets) demonstrate high piezoelectric activity upon being electrically charged. When an external electric field is applied, dielectric barrier discharges (DBDs) occur, resulting in a separation of charges which are subsequently deposited on dielectric surfaces of internal micrometer sized voids. This deposited space charge is responsible for the piezoelectric activity of the material. Previous studies have indicated charging fields larger than predicted by Townsend's model of Paschen breakdown applied to a multilayered electromechanical model; a discrepancy which prompted the present study. The actual breakdown fields for micrometer sized voids were determined by constructing single cell voids using polypropylene spacers with heights ranging from 8 to 75 mu m, "sandwiched" between two polypropylene dielectric barriers and glass slides with semi-transparent electrodes. Subsequently, a bipolar triangular charging waveform with a peak voltage of 6 kV was applied to the samples. The breakdown fields were determined by monitoring the emission of light due to the onset of DBDs using an electron multiplying CCD camera. The breakdown fields at absolute pressures from 101 to 251 kPa were found to be in good agreement with the standard Paschen curves. Additionally, the magnitude of the light emission was found to scale linearly with the amount of gas, i.e., the height of the voids. Emissions were homogeneous over the observed regions of the voids for voids with heights of 25 lm or less and increasingly inhomogeneous for void heights greater than 40 lm at high electric fields.
Y1  - 2014
U6  - https://doi.org/10.1063/1.4871678
SN  - 0021-8979
SN  - 1089-7550
VL  - 115
IS  - 16
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Lange, Ilja
A1  - Reiter, Sina
A1  - Paetzel, Michael
A1  - Zykov, Anton
A1  - Nefedov, Alexei
A1  - Hildebrandt, Jana
A1  - Hecht, Stefan
A1  - Kowarik, Stefan
A1  - Woell, Christof
A1  - Heimel, Georg
A1  - Neher, Dieter
T1  - Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers
JF  - Advanced functional materials
N2  - Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self-assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole-injecting and electron-injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well-defined molecular monolayers.
KW  - ZnO
KW  - self-assembled monolayers
KW  - phosphonic acid
KW  - surface modification
KW  - electrodes
Y1  - 2014
U6  - https://doi.org/10.1002/adfm.201401493
SN  - 1616-301X
SN  - 1616-3028
VL  - 24
IS  - 44
SP  - 7014
EP  - 7024
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Schick, Daniel
A1  - Herzog, Marc
A1  - Bojahr, Andre
A1  - Leitenberger, Wolfram
A1  - Hertwig, Andreas
A1  - Shayduk, Roman
A1  - Bargheer, Matias
T1  - Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction
JF  - Structural dynamics
N2  - Using ultrafast X-ray diffraction, we study the coherent picosecond lattice dynamics of photoexcited thin films in the two limiting cases, where the photoinduced stress profile decays on a length scale larger and smaller than the film thickness. We solve a unifying analytical model of the strain propagation for acoustic impedance-matched opaque films on a semi-infinite transparent substrate, showing that the lattice dynamics essentially depend on two parameters: One for the spatial profile and one for the amplitude of the strain. We illustrate the results by comparison with high-quality ultrafast X-ray diffraction data of SrRuO3 films on SrTiO3 substrates. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Y1  - 2014
U6  - https://doi.org/10.1063/1.4901228
SN  - 2329-7778
VL  - 1
IS  - 6
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - THES
A1  - Trabant, Christoph
T1  - Ultrafast photoinduced phase transitions in complex materials probed by time-resolved resonant soft x-ray diffraction
T1  - Ultraschnelle lichtinduzierte Phasenübergänge in komplexen Materialien untersucht mit zeitaufgelöster resonanter Weichröntgenbeugung
N2  - In processing and data storage mainly ferromagnetic (FM) materials are being used. Approaching physical limits, new concepts have to be found for faster, smaller switches, for higher data densities and more energy efficiency. Some of the discussed new concepts involve the material classes of correlated oxides and materials with antiferromagnetic coupling. Their applicability depends critically on their switching behavior, i.e., how fast and how energy efficient material properties can be manipulated. This thesis presents investigations of ultrafast non-equilibrium phase transitions on such new materials. In transition metal oxides (TMOs) the coupling of different degrees of freedom and resulting low energy excitation spectrum often result in spectacular changes of macroscopic properties (colossal magneto resistance, superconductivity, metal-to-insulator transitions) often accompanied by nanoscale order of spins, charges, orbital occupation and by lattice distortions, which make these material attractive. Magnetite served as a prototype for functional TMOs showing a metal-to-insulator-transition (MIT) at T = 123 K. By probing the charge and orbital order as well as the structure after an optical excitation we found that the electronic order and the structural distortion, characteristics of the insulating phase in thermal equilibrium, are destroyed within the experimental resolution of 300 fs. The MIT itself occurs on a 1.5 ps timescale. It shows that MITs in functional materials are several thousand times faster than switching processes in semiconductors. Recently ferrimagnetic and antiferromagnetic (AFM) materials have become interesting. It was shown in ferrimagnetic GdFeCo, that the transfer of angular momentum between two opposed FM subsystems with different time constants leads to a switching of the magnetization after laser pulse excitation. In addition it was theoretically predicted that demagnetization dynamics in AFM should occur faster than in FM materials as no net angular momentum has to be transferred out of the spin system. We investigated two different AFM materials in order to learn more about their ultrafast dynamics. In Ho, a metallic AFM below T ≈ 130 K, we found that the AFM Ho can not only be faster but also ten times more energy efficiently destroyed as order in FM comparable metals. In EuTe, an AFM semiconductor below T ≈ 10 K, we compared the loss of magnetization and laser-induced structural distortion in one and the same experiment. Our experiment shows that they are effectively disentangled. An exception is an ultrafast release of lattice dynamics, which we assign to the release of magnetostriction. The results presented here were obtained with time-resolved resonant soft x-ray diffraction at the Femtoslicing source of the Helmholtz-Zentrum Berlin and at the free-electron laser in Stanford (LCLS). In addition the development and setup of a new UHV-diffractometer for these experiments will be reported.
