TY - THES A1 - Schmeja, Stefan T1 - Properties of turbulent star-forming clusters : models versus observations T1 - Eigenschaften turbulenter junger Sternhaufen : Modelle kontra Beobachtungen N2 - Stars are born in turbulent molecular clouds that fragment and collapse under the influence of their own gravity, forming a cluster of hundred or more stars. The star formation process is controlled by the interplay between supersonic turbulence and gravity. In this work, the properties of stellar clusters created by numerical simulations of gravoturbulent fragmentation are compared to those from observations. This includes the analysis of properties of individual protostars as well as statistical properties of the entire cluster. It is demonstrated that protostellar mass accretion is a highly dynamical and time-variant process. The peak accretion rate is reached shortly after the formation of the protostellar core. It is about one order of magnitude higher than the constant accretion rate predicted by the collapse of a classical singular isothermal sphere, in agreement with the observations. For a more reasonable comparison, the model accretion rates are converted to the observables bolometric temperature, bolometric luminosity, and envelope mass. The accretion rates from the simulations are used as input for an evolutionary scheme. The resulting distribution in the Tbol-Lbol-Menv parameter space is then compared to observational data by means of a 3D Kolmogorov-Smirnov test. The highest probability found that the distributions of model tracks and observational data points are drawn from the same population is 70%. The ratios of objects belonging to different evolutionary classes in observed star-forming clusters are compared to the temporal evolution of the gravoturbulent models in order to estimate the evolutionary stage of a cluster. While it is difficult to estimate absolute ages, the realtive numbers of young stars reveal the evolutionary status of a cluster with respect to other clusters. The sequence shows Serpens as the youngest and IC 348 as the most evolved of the investigated clusters. Finally the structures of young star clusters are investigated by applying different statistical methods like the normalised mean correlation length and the minimum spanning tree technique and by a newly defined measure for the cluster elongation. The clustering parameters of the model clusters correspond in many cases well to those from observed ones. The temporal evolution of the clustering parameters shows that the star cluster builds up from several subclusters and evolves to a more centrally concentrated cluster, while the cluster expands slower than new stars are formed. N2 - Sterne entstehen im Inneren von turbulenten Molekülwolken, die unter dem Einfluss ihrer eigenen Gravitation fragmentieren und kollabieren. So entsteht ein Sternhaufen aus hundert oder mehr Objekten. Der Sternentstehungsprozess wird durch das Wechselspiel von Überschallturbulenz und Gravitation reguliert. In dieser Arbeit werden verschiedene Eigenschaften solcher Sternhaufen, die mit Hilfe von numerischen Simulationen modelliert wurden, untersucht und mit Beobachtungsdaten verglichen. Dabei handelt es sich sowohl um Eigenschaften einzelner Protosterne, als auch um statistische Parameter des Sternhaufens als Ganzes. Es wird gezeigt, dass die Massenakkretion von Protosternen ein höchst dynamischer und zeitabhängiger Prozess ist. Die maximale Akkretionsrate wird kurz nach der Bildung des Protosterns erreicht, bevor sie annähernd exponentiell abfällt. Sie ist, in Übereinstimmung mit Beobachtungen, etwa um eine Größenordnung höher als die konstante Rate in den klassischen Modellen. Um die Akkretionsraten der Modelle zuverlässiger vergleichen zu können, werden sie mit Hilfe eines Evolutionsschemas in besser beobachtbare Parameter wie bolometrische Temperatur und Leuchtkraft sowie Hüllenmasse umgewandelt. Die dreidimensionale Verteilung dieser Parameter wird anschließend mittels eines Kolmogorov-Smirnov-Tests mit Beobachtungsdaten verglichen. Die relative Anzahl junger Sterne in verschiedenen Entwicklungsstadien wird mit der zeitlichen Entwicklung der Modelle verglichen, um so den Entwicklungsstand des Sternhaufens abschätzen zu können. Während eine genaue Altersbestimmung schwierig ist, kann der Entwicklungsstand eines Haufens relativ zu anderen gut ermittelt werden. Von den untersuchten Objekten stellt sich Serpens als der jüngste und IC 348 als der am weitesten entwickelte Sternhaufen heraus. Zuletzt werden die Strukturen von jungen Sternhaufen an Hand verschiedener statistischer Methoden und eines neuen Maßes für die Elongation eines Haufens untersucht. Auch hier zeigen die Parameter der Modelle eine gute Übereinstimmung mit solchen von beobachteten Objekten, insbesondere, wenn beide eine ähnliche Elongation aufweisen. Die zeitliche Entwicklung der Parameter zeigt, dass sich ein Sternhaufen aus