@article{FoxBargerWakkeretal.2018, author = {Fox, Andrew J. and Barger, Kathleen A. and Wakker, Bart P. and Richter, Philipp and Antwi-Danso, Jacqueline and Casetti-Dinescu, Dana I. and Howk, J. Christopher and Lehner, Nicolas and Crowther, Paul A. and Lockman, Felix J.}, title = {Chemical Abundances in the Leading Arm of the Magellanic Stream}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {854}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aaa9bb}, pages = {14}, year = {2018}, abstract = {The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously, only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph and Green Bank Telescope spectra of four AGN sightlines passing through the LA and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0+(2.0)(2.0)\% 12.6(4.1)(6.0)\% solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H = 35 +/- 7\% solar, but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, two of the three nearby sightlines show high-velocity clouds with H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest.}, language = {en} } @article{ZhangYanRichter2018, author = {Zhang, Heshou and Yan, Huirong and Richter, Philipp}, title = {The influence of atomic alignment on absorption and emission spectroscopy}, series = {Monthly notices of the Royal Astronomical Society}, volume = {479}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sty1594}, pages = {3923 -- 3935}, year = {2018}, abstract = {Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in our Universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionization fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of Photodissociation regions in rho Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability.}, language = {en} } @article{HeintzWatsonJakobssonetal.2018, author = {Heintz, Kasper Elm and Watson, D. and Jakobsson, P. and Fynbo, J. P. U. and Bolmer, J. and Arabsalmani, M. and Cano, Zach and Covino, S. and Gomboc, A. and Japelj, J. and Kaper, L. and Krogager, J. -K. and Pugliese, G. and Sanchez-Ramirez, R. and Selsing, J. and Sparre, Martin and Tanvir, N. R. and Thone, C. C. and de Ugarte Postigo, A. and Vergani, S. D.}, title = {Highly ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts}, series = {Monthly notices of the Royal Astronomical Society}, volume = {479}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/sty1447}, pages = {3456 -- 3476}, year = {2018}, abstract = {We present here a survey of high-ionization absorption lines in the afterglow spectra of long-duration gamma-ray bursts (GRBs) obtained with the VLT/X-shooter spectrograph. Our main goal is to investigate the circumburst medium in the natal regions of GRBs. Our primary focus is on the N vλλ 1238, 1242 line transitions, but we also discuss other high-ionization lines such as O vi, C iv, and Si iv. We find no correlation between the column density of N v and the neutral gas properties such as metallicity, H i column density, and dust depletion; however, the relative velocity of N v, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of H i. This may be explained if the N v gas is part of an H ii region hosting the GRB, where the region's expansion is confined by dense, neutral gas in the GRB's host galaxy. We find tentative evidence (at 2σ significance) that the X-ray derived column density, NH, X, may be correlated with the column density of N v, which would indicate that both measurements are sensitive to the column density of the gas located in the vicinity of the GRB. We investigate the scenario where N v (and also O vi) is produced by recombination after the corresponding atoms have been stripped entirely of their electrons by the initial prompt emission, in contrast to previous models where highly ionized gas is produced by photoionization from the GRB afterglow.}, language = {en} } @article{RichterFoxWakkeretal.2018, author = {Richter, Philipp and Fox, Andrew J. and Wakker, Bart P. and Howk, J. Christopher and Lehner, Nicolas and Barger, Kathleen A. and Lockman, Felix J.}, title = {New constraints on the nature and origin of the leading arm of the magellanic stream}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {865}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aadd0f}, pages = {16}, year = {2018}, abstract = {We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H-2) in the Leading Arm (region II, LA II) of the Magellanic Stream (MS) toward the Seyfert galaxy NGC 3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140 ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multiphase absorber, delivering important constraints on the nature and origin of LA II. We derive a uniform, moderate a abundance in the two main absorber groups at +245 and +190 km s(-1) of alpha/H = 0.30 +/- 0.05 solar, a low nitrogen abundance of N/H = 0.05 +/- 0.01 solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These a, N, and dust abundances in LA II are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H2 absorption, we determine a high thermal pressure of P/k approximate to 1680 K cm(-3) in LA II, in line with the idea that LA II is located in the inner Milky Way halo at a z-height of < 20 kpc, where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario in which LA II stems from the breakup of a metal- and dust-enriched progenitor cloud that was recently (200-500 Myr ago) stripped from the SMC.