N2  - In der Datenspeichertechnologie werden bisher hauptsächlich ferromagnetische Materialien eingesetzt. Da mit diesen aber physikalische Grenzen erreicht werden, werden neue Konzepte gesucht, um schnellere und kleinere Schalter, größere Datendichten und eine höherere Energieeffizienz zu erzeugen. Unter den diskutierten Materialklassen finden sich komplexen Übergangsmetalloxide und Materialien mit antiferromagnetischer Kopplung. Die Anwendbarkeit solcher Materialien hängt stark davon ab, wie schnell sich deren Eigenschaften verändern lassen und wieviel Energie dafür eingesetzt werden muss. Die vorliegende Arbeit beschäftigt sich mit ultraschnellen, Nicht-Gleichgewicht-Phasenübergängen genau in solchen Materialien. In Übergangsmetalloxiden führt die enge Kopplung zwischen den unterschiedlichen Freiheitsgraden zu einem effektiven niederenergetischen Anregungsspektrum. Diese Anregungen sind oft verknüpft mit spektakulären makroskopischen Eigenschaften, wie z.B. dem kolossalen Magnetowiderstand, Hochtemperatur-Supraleitung, Metall- Isolator-Übergang, die oft von nanoskaliger Ordnung von Spins, Ladungen, orbitaler Besetzung sowie Gitterverzerrungen begleitet sind. Dadurch werden diese Materialien interessant für Anwendbarkeit. Magnetit, ein Prototyp eines solchen funktionalen Materials zeigt einen Metall-Isolator-Übergang bei T = 123 K. Untersucht man die Ladungs- und orbitale Ordnung sowie die Struktur nach einer optischen Anregung, so findet man, dass die elektronische Struktur und Gitterverzerrung, die kennzeichnend für die Tieftemperaturphase sind, innerhalb der Zeitauflösung des Experiments von 300 fs zerstört wird. Der eigentliche Metall-Isolator-Übergang zeigt sich erst nach 1.5 ps. Die Ergebnisse zeigen, dass MITs in funktionalen Materialien bis zu tausend Mal schneller geschaltet werden können als in vorhandenen Halbleiter-Schaltern. Seit kurzem rücken auch ferrimagnetische und antiferromagnetische Materialen in den Fokus des Interesses. Es wurde im Ferrimagnet GdFeCo gezeigt, dass der Transfer von Drehimpuls zwischen zwei entgegengesetzten Subsystemen mit unterschiedlichen Zeitkonstanten zu einem Umschalten der Magnetisierung führt. Zudem wurde vorhergesagt, dass Demagnetisierungsdynamiken in antiferromagnetischen Materialien schneller ablaufen soll als in ferromagnetischen, da kein Drehimpuls aus dem Spinsystem abgeführt werden muss. Damit wir mehr über antiferromagnetische Dynamik erfahren haben wir zwei unterschiedliche Antiferromagneten untersucht, um sie mit den bekannten FM zu vergleichen. Im metallischen AFM Holmium fanden wir, dass die magnetische Ordnung schneller und zehnmal energieeffizienter zerstört werden kann als in vergleichbaren FM Metallen. In Europium-Tellurid, einem antiferromagnetischem Halbleiter, haben wir den Zerfall der magnetischen Ordnung im Hinblick auf Wechselwirkungen mit der Struktur untersucht. Wir fanden auf kurzen Zeitskalen eine eher entkoppelte Dynamik. Eine Ausnahme ist ein schneller Beitrag zur Gitterdynamik, den wir mit dem Wegfall von Magnetostriktion erklären. Die hier gezeigten Ergebnisse wurden mit Hilfe zeitaufgelöster resonanter weicher Röntgenbeugung an der Femtoslicing Strahlungsquelle des Helmholtz-Zentrums Berlin und am freien Elektronenlaser LCLS gemessen. Zusätzlich wird über die Entwicklung und den Bau eines UHV-Diffraktometers für diese Experimente berichtet.
