Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 03/05

Clusters of galaxies with masses up to few $10^{15}~M_\odot$ are the largest gravitationally bound objects in the Universe. According to the widely accepted hierarchical model of structure formation the largest structures are formed by mergers of smaller (less massive) objects. Massive clusters are therefore expected to appear in the Universe only ``recently'' (redhshifts $z < 1-2$) and their number is very sensitive to the cosmological parameters, such the amplitude of primordial fluctuations and dynamics of the Universe expansion which is in turn related to dark energy and dark matter content of the Universe.

Three major constituents make a galaxy cluster: stars (readily observable in bright optical galaxies), hot X-ray emitting gas (observed with modern X-ray space observatories) and dark matter (indirectly observed through its influence on galaxies and gas). It is believed that in clusters the mass fractions of baryons (stars and gas) and the dark matter are representative for the Universe as a whole. Stars is the least massive component out of three, making only few \% of the cluster mass, while hot gas and dark matter account for 10\%-15\% and $\sim 75$\% respectively. Since the dark matter can not be seen directly we have to use other components to determine main parameters of a cluster (such as e.g. total mass). In particular instrumental are X-ray observations of the hot gas, which are used to accurately map the mass profile of a cluster. This hot gas is a fully ionized plasma with the temperature of tens of millions degrees and the density $10^{-2}-10^{-4}$ particles per cm$^3$, which emits radiation primarily through bremsstrahlung and excitation/recombination lines of heavy elements.

While we are confident that the parameters of clusters determined from optical and X-ray data are broadly correct, there are a number observed phenomena in galaxy clusters which are yet to be explained and a number of parameters yet to be measured. For instance we do not fully understand if radiative cooling of the gas in the central regions of clusters is compensated by some sources of energy (e.g. from the activity of supermassive black holes). We also do not know what are the values of such fundamental characteristic of the cluster plasma as thermal conductivity or viscosity. In our research we discuss two simple theoretical models aimed at:

evaluating thermal conductivity in the gas using X-ray observations of sharp features in the surface brightness distribution of X-ray emission (so called ``cold fronts'')

estimating the energetics of the feedback from the supermassive black holes through its influence on the distribution of heavy elements in the hot gas.

Die Informationstechnologie revolutioniert das tägliche Leben seit Jahrzehnten. Der stetig steigende Umfang zu speichernder und zu bearbeitender Daten motiviert die Suche nach Schalt- und Speicherbausteinen in der Größenordnung einzelner Moleküle. Diarylethene (DAE) sind so genannte molekulare Schalter und zeigen großes Potential als photochrome optische Speicher im Bereich der sich entwickelnden jungen Molekularelektronik. Sie gehören zu den molekularen Schaltern auf der Basis eines 6-Pi-Elektronensystems. Ihre ausgeprägte Photochromie wird von einer photoinduzierten, reversiblen Ringschlussreaktion bestimmt, die im Pikosekundenbereich (1 ps = 10−12 s) stattfindet. Diese Arbeit stellt eine spektroskopisch bisher unbearbeitete Klasse von Diarylethenen vor, deren strukturbildende Einheit auf einer Maleinsäureimidgruppe basiert.

(i) Durch Kombination verschiedener spektroskopischer Techniken bestehend aus stationärer Absorptions- und Emissionsspektroskopie sowie moderner Methoden der Ultrakurzzeitspektroskopie (Breitband-Anrege-Abfrage Spektroskopie im UV/VIS-Spektralbereich) konnten die kinetischen Abläufe der photoinduzierten Isomerisierungsprozesse im Bereich weniger Pikosekunden detailliert beobachtet und verstanden werden. Die Untersuchungsmethoden konnten für die Reaktionsdynamik nahezu aller untersuchter Moleküle ein detailliertes Reaktionsmodell der Photoreaktion liefern.

(ii) Durch Einsatz unterschiedlichster Substituenten und deren Methylierung bzw. fehlender Methylierung konnte der Mechanismus des Ringschlusses bzw. der Ringöffnungsreaktion untersucht und der Einfluss der Substitution auf die Reaktion verstanden werden. Hierbei zeigt sich, dass die Methylierung einerseits über sterische Wechselwirkung die für die Ringschlussreaktion günstige Ausrichtung der Substituenten steuert und andererseits die Reversibilität des Isomerisierungsprozesses gewährleistet.

(iii) Aus der Substitution des Maleinsäureimids mit verschiedenen Heterozyklen zeigte sich, dass deren aromatische Stabilität die Effizienz der Ringschluss- und Ringöffnungsreaktionen reguliert. Monozyklische Heteroaromaten zeigen eine geringere Stabilität ihres aromatischen Ringsystems (erhöhte Tendenz zur Ringschlussreaktion) im Vergleich zu benzolkondensierten Heteroaromaten (erniedrigte Tendenz zur Ringschlussreaktion). Zusätzliche Elektronenakzeptoren (hier: boc-Schutzgruppe) reduzieren die Aromatenstabilität des Heterozyklus und erhöhen die Ringschlusstendenz.

(iv) Es konnte gezeigt werden, dass die N-ständige Tolyl- bzw. Methyl-Substitution des Maleinsäureimids einen vernachlässigbaren Einfluss auf die Reaktionsdynamik ausübt - diese bleibt faktisch unverändert. Größere Fremdmoleküle (z.B. Proteine) lassen sich somit N-ständig über die Maleinsäureimidgruppe an das jeweilige DAE anbinden, ohne dessen Reaktionsdynamik maßgeblich zu verändern.