}, language = {en} } @article{RichterWinkelWakkeretal.2018, author = {Richter, Philipp and Winkel, Benjamin and Wakker, Bart P. and Pingel, N. M. and Fox, Andrew J. and Heald, G. and Walterbos, Rene A. M. and Fechner, C. and Ben Bekhti, N. and Gentile, G. and Zschaechner, Laura}, title = {Circumgalactic Gas at Its Extreme}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {868}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/aae838}, pages = {12}, year = {2018}, abstract = {We present a detailed analysis of the absorption properties of one of the tidal gas streams around the "Whale" galaxy NGC 4631 in the direction of the quasar 2MASS J12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong H I Ly alpha absorption in the velocity range +550 to +800 km s(-1) related to gas from a NGC 4631 tidal stream known as Spur 2. We measure a column density of log (N(H I/cm(-2))) = 18.68 +/- 0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21 cm data. Metal absorption in Spur 2 is detected in the lines of O I, C II, Si II, and Si III in a complex absorption pattern that reflects the multiphase nature of the gas. We find that the average neutral gas fraction in Spur 2 toward 2MASS J12421031+3214268 is only 14\%. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10(9)M(circle dot). No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured O I/H I column density ratio, we determine an alpha abundance in Spur 2 of 0.131(-0.05)(+0.07) solar ([alpha/H] = -0.90 +/- 0.16), which is substantially lower than what is observed in the NGC 4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC 4631.}, language = {en} } @article{GeHeYan2016, author = {Ge, J. X. and He, J. H. and Yan, Huirong}, title = {Effects of turbulent dust grain motion to interstellar chemistry}, series = {Monthly notices of the Royal Astronomical Society}, volume = {455}, journal = {Monthly notices of the Royal Astronomical Society}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stv2560}, pages = {3570 -- 3587}, year = {2016}, abstract = {Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon interstellar chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modelling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarized as follows: (1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; (2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about 10; (3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviours too. These effects usually begin to emerge from a typical DC model age of about 10(5) yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations on to positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.}, language = {en} } @article{RichterdeBoerWerneretal.2015, author = {Richter, Philipp and de Boer, Klaas S. and Werner, Klaus and Rauch, Thomas}, title = {High-velocity gas toward the LMC resides in the Milky Way halo}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {584}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201527451}, pages = {4}, year = {2015}, abstract = {Aims. To explore the origin of high-velocity gas in the direction of the Large Magellanic Cloud, (LMC) we analyze absorption lines in the ultraviolet spectrum of a Galactic halo star that is located in front of the LMC at d = 9.2(-7.2)(+4.1) kpc distance. Methods. We study the velocity-component structure of low and intermediate metal ions (CII, SiII, SiIII) in the spectrum of RXJ0439.8-6809, as obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST), and measure equivalent widths and column densities for these ions. We supplement our COS data with a Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the nearby LMC star Sk-69 59 and with Hi 21 cm data from the Leiden-Argentina-Bonn (LAB) survey. Results. Metal absorption toward RXJ0439.8-6809 is unambiguously detected in three different velocity components near v(LSR) = 0, + 60, and + 150 km s(-1). The presence of absorption proves that all three gas components are situated in front of the star, thus located in the disk and inner halo of the Milky Way. For the high-velocity cloud (HVC) at v(LSR) = + 150 km s(-1), we derive an oxygen abundance of [O/H] = -0.63 (similar to 0.2 solar) from the neighboring Sk-69 59 sight line, in accordance with previous abundance measurements for this HVC. From the observed kinematics we infer that the HVC hardly participates in the Galactic rotation. Conclusions. Our study shows that the HVC toward the LMC represents a Milky Way halo cloud that traces low column density gas with relatively low metallicity. We rule out scenarios in which the HVC represents material close to the LMC that stems from a LMC outflow.}, language = {en} } @article{DanehkarTodtErcolanoetal.2014, author = {Danehkar, A. and Todt, Helge Tobias and Ercolano, B. and Kniazev, A. Y.}, title = {Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula Abell 48(star)}, series = {Monthly notices of the Royal Astronomical Society}, volume = {439}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu203}, pages = {3605 -- 3615}, year = {2014}, abstract = {Recent observations reveal that the central star of the planetary nebula Abell 48 exhibits spectral features similar to massive nitrogen-sequence Wolf-Rayet stars. This raises a pertinent question, whether it is still a planetary nebula or rather a ring nebula of a massive star. In this study, we have constructed a three-dimensional photoionization model of Abell 48, constrained by our new optical integral field spectroscopy. An analysis of the spatially resolved velocity distributions allowed us to constrain the geometry of Abell 48. We used the collisionally excited lines to obtain the nebular physical conditions and ionic abundances of nitrogen, oxygen, neon, sulphur and argon, relative to hydrogen. We also determined helium temperatures and ionic abundances of helium and carbon from the optical recombination lines. We obtained a good fit to the observations for most of the emission-line fluxes in our photoionization model. The ionic abundances deduced from our model are in decent agreement with those derived by the empirical analysis. However, we notice obvious discrepancies between helium temperatures derived from the model and the empirical analysis, as overestimated by our model. This could be due to the presence of a small fraction of cold metal-rich structures, which were not included in our model. It is found that the observed nebular line fluxes were best reproduced by using a hydrogen-deficient expanding model atmosphere as the ionizing source with an effective temperature of T-eff = 70 kK and a stellar luminosity of L-star = 5500 L-circle dot, which corresponds to a relatively low-mass progenitor star (similar to 3 M-circle dot) rather than a massive Pop I star.