KW  - Ultraschnell
KW  - Weichröntgenbeugung
KW  - nichtgleichgewichts Dynamik
KW  - Phasenübergänge
KW  - Antiferromagnetisch
KW  - Ultrafast
KW  - soft x-ray diffraction
KW  - photoinduced dynamics
KW  - phase transitions
KW  - antiferromagnetic
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-71377
ER  - 
TY  - JOUR
A1  - Gaal, Peter
A1  - Schick, Daniel
A1  - Herzog, Marc
A1  - Bojahr, Andre
A1  - Shayduk, Roman
A1  - Goldshteyn, Jevgeni
A1  - Leitenberger, Wolfram
A1  - Vrejoiu, Ionela
A1  - Khakhulin, Dmitry
A1  - Wulff, Michael
A1  - Bargheer, Matias
T1  - Ultrafast switching of hard X-rays
JF  - Journal of synchrotron radiation
N2  - A new concept for shortening hard X-ray pulses emitted from a third-generation synchrotron source down to few picoseconds is presented. The device, called the PicoSwitch, exploits the dynamics of coherent acoustic phonons in a photo-excited thin film. A characterization of the structure demonstrates switching times of <= 5 ps and a peak reflectivity of similar to 10(-3). The device is tested in a real synchrotron-based pump-probe experiment and reveals features of coherent phonon propagation in a second thin film sample, thus demonstrating the potential to significantly improve the temporal resolution at existing synchrotron facilities.
KW  - ultrafast X-ray diffraction
KW  - thin film
KW  - coherent phonons
KW  - X-ray switching
KW  - pulse shortening
KW  - optical pump X-ray probe
KW  - time-resolved
Y1  - 2014
U6  - https://doi.org/10.1107/S1600577513031949
SN  - 0909-0495
SN  - 1600-5775
VL  - 21
SP  - 380
EP  - 385
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - THES
A1  - Goswami, Bedartha
T1  - Uncertainties in climate data analysis
T1  - Unsicherheiten in der Analyse von Klimadaten
BT  - Perspectives on working with measurement errors and other unknowns
BT  - Über Möglichkeiten der Arbeit mit Meßfehlern und anderen Unbekannten
N2  - Scientific inquiry requires that we formulate not only what we know, but also what we do not know and by how much. In climate data analysis, this involves an accurate specification of measured quantities and a consequent analysis that consciously propagates the measurement errors at each step. The dissertation presents a thorough analytical method to quantify errors of measurement inherent in paleoclimate data. An additional focus are the uncertainties in assessing the coupling between different factors that influence the global mean temperature (GMT).

Paleoclimate studies critically rely on `proxy variables' that record climatic signals in natural archives. However, such proxy records inherently involve uncertainties in determining the age of the signal. We present a generic Bayesian approach to analytically determine the proxy record along with its associated uncertainty, resulting in a time-ordered sequence of correlated probability distributions rather than a precise time series. We further develop a recurrence based method to detect dynamical events from the proxy probability distributions. The methods are validated with synthetic examples and
demonstrated with real-world proxy records. The proxy estimation step reveals the interrelations between proxy variability and uncertainty. The recurrence analysis of the East Asian Summer Monsoon during the last 9000 years confirms the well-known `dry' events at 8200 and 4400 BP, plus an additional significantly dry event at 6900 BP.

We also analyze the network of dependencies surrounding GMT. We find an intricate, directed network with multiple links between the different factors at multiple time delays. We further uncover a significant feedback from the GMT to the El Niño Southern Oscillation at quasi-biennial timescales. The analysis highlights the need of a more nuanced formulation of influences between different climatic factors, as well as the limitations in trying to estimate such dependencies.
N2  - Wissenschaftliche Untersuchungen setzen nicht nur eine Formulierung des gegenwärtigen Kenntnisstandes mitsamt ihrer Unsicherheiten voraus, sondern ebenso eine Eingrenzung des Unbekannten. Bezogen auf die Analyse von Klimadaten beinhaltet dies eine präzise Spezifikation gemessener Größen sowie eine durchgängige Berücksichtigung ihrer Messunsicherheiten in allen Schritten der Analyse. Diese Dissertation präsentiert eine analytische Methode zur Quantifizierung der in Paläoklimadaten inhärenten Messunsicherheiten. Ein weiterer Schwerpunkt liegt auf der Untersuchung von Unsicherheiten in der Kopplungsstruktur zwischen Klimafaktoren die bekanntermaßen die globale Durchschnittstemperatur (GMT global mean temperature) beeinflussen.

Die Paläoklimaforschung beruht in kritischem Maße auf der Analyse von Proxydaten welche die Klimaentwicklung dokumentieren. Allerdings sind Proxydaten mit inhärenten Datierungsunsicherheiten behaftet. Basierend auf einem generischen Bayes’schen Ansatz wird in dieser Dissertation eine analytische Methode vorgestellt um aus den zeitlich unsicheren Proxydaten zeitlich präzise Folgen korrelierter Wahrscheinlichkeitsverteilungen zu erhalten. Von diesen Verteilungen werden Proxyzeitreihen zusammen mit ihren Unsicherheiten berechnet. Weiterhin wird eine rekurrenzbasierte Methode zur Analyse von Proxydaten entwickelt welche anhand dieser Wahrscheinlichkeitsverteilungen plötzliche Änderungen in der Dynamik des Systems ermittelt. Beide Methoden werden mit Hilfe synthetischer Beispieldaten validiert und mit realen Proxydaten demonstriert. Diese statistische Analyse von Proxydaten deckt unteranderem die Beziehungen zwischen der Variabilität der Daten und zugehöriger Unsicherheiten der Proxyzeitreihen auf. Die Re- kurrenzanalyse des Ostasiatischen Sommermonsuns bestätigt die bekannten Trockenzeiten der letzten 9.000 Jahre um 8.200 bzw. 4.400 Jahre vor unserer Zeit und deckt eine zusätzliche Trockenzeit um etwa 6.900 Jahre vor unserer Zeit auf.

