Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 04/05

The Carina Nebula Complex is known to be an active star-formation region. This work presents a large catalogue of point-like sources assembled from archive data of the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. This catalogue covers a region of 2.3 deg × 3.0 deg, which makes it the most extendended mid-infrared survey undertaken of the Carina Nebula Complex to date. From the catalogue a subsample of candidate young stellar objects is extracted utilising the fact that young stellar objects exhibit typical mid-infrared excesses.

These catalogues are employed to characterise the young stellar population of the Carina Nebula Complex. Using them, it was possible to identify three new extended green objects and five compact green objects and find the probable sources for 28 further objects connected with jets emitted from young stars, such as molecular hydrogen emission-line objects and Herbig-Haro jets. For 17 of them, observational data from the near-infrared (from HAWK-I and 2MASS) to the far-infrared (from the Herschel Space Observatory) could be collected and their spectral energy distributions fitted. From the fit parameters, stellar characteristics such as stellar and disk masses could be estimated. No young stellar objects with masses above 10 M_sol could be evidenced, pointing towards an intermediate-mass population currently forming.

It could be shown that the Gum 31 region on the outer periphery of the Carina Nebula Complex is not only part of the complex but also an important centre of star formation. A large sample of candidate young stellar object was obtained from the WISE All-Sky Data Release, which allowed a detailed comparison with both the candidate young stellar objects from the IRAC catalogue and those identified from Herschel observations. Evidence could be found that two modes of triggered star formation are going on in the HII region: Young stellar objects are found in and in front of dust pillars, which is an indicator of radiative triggering, and a ‘collect and collapse’ model of the region was shown to produce results in agreement with the observations.

An objective and large-scale search for clusters of young stellar objects in the complex was performed using a nearest-neighbour algorithm. This search derived 22 clusters not described before. Nine of those are new detections in the fields of previous studies of clusters while the majority are found in fields surveyed for clusters for the first time here. Clusters are also found in agreement with previous studies where study fields overlap, thus corroborating the validity of the study. It is found that ∼40% of the young stellar objects in the Carina Nebula Complex occur in clusters while up to 60% are part of a distributed population. A total population for the 2.3 deg × 3.0 deg study field of ∼200 000 young stars is estimated.

The morphological analysis of galaxy clusters in X-rays allows a reliable determination of their dynamical state. Substructures on (sub-)Mpc scale influence the gravitational potential of a cluster and manifest themselves in the X-ray surface brightness distribution as secondary peaks or overall irregular shape. They lead to deviations from the hydrostatic equilibrium and spherical shape, two assumptions which are widely used in galaxy cluster studies to derive global astrophysical properties. Analyzing the X-ray morphology of clusters thus yields valuable information, provided that the employed substructure measures are well-tested and well-calibrated.

In this work, the X-ray morphology of galaxy clusters is quantified using three common substructure parameters (power ratios, center shift and the asymmetry parameter), which are subsequently employed to study the disturbed cluster fraction as a function of redshift. To ensure a reliable application of these substructure parameters on a variety of X-ray images, a detailed parameter study is conducted. It focuses on the performance and reliability of the parameters for varying data quality using simulated and observed X-ray images. In particular, when applying them to X-ray images with low photon counts such as observations of distant clusters or survey data, it is important to know the characteristics of the parameters. Comparing the three substructure measures, the center shift parameter is most robust against Poisson noise and allows a reliable determination of the clusters' dynamical state even for low-count observations. Power ratios, especially the hexapole P3/P0, and the asymmetry parameter, on the other hand, are severely affected by noise, which results in spuriously high substructure signals. Furthermore, this work presents methods to minimize the noise bias.

The results of the parameter study provide a step forward in the morphological analysis of high-redshift clusters and are employed in the framework of this thesis to quantify the evolution of the disturbed cluster fraction. The sample used for this analysis comprises 78 low-z (z < 0.3) and 51 high-z (0.3 < z < 1.08) galaxy clusters with varying photon statistics. The low-redshift objects were observed with the XMM-Newton observatory, contain a high number of photon counts and are part of several well-known and representative samples. For z > 0.3, the high-redshift subsets of the 400d2 and SPT survey catalog are used. These objects were mainly observed with the Chandra observatory and have low photon counts. To ensure a fair comparison, which is independent of the data quality, the photon statistics of the low- and high-redshift observations are aligned before performing the morphological analysis.

In agreement with the hierarchical structure formation model, a mild positive evolution with redshift, i.e. a larger fraction of clusters with disturbed X-ray morphologies at higher redshift, is found. Owing to the low photon counts and small number of high-redshift observations, the statistical significance of this result is low. For two of the three substructure parameters (power ratios and center shift) the findings are also consistent within the significance limits with no evolution, but a negative evolution of the disturbed cluster fraction can be excluded for all parameters.

Idealized numerical model experiments are presented to investigate the convective generation of vertical vorticity in a tropical depression.

The calculations are motivated by observations made during the recent PREDICT field experiment to study tropical cyclogenesis, and by a desire to understand the aggregation of vorticity debris produced by deep convection in models of tropical cyclogenesis to form a monopole vortex.

One aim is to isolate and quantify the effects of low to mid level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. Another aim is to isolate the effects of a unidirectional boundary layer wind profile on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. A third aim is to isolate the effects of a vortex boundary-layer wind profile on tropical deep convection, focussing especially on the morphology of vertical vorticity that develops.

The growing convective updraughts, that are initiated by a near surface thermal perturbation, amplify locally the ambient rotation at low levels by more than an order of magnitude and this vorticity persists long after the updraught has decayed, supporting the results of an earlier study.

The results of calculations with dry air aloft do not support a common perception that the dry air produces stronger downdraughts.

In calculations where the vertical wind shear changes sign at some level near the top of the boundary layer, as occurs in warm-cored disturbances such as tropical depressions or tropical cyclones, it was found that the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This feature is quite unlike the structure in a typical middle-latitude `supercell' storm. These results provide an essential first step to understanding the interaction between deep convective elements in a tropical depression or tropical cyclone.

An increase in the magnitude of boundary-layer shear was found to have the dual effect of weakening the development of the initial thermal, which is detrimental to vertical vorticity production by stretching and tilting, while at the same time increasing the magnitude of horizontal vorticity that can be tilted.

The results provide a basis for appraising a recent conjecture concerning the role of storm splitting in explaining the contraction of the eyewall in tropical cyclones.