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Until now, spatially resolved Raman Spectroscopy has required to scan a sample under investigation in a time-consuming step-by-step procedure. Here, we present a technique that allows the capture of an entire Raman image with only one single exposure. The Raman scattering arising from the sample was collected with a fiber-coupled high-performance astronomy spectrograph. The probe head consisting of an array of 20 x 20 multimode fibers was linked to the camera port of a microscope. To demonstrate the high potential of this new concept, Raman images of reference samples were recorded. Entire chemical maps were received without the need for a scanning procedure.
Unstetige Galerkin-Diskretisierung niedriger Ordnung in einem atmosphärischen Multiskalenmodell
(2014)
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.
Today, it is well known that galaxies like the Milky Way consist not only of stars but also of gas and dust. The galactic halo, a sphere of gas that surrounds the stellar disk of a galaxy, is especially interesting. It provides a wealth of information about in and outflowing gaseous material towards and away from galaxies and their hierarchical evolution. For the Milky Way, the so-called high-velocity clouds (HVCs), fast moving neutral gas complexes in the halo that can be traced by absorption-line measurements, are believed to play a crucial role in the overall matter cycle in our Galaxy. Over the last decades, the properties of these halo structures and their connection to the local circumgalactic and intergalactic medium (CGM and IGM, respectively) have been investigated in great detail by many different groups. So far it remains unclear, however, to what extent the results of these studies can be transferred to other galaxies in the local Universe. In this thesis, we study the absorption properties of Galactic HVCs and compare the HVC absorption characteristics with those of intervening QSO absorption-line systems at low redshift. The goal of this project is to improve our understanding of the spatial extent and physical conditions of gaseous galaxy halos in the local Universe. In the first part of the thesis we use HST /STIS ultraviolet spectra of more than 40 extragalactic background sources to statistically analyze the absorption properties of the HVCs in the Galactic halo. We determine fundamental absorption line parameters including covering fractions of different weakly/intermediately/highly ionized metals with a particular focus on SiII and MgII. Due to the similarity in the ionization properties of SiII and MgII, we are able to estimate the contribution of HVC-like halo structures to the cross section of intervening strong MgII absorbers at z = 0. Our study implies that only the most massive HVCs would be regarded as strong MgII absorbers, if the Milky Way halo would be seen as a QSO absorption line system from an exterior vantage point. Combining the observed absorption-cross section of Galactic HVCs with the well-known number density of intervening strong MgII absorbers at z = 0, we conclude that the contribution of infalling gas clouds (i.e., HVC analogs) in the halos of Milky Way-type galaxies to the cross section of strong MgII absorbers is 34%. This result indicates that only about one third of the strong MgII absorption can be associated with HVC analogs around other galaxies, while the majority of the strong MgII systems possibly is related to galaxy outflows and winds. The second part of this thesis focuses on the properties of intervening metal absorbers at low redshift. The analysis of the frequency and physical conditions of intervening metal systems in QSO spectra and their relation to nearby galaxies offers new insights into the typical conditions of gaseous galaxy halos. One major aspect in our study was to regard intervening metal systems as possible HVC analogs. We perform a detailed analysis of absorption line properties and line statistics for 57 metal absorbers along 78 QSO sightlines using newly-obtained ultraviolet spectra obtained with HST /COS. We find clear evidence for bimodal distribution in the HI column density in the absorbers, a trend that we interpret as sign for two different classes of absorption systems (with HVC analogs at the high-column density end). With the help of the strong transitions of SiII λ1260, SiIII λ1206, and CIII λ977 we have set up Cloudy photoionization models to estimate the local ionization conditions, gas densities, and metallicities. We find that the intervening absorption systems studied by us have, on average, similar physical conditions as Galactic HVC absorbers, providing evidence that many of them represent HVC analogs in the vicinity of other galaxies. We therefore determine typical halo sizes for SiII, SiIII, and CIII for L = 0.01L∗ and L = 0.05L∗ galaxies. Based on the covering fractions of the different ions in the Galactic halo, we find that, for example, the typical halo size for SiIII is ∼ 160 kpc for L = 0.05L∗ galaxies. We test the plausibility of this result by searching for known galaxies close to the QSO sightlines and at similar redshifts as the absorbers. We find that more than 34% of the measured SiIII absorbers have galaxies associated with them, with the majority of the absorbers indeed being at impact parameters ρ ≤160 kpc.