Die Kopplungsstruktur zwischen Klimafaktoren die bekanntermaßen die GMT beeinflussen lässt sich als ein verworrenes, gerichtetes Netzwerk mit multiplen Links, welche zu verschiedenen Zeitskalen gehören, darstellen. Speziell ergibt sich eine signifikante Wechselwirkung zwischen der GMT und dem ENSO-Phänomen (El Niño-Southern Oscillation) auf einer quasi-zweijährigen Zeitskala. Einerseits beleuchtet diese Analyse die Notwendigkeit Einflüsse verschiedener Klimafaktoren auf die GMT nuancierter zu formulieren, andererseits werden die Grenzen der Quantifizierung dieser Einflüsse aufgezeigt.
KW  - data analysis
KW  - Bayesian estimation
KW  - Bayessche Schätzer
KW  - Datenanalyse
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-78312
ER  - 
TY  - THES
A1  - Orgis, Thomas
T1  - Unstetige Galerkin-Diskretisierung niedriger Ordnung in einem atmosphärischen Multiskalenmodell
T1  - Low-order discontinuous Galerkin discretization in an atmospheric multi-scale model
N2  - Die Dynamik der Atmosphäre der Erde umfasst einen Bereich von mikrophysikalischer Turbulenz über konvektive Prozesse und Wolkenbildung bis zu planetaren Wellenmustern. Für Wettervorhersage und zur Betrachtung des Klimas über Jahrzehnte und Jahrhunderte ist diese Gegenstand der Modellierung mit numerischen Verfahren. Mit voranschreitender Entwicklung der Rechentechnik sind Neuentwicklungen der dynamischen Kerne von Klimamodellen, die mit der feiner werdenden Auflösung auch entsprechende Prozesse auflösen können, notwendig. Der dynamische Kern eines Modells besteht in der Umsetzung (Diskretisierung) der grundlegenden dynamischen Gleichungen für die Entwicklung von Masse, Energie und Impuls, so dass sie mit Computern numerisch gelöst werden können. Die vorliegende Arbeit untersucht die Eignung eines unstetigen Galerkin-Verfahrens niedriger Ordnung für atmosphärische Anwendungen. Diese Eignung für Gleichungen mit Wirkungen von externen Kräften wie Erdanziehungskraft und Corioliskraft ist aus der Theorie nicht selbstverständlich. Es werden nötige Anpassungen beschrieben, die das Verfahren stabilisieren, ohne sogenannte „slope limiter” einzusetzen. Für das unmodifizierte Verfahren wird belegt, dass es nicht geeignet ist, atmosphärische Gleichgewichte stabil darzustellen. Das entwickelte stabilisierte Modell reproduziert eine Reihe von Standard-Testfällen der atmosphärischen Dynamik mit Euler- und Flachwassergleichungen in einem weiten Bereich von räumlichen und zeitlichen Skalen. Die Lösung der thermischen Windgleichung entlang der mit den Isobaren identischen charakteristischen Kurven liefert atmosphärische Gleichgewichtszustände mit durch vorgegebenem Grundstrom einstellbarer Neigung zu(barotropen und baroklinen)Instabilitäten, die für die Entwicklung von Zyklonen wesentlich sind. Im Gegensatz zu früheren Arbeiten sind diese Zustände direkt im z-System(Höhe in Metern)definiert und müssen nicht aus Druckkoordinaten übertragen werden.Mit diesen Zuständen, sowohl als Referenzzustand, von dem lediglich die Abweichungen numerisch betrachtet werden, und insbesondere auch als Startzustand, der einer kleinen Störung unterliegt, werden verschiedene Studien der Simulation von barotroper und barokliner Instabilität durchgeführt. Hervorzuheben ist dabei die durch die Formulierung von Grundströmen mit einstellbarer Baroklinität ermöglichte simulationsgestützte Studie des Grades der baroklinen Instabilität verschiedener Wellenlängen in Abhängigkeit von statischer Stabilität und vertikalem Windgradient als Entsprechung zu Stabilitätskarten aus theoretischen Betrachtungen in der Literatur.
N2  - The dynamics of the Earth’s atmosphere encompass a range from microphysical turbulence over convective processes and cloud formation up to planetary wave patterns. For weather forecasting and the investigation of climate over decades and centuries, these are subject to modelling with numerical methods. With progressing development of computer technology, re-development of the dynamical cores of climate models is in order to properly handle processes covered by the increasing resolution. The dynamical core of a model consists of the adaptation(discretization)of the basic equations for the dynamics of mass, energy and momentum for solving them numerically employing computers. The presented work investigates the applicability of a low-order Discontinuous Galerkin (DG) method for atmospheric applications. With equations that include external forces like gravitation and the Coriolis force, that is not given by theory. Necessary changes for stabilizing the method without resorting to slope limiters are presented. For the unmodified method, the basic inability to properly keep atmospheric balances is demonstrated. The developed stabilized model reproduces a set of standard test cases in a wide range of spatial and temporal scales. The solution of the termal wind equation along its characteristics curves, those being identical to the isobars, produces balanced atmospheric states with tunable (barotropic and baroclinic) instability via a prescribed zonal wind field. The constructed instability directly relates to the generation of cyclones. In contrast to earlier works, these balanced states are directly given in the z system (height in meters), without need for elaborate conversion from pressure coordinates. With these constructed states, both as reference state, the deviations from which being considered numerically, and as especially as initial condition subject to a small perturbation, several studies of barotropic and baroclinic instability are conducted via simulations. Particularily, the construction of steady states with configurable zonal flows of certain baroclinity facilitates a simulation-based study of baroclinic instability of differing wavelengths, depending on static stability and vertical wind gradient, in correspondence with stability maps from theoretical considerations in the literature.