mehreren kleineren Gruppen bildet, die zusammenwachsen und einen zum Zentrum hin konzentrierten Haufen bilden. Dabei werden neue Sterne schneller gebildet als sich der Sternhaufen ausdehnt. KW - Sternentstehung KW - Sternhaufen KW - Turbulenz KW - Interstellare Materie KW - Numerisches Verfahren KW - star formation KW - star clusters KW - turbulence KW - interstellar medium KW - numerical simulations Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-7364 ER - TY - JOUR A1 - Kaiser, Bjoern Onno A1 - Cacace, Mauro A1 - Scheck-Wenderoth, Magdalena A1 - Lewerenz, Bjoern T1 - Characterization of main heat transport processes in the Northeast German Basin constraints from 3-D numerical models JF - Geochemistry, geophysics, geosystems N2 - To investigate and quantify main physical heat driving processes affecting the present-day subsurface thermal field, we study a complex geological setting, the Northeast German Basin (NEGB). The internal geological structure of the NEGB is characterized by the presence of a relatively thick layer of Permian Zechstein salt (up to 5000 m), which forms many salt diapirs and pillows locally reaching nearly the surface. By means of three-dimensional numerical simulations we explore the role of heat conduction, pressure, and density driven groundwater flow as well as fluid viscosity related effects. Our results suggest that the regional temperature distribution within the basin results from interactions between regional pressure forces as driven by topographic gradients and thermal diffusion locally enhanced by thermal conductivity contrasts between the different sedimentary rocks with the highly conductive salt playing a prominent role. In contrast, buoyancy forces triggered by temperature-dependent fluid density variations are demonstrated to affect only locally the internal thermal configuration. Locations, geometry, and wavelengths of convective thermal anomalies are mainly controlled by the permeability field and thickness values of the respective geological layers. KW - advection KW - convection KW - coupled fluid and heat transport KW - numerical simulations KW - Northeast German Basin KW - salt structures Y1 - 2011 U6 - https://doi.org/10.1029/2011GC003535 SN - 1525-2027 VL - 12 IS - 13 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kaiser, Björn Onno A1 - Cacace, Mauro A1 - Scheck-Wenderoth, Magdalena T1 - Quaternary channels within the Northeast German Basin and their relevance on double diffusive convective transport processes - constraints from 3-D thermohaline numerical simulations JF - Geochemistry, geophysics, geosystems N2 - The internal geological structure of the Northeast German Basin (NEGB) is affected by intense salt diapirism and by the presence of several stratified aquifer complexes of regional relevance. The shallow Quaternary to late Tertiary freshwater aquifer is separated from the underlying Mesozoic saline aquifers by an embedded Tertiary clay enriched aquitard (Rupelian Aquitard). An important feature of this aquitard is that hydraulic connections between the upper and lower aquifers do exist in areas where the Rupelian Aquitard is missing (hydrogeological windows). Three-dimensional thermohaline numerical simulations are carried out to investigate the effects of such hydrogeological windows in the Rupelian Aquitard on the resulting groundwater, temperature, and salinity distributions. Numerical results suggest that hydrogeological windows act as preferential domains of hydraulic interconnectivity between the different aquifers at depth and enable vigorous heat and mass transport which causes a mixing of warm and saline groundwater with cold and less saline groundwater within both aquifers. In areas where the Rupelian Aquitard confines the Mesozoic aquifer, dissolved solutes from major salt structures are transported laterally giving rise to plumes of variable salinity content ranging from few hundreds of meters to several tens of kilometers. Furthermore, destabilizing thermal buoyancy forces may overwhelm counteracting stabilizing salinity induced forces offside of salt domes. This may result in buoyant upward groundwater flow transporting heat and mass to shallower levels within the same Mesozoic Aquifer. KW - double diffusive convection KW - thermohaline processes KW - numerical simulations KW - salt structures KW - Northeast German Basin KW - quarternary channels Y1 - 2013 U6 - https://doi.org/10.1002/ggge.20192 SN - 1525-2027 VL - 14 IS - 8 SP - 3156 EP - 3175 PB - American Geophysical Union CY - Washington ER - TY - GEN A1 - Feldmann, Johannes A1 - Levermann, Anders T1 - From cyclic ice streaming to Heinrich-like events BT - the grow-and-surge instability in the Parallel Ice Sheet Model T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - Here we report on a cyclic, physical ice-discharge instability in the Parallel Ice Sheet Model, simulating the flow of a three-dimensional, inherently