}, language = {en} } @article{FoxWakkerBargeretal.2014, author = {Fox, Andrew J. and Wakker, Bart P. and Barger, Kathleen A. and Hernandez, Audra K. and Richter, Philipp and Lehner, Nicolas and Bland-Hawthorn, Joss and Charlton, Jane C. and Westmeier, Tobias and Thom, Christopher and Tumlinson, Jason and Misawa, Toru and Howk, J. Christopher and Haffner, L. Matthew and Ely, Justin and Rodriguez-Hidalgo, Paola and Kumari, Nimisha}, title = {The COS/UVES absorption survey of the magellanic stream. III. Ionization, total mass, and inflow rate onto the milky way}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {787}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/787/2/147}, pages = {31}, year = {2014}, abstract = {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.}, language = {en} } @article{RichterFoxWakkeretal.2013, author = {Richter, Philipp and Fox, Andrew J. and Wakker, Bart P. and Lehner, Nicolas and Howk, J. Christopher and Bland-Hawthorn, Joss and Ben Bekhti, Nadya and Fechner, Cora}, title = {The COS/UVES absorption survey of the magellanic stream - II. Evidence for a complex enrichment history of the stream from the fairall 9 sightline}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {772}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/772/2/111}, pages = {19}, year = {2013}, abstract = {We present a multi-wavelength study of the Magellanic Stream (MS), a massive gaseous structure in the Local Group that is believed to represent material stripped from the Magellanic Clouds. We use ultraviolet, optical and radio data obtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances and physical conditions in the Stream toward the quasar Fairall 9. Line absorption in the MS from a large number of metal ions and from molecular hydrogen is detected in up to seven absorption components, indicating the presence of multi-phase gas. From the analysis of unsaturated S II absorption, in combination with a detailed photoionization model, we obtain a surprisingly high alpha abundance in the Stream toward Fairall 9 of [S/H] = -0.30 +/- 0.04 (0.50 solar). This value is five times higher than what is found along other MS sightlines based on similar COS/UVES data sets. In contrast, the measured nitrogen abundance is found to be substantially lower ([N/H] = -1.15 +/- 0.06), implying a very low [N/alpha] ratio of -0.85 dex. The substantial differences in the chemical composition of MS toward Fairall 9 compared to other sightlines point toward a complex enrichment history of the Stream. We favor a scenario, in which the gas toward Fairall 9 was locally enriched with a elements by massive stars and then was separated from the Magellanic Clouds before the delayed nitrogen enrichment from intermediate-mass stars could set in. Our results support (but do not require) the idea that there is a metal-enriched filament in the Stream toward Fairall 9 that originates in the LMC.}, language = {en} } @article{FoxRichterWakkeretal.2013, author = {Fox, Andrew J. and Richter, Philipp and Wakker, Bart P. and Lehner, Nicolas and Howk, J. Christopher and Ben Bekhti, Nadya and Bland-Hawthorn, Joss and Lucas, Stephen}, title = {The COS/UVES absorption survey of the magellanic stream - I. One-tenth solar abundances along the body of the stream}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {772}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/772/2/110}, pages = {16}, year = {2013}, abstract = {The Magellanic Stream (MS) is a massive and extended tail of multi-phase gas stripped out of the Magellanic Clouds and interacting with the Galactic halo. In this first paper of an ongoing program to study the Stream in absorption, we present a chemical abundance analysis based on HST/COS and VLT/UVES spectra of four active galactic nuclei (RBS 144, NGC 7714, PHL 2525, and HE 0056-3622) lying behind the MS. Two of these sightlines yield good MS metallicity measurements: toward RBS 144 we measure a low MS metallicity of [S/H] = [S II/H I] = -1.13 +/- 0.16 while toward NGC 7714 we measure [O/H] = [O I/H I] = -1.24 +/- 0.20. Taken together with the published MS metallicity toward NGC 7469, these measurements indicate a uniform abundance of approximate to 0.1 solar along the main body of the Stream. This provides strong support to a scenario in which most of the Stream was tidally stripped from the SMC approximate to 1.5-2.5 Gyr ago (a time at which the SMC had a metallicity of approximate to 0.1 solar), as predicted by several N-body simulations. However, in Paper II of this series, we report a much higher metallicity (S/H = 0.5 solar) in the inner Stream toward Fairall 9, a direction sampling a filament of the MS that Nidever et al. claim can be traced kinematically to the Large Magellanic Cloud, not the Small Magellanic Cloud. This shows that the bifurcation of the Stream is evident in its metal enrichment, as well as its spatial extent and kinematics. Finally we measure a similar low metallicity [O/H] = [O I/H I] = -1.03 +/- 0.18 in the v(LSR) = 150 km s(-1) cloud toward HE 0056-3622, which belongs to a population of anomalous velocity clouds near the south Galactic pole. This suggests these clouds are associated with the Stream or more distant structures (possibly the Sculptor Group, which lies in this direction at the same velocity), rather than tracing foreground Galactic material.}, language = {en} }