KW  - Atmosphärenmodellierung
KW  - Unstetiges Galerkin-Verfahren
KW  - Multiskale
KW  - Barokline Instabilität
KW  - thermische Windgleichung
KW  - atmospheric modelling
KW  - discontinuous Galerkin method
KW  - multi-scale
KW  - baroclinic instability
KW  - thermal wind equation
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-70687
ER  - 
TY  - JOUR
A1  - Levnajic, Zoran
A1  - Pikovskij, Arkadij
T1  - Untangling complex dynamical systems via derivative-variable correlations
JF  - Scientific reports
N2  - Inferring the internal interaction patterns of a complex dynamical system is a challenging problem. Traditional methods often rely on examining the correlations among the dynamical units. However, in systems such as transcription networks, one unit's variable is also correlated with the rate of change of another unit's variable. Inspired by this, we introduce the concept of derivative-variable correlation, and use it to design a new method of reconstructing complex systems (networks) from dynamical time series. Using a tunable observable as a parameter, the reconstruction of any system with known interaction functions is formulated via a simple matrix equation. We suggest a procedure aimed at optimizing the reconstruction from the time series of length comparable to the characteristic dynamical time scale. Our method also provides a reliable precision estimate. We illustrate the method's implementation via elementary dynamical models, and demonstrate its robustness to both model error and observation error.
Y1  - 2014
U6  - https://doi.org/10.1038/srep05030
SN  - 2045-2322
VL  - 4
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - THES
A1  - Hutter, Anne
T1  - Unveiling the epoch of reionization by simulations and high-redshift galaxies
T1  - Untersuchungen der Epoche der Reionisation mithilfe
von Simulationen und Beobachtungen hoch rotverschobener Galaxien
N2  - The Epoch of Reionization marks after recombination the second major change in the ionization state of the universe, going from a neutral to an ionized state. It starts with the appearance of the first stars and galaxies; a fraction of high-energy photons emitted from galaxies permeate into the intergalactic medium (IGM) and gradually ionize the hydrogen, until the IGM is completely ionized at z~6 (Fan et al., 2006). While the progress of reionization is driven by galaxy evolution, it changes the ionization and thermal state of the IGM substantially and affects subsequent structure and galaxy formation by various feedback mechanisms. 
Understanding this interaction between reionization and galaxy formation is further impeded by a lack of understanding of the high-redshift galactic properties such as the dust distribution and the escape fraction of ionizing photons. Lyman Alpha Emitters (LAEs) represent a sample of high-redshift galaxies that are sensitive to all these galactic properties and the effects of reionization. 
In this thesis we aim to understand the progress of reionization by performing cosmological simulations, which allows us to investigate the limits of constraining reionization by high-redshift galaxies as LAEs, and examine how galactic properties and the ionization state of the IGM affect the visibility and observed quantities of LAEs and Lyman Break galaxies (LBGs).  

In the first part of this thesis we focus on performing radiative transfer calculations to simulate reionization. We have developed a mapping-sphere-scheme, which, starting from spherically averaged temperature and density fields, uses our 1D radiative transfer code and computes the effect of each source on the IGM temperature and ionization (HII, HeII, HeIII) profiles, which are subsequently mapped onto a grid. Furthermore we have updated the 3D Monte-Carlo radiative transfer pCRASH, enabling detailed reionization simulations which take individual source characteristics into account.

In the second part of this thesis we perform a reionization simulation by post-processing a smoothed-particle hydrodynamical (SPH) simulation (GADGET-2) with 3D radiative transfer (pCRASH), where the ionizing sources are modelled according to the characteristics of the stellar populations in the hydrodynamical simulation. Following the ionization fractions of hydrogen (HI) and helium (HeII, HeIII), and temperature in our simulation, we find that reionization starts at z~11 and ends at z~6, and high density regions near sources are ionized earlier than low density regions far from sources.

In the third part of this thesis we couple the cosmological SPH simulation and the radiative transfer simulations with a physically motivated, self-consistent model for LAEs, in order to understand the importance of the ionization state of the IGM, the escape fraction of ionizing photons from galaxies and dust in the interstellar medium (ISM) on the visibility of LAEs. Comparison of our models results with the LAE Lyman Alpha (Lya) and UV luminosity functions at z~6.6 reveals a three-dimensional degeneracy between the ionization state of the IGM, the ionizing photons escape fraction and the ISM dust distribution, which implies that LAEs act not only as tracers of reionization but also of the ionizing photon escape fraction and of the ISM dust distribution. This degeneracy does not even break down when we compare simulated with observed clustering of LAEs at z~6.6. However, our results show that reionization has the largest impact on the amplitude of the LAE angular correlation functions, and its imprints are clearly distinguishable from those of properties on galactic scales. These results show that reionization cannot be constrained tightly by exclusively using LAE observations. Further observational constraints, e.g. tomographies of the redshifted hydrogen 21cm line, are required.