buttressed ice-sheet-shelf system which periodically surges on a millennial timescale. The thermomechanically coupled model on 1 km horizontal resolution includes an enthalpy-based formulation of the thermodynamics, a nonlinear stress-balance-based sliding law and a very simple subglacial hydrology. The simulated unforced surging is characterized by rapid ice streaming through a bed trough, resulting in abrupt discharge of ice across the grounding line which is eventually calved into the ocean. We visualize the central feedbacks that dominate the subsequent phases of ice buildup, surge and stabilization which emerge from the interaction between ice dynamics, thermodynamics and the subglacial till layer. Results from the variation of surface mass balance and basal roughness suggest that ice sheets of medium thickness may be more susceptible to surging than relatively thin or thick ones for which the surge feedback loop is damped. We also investigate the influence of different basal sliding laws (ranging from purely plastic to nonlinear to linear) on possible surging. The presented mechanisms underlying our simulations of self-maintained, periodic ice growth and destabilization may play a role in large-scale ice-sheet surging, such as the surging of the Laurentide Ice Sheet, which is associated with Heinrich events, and ice-stream shutdown and reactivation, such as observed in the Siple Coast region of West Antarctica. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 652 KW - grounding-line migration KW - last glacial period KW - West Antarctica KW - North Atlantic KW - numerical simulations KW - iceberg discharges KW - creep stability KW - basal mechanics KW - climate KW - ocean Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-418777 SN - 1866-8372 IS - 652 ER - TY - GEN A1 - Kirchartz, Thomas A1 - Márquez, José A. A1 - Stolterfoht, Martin A1 - Unold, Thomas T1 - Photoluminescence-based characterization of halide perovskites for photovoltaics T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non-radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1419 KW - metal halide perovskites KW - numerical simulations KW - photoluminescence KW - photon recycling Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-519702 SN - 1866-8372 IS - 26 ER - TY - JOUR A1 - Kirchartz, Thomas A1 - Márquez, José A. A1 - Stolterfoht, Martin A1 - Unold, Thomas T1 - Photoluminescence-based characterization of halide perovskites for photovoltaics JF - Advanced Energy Materials N2 - Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non-radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials. KW - metal halide perovskites KW - numerical simulations KW - photoluminescence KW - photon recycling Y1 - 2020 U6 - https://doi.org/10.1002/aenm.201904134 SN - 1614-6832 SN - 1614-6840 VL - 10 IS - 26 SP - 1 EP - 21 PB - Wiley CY - Weinheim ER - TY - JOUR A1 - Li, Zhen A1 - Spangenberg, Erik A1 - Schicks, Judith Maria A1 - Kempka, Thomas T1 - Numerical Simulation of Coastal Sub-Permafrost Gas Hydrate Formation in the Mackenzie Delta, Canadian Arctic JF - Energies N2 - The Mackenzie Delta (MD) is a permafrost-bearing region along the coasts of the Canadian Arctic which exhibits high sub-permafrost gas hydrate (GH) reserves. The GH occurring at the Mallik site in the MD is dominated by thermogenic methane (CH4), which migrated from deep conventional hydrocarbon reservoirs, very likely through the present fault systems. Therefore, it is assumed that fluid flow transports dissolved CH4 upward and out of the deeper overpressurized reservoirs via the existing polygonal fault system and then forms the GH accumulations in the Kugmallit-Mackenzie Bay Sequences. We investigate the feasibility of this mechanism with a thermo-hydraulic-chemical numerical model, representing a cross section of the Mallik site. We present the first simulations that consider permafrost formation and thawing, as well as the formation of GH accumulations sourced from the upward migrating CH4-rich formation fluid. The simulation results show that temperature distribution, as well as the thickness and base of the ice-bearing permafrost are consistent with corresponding field observations. The primary driver for the spatial GH distribution is the permeability of the host sediments. Thus, the hypothesis on GH formation by dissolved CH4 originating from deeper geological reservoirs is successfully validated. Furthermore, our results demonstrate that the permafrost has been substantially heated to 0.8-1.3 degrees C, triggered by the global temperature increase of about 0.44 degrees C and further enhanced by the Arctic Amplification effect at the Mallik site from the early 1970s to the mid-2000s. KW - gas hydrate KW - permafrost KW - methane KW - faults KW - climate change KW - Mallik KW - numerical simulations Y1 - 2022 U6 - https://doi.org/10.3390/en15144986 SN - 1996-1073 VL - 15 IS - 14 PB - MDPI CY - Basel ER -