This dissertation deals with a climatological investigation of different orographic precipitation enhancement processes in the Alps. On the one hand, the database consists of observations of more than 1000 weather stations in Bavaria and western Austria for the time period of 1991 - 2000. On the other hand, a MM5-climate-mode-simulation driven with ERA-40 reanalysis data provides information about the environmental meteorological conditions. After a short introduction into the basics of orographic precipitation enhancement and into some simple linear models, the different station data are converted into a uniform temporal resolution of 6 hours. To estimate the relative contribution of different precipitation types, we distinguish between cold fronts, warm fronts, convection and a class carrying unclassified events. Convective precipitation in connection with fronts is attributed to the respective frontal class. Unclassified events predominantly consist of postfrontal upslope precipitation and quasi-stationary fronts. In addition, the wind direction at Alpine crest level (700 hPa) is considered.

In a first step, the precipitation differences between the Alpine foreland and the northern Alps are analyzed. The investigation of the climatological importance of the 4 precipitation types in various regions shows that summertime convection and orographic lifting make the largest contribution to the precipitation gradient towards the Alps. Convective precipitation occurs predominantly in association with southwesterly flow and is more abundant in the Alps than in the adjacent forelands because convection is primarily triggered over the Alps. Orographic lifting is most active in case of northwesterly and northerly winds and intensifies both frontal and postfrontal precipitation. The climatological importance of orographic lifting is much larger in winter than in summer. A reversed precipitation gradient with systematically more precipitation in the foreland than in the Alps is found for fronts associated with a wind direction of exactly 270$^\circ$. Under these circumstances, the wind blows parallel to the mountain range and lee effects related to upstream topography reduce the precipitation intensity in the Alps. The climatological precipitation maximum is shifted from westerly towards northerly winds when moving from west to east in the northern Alps.

The second part of this work comprises an investigation of the climatological precipitation decrease from the northern Alps to the inner-Alpine valleys. The comparison between the precipitation distribution of 3 regions in the northern Alps and 3 regions in the central Alps shows that especially for cold fronts and unclassified events in connection with northwesterly or northerly flow, a distinctive precipitation surplus can be found in the northern Alps. For convective precipitation and southwesterly or westerly winds the inner-Alpine regions show high convective activity. For southerly wind directions the showers formed over the central Alps are advected towards the northern Alps, where they sometimes even intensify due to a convergence with the inflow from the Alpine foreland. There is a strong west-east precipitation gradient in the central Alps. The decreasing crest level of the northern Alps and the decreasing north-south-extension of the Alps together with the topographical structure of the valleys are the main reasons for the higher precipitation amounts in the eastern parts of the central Alps than in the western parts. Additionally, an investigation of the precipitation gradient in dependence on the temperature level is made. However, a classification of the precipitation events into 3 classes (snow line $>$ 2500 m, 2500 m - 1000 m, $<$ 1000 m) does not show any clear results.

The next part investigates altitudinal precipitation differences in the Alps. The analysis is performed for four station pairs, consisting of a mountain station and a nearby valley station each. The c

Star formation (SF) is still an unsolved problem in astrophysics.

Gas cooling is the principal mechanism leading to

the condensation of gas and consequently to star and structure

formation.

In a metal-free environment, the main available coolants are H, He,

H$_2$ and HD; once the gas is enriched with metals, these also

become important.

In this work, in order to properly determine the SF in the early

Universe, we compute fine-structure transition metal cooling and

implement and test molecular chemistry.

Moreover, we investigate its redshift ($z$) evolution

and compare different modeling running very high-resolution,

three-dimensional, N-body/hydrodynamic simulations including

non-equilibrium, atomic and molecular chemistry, SF

prescriptions and feedback effects.

We also study how the primordial SF changes accordingly to

different semi-analytical approaches, cosmological parameters, initial

set-ups and critical metallicity ($Z_{crit}$) for the transition from

a metal-free SF regime to a standard enriched one.\\

Our main findings are:

the H$_2$ molecule is the most relevant coolant in early times;

inclusion of HD cooling results in a $\sim 10\%-20\%$ higher gas

clumping;

metal cooling at low temperatures can have a significant impact on the

formation and evolution of first objects;

typical numerical ``sub-grid'' models fail in following the cooling

of primordial gas and predict too early SF ($z\sim 30$);

considering molecular cooling, we get a postponed epoch ($z\sim 15$)

for the same initial conditions;

rare, high-density peak can host SF even at $z\gtrsim 40$;

metal-free SF regime is completely negligible

with respect to the global SF rate, because of the very

short first star life-times;

it has some relevance only for $\Delta z\simeq 1$ (at $z\sim 16$);

primordial pollution up to $\sim 10^{-3}\,Z_\odot$\footnote[1]{

The solar metallicity is $Z_\odot\simeq 0.02$.

}, or higher, is

extremely rapid and allows for a very fast transition to standard SF

regimes;

the different SF rates and metal enrichment got for

different $Z_{crit}$ are well distinguishable and span about one order

of magnitude for $Z_{crit}/Z_\odot \in [10^{-6}, 10^{-3}]$.

Given the importance of the initial stellar mass distribution

function, an analytical model describing its possible derivation from

turbulent dissipation is presented.