In addition we also use our LAE model to probe the question when a galaxy is visible as a LAE or a LBG. Within our model galaxies above a critical stellar mass can produce enough luminosity to be visible as a LBG and/or a LAE. By finding an increasing duty cycle of LBGs with Lya emission as the UV magnitude or stellar mass of the galaxy rises, our model reveals that the brightest (and most massive) LBGs most often show Lya emission. 
Predicting the Lya equivalent width (Lya EW) distribution and the fraction of LBGs showing Lya emission at z~6.6, we reproduce the observational trend of the Lya EWs with UV magnitude. However, the Lya EWs of the UV brightest LBGs exceed observations and can only be reconciled by accounting for an increased Lya attenuation of massive galaxies, which implies that the observed Lya brightest LAEs do not necessarily coincide with the UV brightest galaxies. We have analysed the dependencies of LAE observables on the properties of the galactic and intergalactic medium and the LAE-LBG connection, and this enhances our understanding of the nature of LAEs.
N2  - Die Epoche der Reionisation markiert die nach der Rekombination zweite grundlegende Änderung des Ionisationszustandes des Universums, nämlich den Übergang von einem neutralen zu einem ionisierten Zustand. Die Epoche der Reionisation beginnt mit dem Erscheinen der ersten Sterne und Galaxien. Von den Galaxien ausgesendete energiereiche Photonen durchdringen das intergalaktische Medium (IGM) und ionisieren den vorhandenen Wasserstoff schrittweise, bis das IGM bei z~6 (Fan et al., 2006) vollständig ionisiert ist. Während der Verlauf der Reionisation zum einen durch die Galaxienentwicklung bestimmt wird, verändert die Reionisation zum anderen den Ionisations- und thermischen Zustand des IGMs und beeinflusst damit  die darauffolgende Struktur- und Galaxienentwicklung durch verschiedene Rückkopplungsmechanismen. 
Die geringen Kenntnisse der Eigenschaften von Galaxien bei hohen Rotverschiebungen wie der Staubverteilung und des Anteils an ionisierenden Photonen, die die Galaxien verlassen können, erschweren jedoch das Verständnis des Wechselspiels zwischen Reionisation und Galaxienentwicklung.
Lyman Alpha Emitter (LAE) stellen Galaxien bei hoher Rotverschiebung dar, deren Sichtbarkeit diesen Eigenschaften und den Effekten der Reionisa\-tion unterliegen.
Diese Arbeit zielt darauf ab, den Verlauf der Reionisation anhand von kosmologischen Simulationen zu verstehen. Insbesondere interessiert, inwieweit der Verlauf der Reionisation durch Galaxien bei hohen Rotverschiebungen eingeschränkt werden kann, und wie die Eigenschaften der Galaxien und der Ionisationszustand des IGMs die Sichtbarkeit und die beobachtbaren Größen der LAE und Lyman Break Galaxien (LBG) beeinflussen können.

Im ersten Teil dieser Arbeit werden verschiedene Ansätze zur Lösung des Strahlungstransportes neu- und weiterentwickelt mit dem Ziel, die Epoche der Reionisation zu simulieren. Dazu wurde zum einen eine Methode entwickelt, die als Berechnungsgrundlage sphärisch gemittelte Temperatur- und Dichtefelder benutzt. Mithilfe des in dieser Arbeit entwickelten eindimensionalen Strahlungstransportcodes werden die Auswirkungen jeder Quelle auf die dementsprechenden Temperatur- und Ionisa\-tionsprofile (HII, HeII, HeIII) berechnet und diese auf ein Gitter abgebildet. Zum anderen wurde der dreidimensionale Monte-Carlo-Strahlungstransportcode pCRASH so erweitert, sodass detaillierte Reionisationsimulationen, die individulle Quelleneigenschaften berücksichtigen, durchgeführt werden können.

Im zweiten Teil dieser Arbeit wird die Epoche der Reionisation in sich konsistent simuliert, indem aufbauend auf einer gasdynamischen Simulation (smoothed particle hydrodynamics (SPH), GADGET-2) mithilfe von pCRASH Strahlungstransportrechnungen ausgeführt werden. Dabei werden die ionisierenden Quellen gemäß der Eigenschaften der Sternpopulationen in der gasdynamischen Simulation modelliert. Die Entwicklung der IGM-Ionisationsanteile an Wasserstoff (HII) und Helium (HeII, HeIII) sowie der Temperatur werden in der Simulation verfolgt. Es zeigt sich, dass Reionisation erstens bei z~11 beginnt und bei z~6 endet, und zweitens von überdichten zu unterdichten Gebieten des Kosmos hin verläuft.