The atmosphere over the Arctic Ocean is strongly influenced by the distribution of sea ice and open water. Leads in the sea ice produce strong convective fluxes of sensible and latent heat and release aerosol particles into the atmosphere. They increase the occurrence of clouds and modify the structure and characteristics of the atmospheric boundary layer (ABL) and thereby influence the Arctic climate.
In the course of this study aircraft measurements were performed over the western Arctic Ocean as part of the campaign PAMARCMIP 2012 of the Alfred Wegener Institute for Polar and Marine Research (AWI). Backscatter from aerosols and clouds within the lower troposphere and the ABL were measured with the nadir pointing Airborne Mobile Aerosol Lidar (AMALi) and dropsondes were launched to obtain profiles of meteorological variables. Furthermore, in situ measurements of aerosol properties, meteorological variables and turbulence were part of the campaign. The measurements covered a broad range of atmospheric and sea ice conditions.
In this thesis, properties of the ABL over Arctic sea ice with a focus on the influence of open leads are studied based on the data from the PAMARCMIP campaign. The height of the ABL is determined by different methods that are applied to dropsonde and AMALi backscatter profiles. ABL heights are compared for different flights representing different conditions of the atmosphere and of sea ice and open water influence. The different criteria for ABL height that are applied show large variation in terms of agreement among each other, depending on the characteristics of the ABL and its history. It is shown that ABL height determination from lidar backscatter by methods commonly used under mid-latitude conditions is applicable to the Arctic ABL only under certain conditions. Aerosol or clouds within the ABL are needed as a tracer for ABL height detection from backscatter. Hence an aerosol source close to the surface is necessary, that is typically found under the present influence of open water and therefore convective conditions. However it is not always possible to distinguish residual layers from the actual ABL. Stable boundary layers are generally difficult to detect.
To illustrate the complexity of the Arctic ABL and processes therein, four case studies are analyzed each of which represents a snapshot of the interplay between atmosphere and underlying sea ice or water surface. Influences of leads and open water on the aerosol and clouds within the ABL are identified and discussed. Leads are observed to cause the formation of fog and cloud layers within the ABL by humidity emission. Furthermore they decrease the stability and increase the height of the ABL and consequently facilitate entrainment of air and aerosol layers from the free troposphere.
Probably no other field of statistical physics at the borderline of soft matter and biological physics has caused such a flurry of papers as polymer translocation since the 1994 landmark paper by Bezrukov, Vodyanoy, and Parsegian and the study of Kasianowicz in 1996. Experiments, simulations, and theoretical approaches are still contributing novel insights to date, while no universal consensus on the statistical understanding of polymer translocation has been reached. We here collect the published results, in particular, the famous–infamous debate on the scaling exponents governing the translocation process. We put these results into perspective and discuss where the field is going. In particular, we argue that the phenomenon of polymer translocation is non-universal and highly sensitive to the exact specifications of the models and experiments used towards its analysis.
Probably no other field of statistical physics at the borderline of soft matter and biological physics has caused such a flurry of papers as polymer translocation since the 1994 landmark paper by Bezrukov, Vodyanoy, and Parsegian and the study of Kasianowicz in 1996. Experiments, simulations, and theoretical approaches are still contributing novel insights to date, while no universal consensus on the statistical understanding of polymer translocation has been reached. We here collect the published results, in particular, the famous–infamous debate on the scaling exponents governing the translocation process. We put these results into perspective and discuss where the field is going. In particular, we argue that the phenomenon of polymer translocation is non-universal and highly sensitive to the exact specifications of the models and experiments used towards its analysis.