Im dritten Teil der Arbeit werden kosmologische SPH - und Strahlungstransportsimulationen mit einem physikalisch motivierten, selbst-konsistenten Modell für LAEs kombiniert, um den Einfluss des Ionisationszustandes des IGMs, des Anteils der ionisierenden Photonen, die die Galaxien verlassen können, und der Staubverteilung im interstellaren Medium (ISM) auf die sichtbaren Eigenschaften der LAEs zu verstehen. Der Vergleich der Simulationsergebnisse mit den beobachteten LAE Lyman Alpha- und UV-Leuchtkraftfunktionen bei z~6.6 offenbart eine dreidimensionale Entartung zwischen dem Ionisationszustand des IGMs, dem Anteil der ionisierenden Photonen, die die Galaxien verlassen können, und der Staubverteilung im ISM. Dies bedeutet, dass die Sichtbarkeit von LAEs nicht nur ein Indikator für den Ionisationszustand des IGM ist, sondern auch für den Anteil an ionisierenden Photonen, die die Galaxien verlassen können, und für die Staubverteilung im ISM. Diese Entartung lässt sich auch nicht auflösen, wenn Simulations- und Beobachtungsdaten der räumlichen Verteilung der LAEs bei z~6.6 gemessen mit der winkelabhängigen Zweipunktkorrelationsfunktion verglichen werden. Jedoch zeigt unser Modell, dass die Reionisation den größten Effekt auf die Amplitude der Winkelkorrelation hat und dass sich ihre Spuren klar von den Effekten auf galaktischen Skalen (den Anteil der ionisierenden Photonen, die die Galaxien verlassen können, und der Staubverteilung im ISM) unterscheiden lassen. Somit kann Reionisation nicht alleine durch LAE Beobachtungen eingeschränkt werden, und es werden weitere Beobachtungen, wie z.B. die Tomographie der rotverschobenen 21cm Wasserstofflinie, benötigt.
KW  - cosmology
KW  - reionization
KW  - high-redshift galaxies
KW  - radiative transfer
KW  - simulation
KW  - Kosmologie
KW  - Reionisation
KW  - hoch rotverschobene Galaxien
KW  - Strahlungstransport
KW  - Simulationen
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-76998
ER  - 
TY  - JOUR
A1  - Xu, Jingsan
A1  - Brenner, Thomas J. K.
A1  - Chen, Zupeng
A1  - Neher, Dieter
A1  - Antonietti, Markus
A1  - Shalom, Menny
T1  - Upconversion-agent induced improvement of g-C3N4 photocatalyst under visible light
JF  - ACS applied materials & interfaces
N2  - Herein, we report the use of upconversion agents to modify graphite carbon nitride (g-C3N4) by direct thermal condensation of a mixture of ErCl3 center dot 6H(2)O and the supramolecular precursor cyanuric acid-melamine. We show the enhancement of g-C3N4 photoactivity after Er3+ doping by monitoring the photodegradation of Rhodamine B dye under visible light. The contribution of the upconversion agent is demonstrated by measurements using only a red laser. The Er3+ doping alters both the electronic and the chemical properties of g-C3N4. The Er3+ doping reduces emission intensity and lifetime, indicating the formation of new, nonradiative deactivation pathways, probably involving charge-transfer processes.
KW  - metal-free photocatalysis
KW  - upconversion
KW  - carbon nitride
KW  - RhB photodegradation
Y1  - 2014
U6  - https://doi.org/10.1021/am5051263
SN  - 1944-8244
VL  - 6
IS  - 19
SP  - 16481
EP  - 16486
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Prokhorov, Boris E.
A1  - Foerster, M.
A1  - He, M.
A1  - Namgaladze, Alexander A.
A1  - Holschneider, Matthias
T1  - Using MFACE as input in the UAM to specify the MIT dynamics
JF  - Journal of geophysical research : Space physics
N2  - The magnetosphere-ionosphere-thermosphere (MIT) dynamic system significantly depends on the highly variable solar wind conditions, in particular, on changes of the strength and orientation of the interplanetary magnetic field (IMF). The solar wind and IMF interactions with the magnetosphere drive the MIT system via the magnetospheric field-aligned currents (FACs). The global modeling helps us to understand the physical background of this complex system. With the present study, we test the recently developed high-resolution empirical model of field-aligned currents MFACE (a high-resolution Model of Field-Aligned Currents through Empirical orthogonal functions analysis). These FAC distributions were used as input of the time-dependent, fully self-consistent global Upper Atmosphere Model (UAM) for different seasons and various solar wind and IMF conditions. The modeling results for neutral mass density and thermospheric wind are directly compared with the CHAMP satellite measurements. In addition, we perform comparisons with the global empirical models: the thermospheric wind model (HWM07) and the atmosphere density model (Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Extended 2000). The theoretical model shows a good agreement with the satellite observations and an improved behavior compared with the empirical models at high latitudes. Using the MFACE model as input parameter of the UAM model, we obtain a realistic distribution of the upper atmosphere parameters for the Northern and Southern Hemispheres during stable IMF orientation as well as during dynamic situations. This variant of the UAM can therefore be used for modeling the MIT system and space weather predictions.
Y1  - 2014
U6  - https://doi.org/10.1002/2014JA019981
SN  - 2169-9380
SN  - 2169-9402
VL  - 119
IS  - 8
PB  - American Geophysical Union
CY  - Washington
ER  - 
TY  - JOUR
A1  - Archer, A.
A1  - Barnacka, Anna
A1  - Beilicke, M.
A1  - Benbow, W.
A1  - Berger, K.
A1  - Bird, R.
A1  - Biteau, Jonathan
A1  - Buckley, J. H.
A1  - Bugaev, V.
A1  - Byrum, K.
A1  - Cardenzana, J. V.
A1  - Cerruti, M.
A1  - Chen, W.
A1  - Chen, Xiaoming
A1  - Ciupik, L.
A1  - Connolly, M. P.
A1  - Cui, W.
A1  - Dickinson, H. J.
A1  - Dumm, J.
A1  - Eisch, J. D.
A1  - Falcone, A.
A1  - Federici, Simone
A1  - Feng, Q.
A1  - Finley, J. P.
A1  - Fleischhack, H.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Galante, N.
A1  - Griffin, S.
A1  - Griffiths, S. T.
A1  - Grube, J.
A1  - Gyuk, G.
A1  - Hakansson, Nils
A1  - Hanna, D.
A1  - Holder, J.
A1  - Hughes, G.
A1  - Johnson, C. A.
A1  - Kaaret, P.
A1  - Kar, P.
A1  - Kertzman, M.
A1  - Khassen, Y.
A1  - Kieda, D.
A1  - Krawczynski, H.
A1  - Kumar, S.
A1  - Lang, M. J.
A1  - Maier, G.
A1  - McArthur, S.
A1  - McCann, A.
A1  - Meagher, K.
A1  - Moriarty, P.
A1  - Mukherjee, R.
A1  - Nieto, D.
A1  - Ong, R. A.
A1  - Otte, A. N.
A1  - Park, N.
A1  - Perkins, J. S.
A1  - Pohl, Manuela
A1  - Popkow, A.
A1  - Prokoph, H.
A1  - Pueschel, Elisa
A1  - Quinn, J.
A1  - Ragan, K.
A1  - Rajotte, J.
A1  - Reyes, L. C.
A1  - Reynolds, P. T.
A1  - Richards, G. T.
A1  - Roache, E.
A1  - Sembroski, G. H.
A1  - Shahinyan, K.
A1  - Smith, A. W.
A1  - Staszak, D.
A1  - Telezhinsky, Igor O.
A1  - Tucci, J. V.
A1  - Tyler, J.
A1  - Varlotta, A.
A1  - Vincent, S.
A1  - Wakely, S. P.
A1  - Weinstein, A.
A1  - Welsing, R.
A1  - Wilhelm, Alina
A1  - Williams, D. A.
A1  - Zajczyk, A.
A1  - Zitzer, B.
T1  - Very-high energy observations of the galactic center region by veritas IN 2010-2012
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2  - The Galactic center is an interesting region for high-energy (0.1-100 GeV) and very-high-energy (E > 100 GeV) gamma-ray observations. Potential sources of GeV/TeV gamma-ray emission have been suggested, e.g., the accretion of matter onto the supermassive black hole, cosmic rays from a nearby supernova remnant (e.g., Sgr A East), particle acceleration in a plerion, or the annihilation of dark matter particles. The Galactic center has been detected by EGRET and by Fermi/LAT in the MeV/GeV energy band. At TeV energies, the Galactic center was detected with moderate significance by the CANGAROO and Whipple 10 m telescopes and with high significance by H.E.S.S., MAGIC, and VERITAS. We present the results from three years of VERITAS observations conducted at large zenith angles resulting in a detection of the Galactic center on the level of 18 standard deviations at energies above similar to 2.5 TeV. The energy spectrum is derived and is found to be compatible with hadronic, leptonic, and hybrid emission models discussed in the literature. Future, more detailed measurements of the high-energy cutoff and better constraints on the high-energy flux variability will help to refine and/or disentangle the individual models.
KW  - astroparticle physics
KW  - black hole physics
KW  - Galaxy: center
KW  - gamma rays: galaxies
KW  - methods: data analysis
KW  - radiation mechanisms: non-thermal
Y1  - 2014
U6  - https://doi.org/10.1088/0004-637X/790/2/149
SN  - 0004-637X
SN  - 1538-4357
VL  - 790
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Steffen, M.
A1  - Hubrig, Swetlana
A1  - Todt, Helge Tobias
A1  - Schoeller, M.
A1  - Hamann, Wolf-Rainer
A1  - Sandin, Christer
A1  - Schönberner, Detlef
T1  - Weak magnetic fields in central stars of planetary nebulae?
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion, stellar or substellar, can account for the variety of the observed nebular morphologies.
 Aims. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping planetary nebulae, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars.
 Methods. We obtained low-resolution polarimetric spectra with FORS2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars.
 Results. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418 as well as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113 and the weak emission line central star of the elliptical nebula Hen 2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features.
 Conclusions. Since our analysis indicates only weak fields, if any, in a few targets of our sample, we conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields.
KW  - planetary nebulae: general
KW  - stars: magnetic field
KW  - stars: AGB and post-AGB
KW  - binaries: close
KW  - techniques: polarimetric
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201423842
SN  - 0004-6361
SN  - 1432-0746
VL  - 570
PB  - EDP Sciences
CY  - Les Ulis
ER  -