It is generally agreed upon that stars typically form in open clusters and stellar associations, but little is known about the structure of the open cluster system. Do open clusters and stellar associations form isolated or do they prefer to form in groups and complexes? Open cluster groups and complexes could verify star forming regions to be larger than expected, which would explain the chemical homogeneity over large areas in the Galactic disk. They would also define an additional level in the hierarchy of star formation and could be used as tracers for the scales of fragmentation in giant molecular clouds? Furthermore, open cluster groups and complexes could affect Galactic dynamics and should be considered in investigations and simulations on the dynamical processes, such as radial migration, disc heating, differential rotation, kinematic resonances, and spiral structure.
In the past decade there were a few studies on open cluster pairs (de La Fuente Marcos & de La Fuente Marcos 2009a,b,c) and on open cluster groups and complexes (Piskunov et al. 2006). The former only considered spatial proximity for the identification of the pairs, while the latter also required tangential velocities to be similar for the members. In this work I used the full set of 6D phase-space information to draw a more detailed picture on these structures. For this purpose I utilised the most homogeneous cluster catalogue available, namely the Catalogue of Open Cluster Data (COCD; Kharchenko et al. 2005a,b), which contains parameters for 650 open clusters and compact associations, as well as for their uniformly selected members. Additional radial velocity (RV) and metallicity ([M/H]) information on the members were obtained from the RAdial Velocity Experiment (RAVE; Steinmetz et al. 2006; Kordopatis et al. 2013) for 110 and 81 clusters, respectively. The RAVE sample was cleaned considering quality parameters and flags provided by RAVE (Matijevič et al. 2012; Kordopatis et al. 2013). To ensure that only real members were included for the mean values, also the cluster membership, as provided by Kharchenko et al. (2005a,b), was considered for the stars cross-matched in RAVE.
6D phase-space information could be derived for 432 out of the 650 COCD objects and I used an adaption of the Friends-of-Friends algorithm, as used in cosmology, to identify potential groupings. The vast majority of the 19 identified groupings were pairs, but I also found four groups of 4-5 members and one complex with 15 members. For the verification of the identified structures, I compared the results to a randomly selected subsample of the catalogue for the Milky Way global survey of Star Clusters (MWSC; Kharchenko et al. 2013), which became available recently, and was used as reference sample. Furthermore, I implemented Monte-Carlo simulations with randomised samples created from two distinguished input distributions for the spatial and velocity parameters. On the one hand, assuming a uniform distribution in the Galactic disc and, on the other hand, assuming the COCD data distributions to be representative for the whole open cluster population.
The results suggested that the majority of identified pairs are rather by chance alignments, but the groups and the complex seemed to be genuine. A comparison of my results to the pairs, groups and complexes proposed in the literature yielded a partial overlap, which was most likely because of selection effects and different parameters considered. This is another verification for the existence of such structures.
The characteristics of the found groupings favour that members of an open cluster grouping originate from a common giant molecular cloud and formed in a single, but possibly sequential, star formation event. Moreover, the fact that the young open cluster population showed smaller spatial separations between nearest neighbours than the old cluster population indicated that the lifetime of open cluster groupings is most likely comparable to that of the Galactic open cluster population itself. Still even among the old open clusters I could identify groupings, which suggested that the detected structure could be in some cases more long lived as one might think.
In this thesis I could only present a pilot study on structures in the Galactic open cluster population, since the data sample used was highly incomplete. For further investigations a far more complete sample would be required. One step in this direction would be to use data from large current surveys, like SDSS, RAVE, Gaia-ESO and VVV, as well as including results from studies on individual clusters. Later the sample can be completed by data from upcoming missions, like Gaia and 4MOST. Future studies using this more complete open cluster sample will reveal the effect of open cluster groupings on star formation theory and their significance for the kinematics, dynamics and evolution of the Milky Way, and thereby of spiral galaxies.
The H.E.S.S. array is a third generation Imaging Atmospheric Cherenkov Telescope (IACT) array. It is located in the Khomas Highland in Namibia, and measures very high energy (VHE) gamma-rays. In Phase I, the array started data taking in 2004 with its four identical 13 m telescopes. Since then, H.E.S.S. has emerged as the most successful IACT experiment to date. Among the almost 150 sources of VHE gamma-ray radiation found so far, even the oldest detection, the Crab Nebula, keeps surprising the scientific community with unexplained phenomena such as the recently discovered very energetic flares of high energy gamma-ray radiation. During its most recent flare, which was detected by the Fermi satellite in March 2013, the Crab Nebula was simultaneously observed with the H.E.S.S. array for six nights. The results of the observations will be discussed in detail during the course of this work. During the nights of the flare, the new 24 m × 32 m H.E.S.S. II telescope was still being commissioned, but participated in the data taking for one night. To be able to reconstruct and analyze the data of the H.E.S.S. Phase II array, the algorithms and software used by the H.E.S.S. Phase I array had to be adapted. The most prominent advanced shower reconstruction technique developed by de Naurois and Rolland, the template-based model analysis, compares real shower images taken by the Cherenkov telescope cameras with shower templates obtained using a semi-analytical model. To find the best fitting image, and, therefore, the relevant parameters that describe the air shower best, a pixel-wise log-likelihood fit is done. The adaptation of this advanced shower reconstruction technique to the heterogeneous H.E.S.S. Phase II array for stereo events (i.e. air showers seen by at least two telescopes of any kind), its performance using MonteCarlo simulations as well as its application to real data will be described.
In dieser Arbeit werden nichtlineare Kopplungsmechanismen von akustischen Oszillatoren untersucht, die zu Synchronisation führen können. Aufbauend auf die Fragestellungen vorangegangener Arbeiten werden mit Hilfe theoretischer und experimenteller Studien sowie mit Hilfe numerischer Simulationen die Elemente der Tonentstehung in der Orgelpfeife und die Mechanismen der gegenseitigen Wechselwirkung von Orgelpfeifen identifiziert. Daraus wird erstmalig ein vollständig auf den aeroakustischen und fluiddynamischen Grundprinzipien basierendes nichtlinear gekoppeltes Modell selbst-erregter Oszillatoren für die Beschreibung des Verhaltens zweier wechselwirkender Orgelpfeifen entwickelt. Die durchgeführten Modellrechnungen werden mit den experimentellen Befunden verglichen. Es zeigt sich, dass die Tonentstehung und die Kopplungsmechanismen von Orgelpfeifen durch das entwickelte Oszillatormodell in weiten Teilen richtig beschrieben werden. Insbesondere kann damit die Ursache für den nichtlinearen Zusammenhang von Kopplungsstärke und Synchronisation des gekoppelten Zwei-Pfeifen Systems, welcher sich in einem nichtlinearen Verlauf der Arnoldzunge darstellt, geklärt werden. Mit den gewonnenen Erkenntnissen wird der Einfluss des Raumes auf die Tonentstehung bei Orgelpfeifen betrachtet. Dafür werden numerische Simulationen der Wechselwirkung einer Orgelpfeife mit verschiedenen Raumgeometrien, wie z. B. ebene, konvexe, konkave, und gezahnte Geometrien, exemplarisch untersucht. Auch der Einfluss von Schwellkästen auf die Tonentstehung und die Klangbildung der Orgelpfeife wird studiert. In weiteren, neuartigen Synchronisationsexperimenten mit identisch gestimmten Orgelpfeifen, sowie mit Mixturen wird die Synchronisation für verschiedene, horizontale und vertikale Pfeifenabstände in der Ebene der Schallabstrahlung, untersucht. Die dabei erstmalig beobachteten räumlich isotropen Unstetigkeiten im Schwingungsverhalten der gekoppelten Pfeifensysteme, deuten auf abstandsabhängige Wechsel zwischen gegen- und gleichphasigen Sychronisationsregimen hin. Abschließend wird die Möglichkeit dokumentiert, das Phänomen der Synchronisation zweier Orgelpfeifen durch numerische Simulationen, also der Behandlung der kompressiblen Navier-Stokes Gleichungen mit entsprechenden Rand- und Anfangsbedingungen, realitätsnah abzubilden. Auch dies stellt ein Novum dar.