Published papers

• Chira, R.-A., Beuther, H., Linz, H., Schuller, F., Walmsley, C. M., Menten, K. M., Bonfman, L., 2013
  Characterization of infrared dark clouds. NH3 observations of an absorption-contrast selected IRDC sample

Master's Thesis

What is the most reliable tracer of core collapse in dense clusters?

Abstract

Context. In the last years surveys and studies have emphasised the importance of filamentary networks within molecular clouds as sites of star formation. Since such environments are more complex than those of isolated cores, it is essential to understand how the observed line profiles from collapsing cores are affected by filaments.
Aims. Emission lines are modelled from collapsing cores embedded in filaments using radiative transfer calculations. Line profile asymmetries are studied and compared to those expected for isolated cores.
Methods. In three cores I model the (1-0), (2-1), (3-2), (4-3), and (5-4) transition lines of six molecular tracers. Three of them, HCN, HCO+ and CS, are optically thick while the other three, N2H+, 13HCO+ and 13CO, are supposed to be optically thin.
Results. I found that less than 50% of simulated (1-0) line profiles show blue infall asymmetries. The numbers of blue asymmetric line profiles increases at higher transitions to about 90% in the (4-3) transitions. The origin of non-blue asymmetric line profile features was localised in the filaments around the embedded cores by using Optical Depth Surfaces.
Conclusions. Even in irregular, embedded cores infalling gas motions can be traced by blue asymmetric line profiles of optically thick lines. The best tracer of our sample is the (4-3) transition of HCN, but the (3-2) and (5-4) transitions of both HCN and HCO+ are also good tracers.

Thesis  [ 12.1MB pdf ]

Bachelor's Thesis

Characterisation of Infrared Dark Clouds

Abstract

The Initial Mass Function (IMF) is a distribution showing how numerous stars with specific masses are. It is observed that there is a peak of the IMF between 0.1-1 M_sun. Thus, low-mass stars like our sun are more common than high-mass stars. But high-mass stars contain the majority of the galaxy's luminosity. So, they are the only visible objects in other galaxies which can be effectively observed. Thus, the understanding of high-mass star formation is not only important for the generel understanding of star formation, but also for the understanding of galaxy structures. In a crude view, high-mass star formation is similar to the formation of low-mass stars. But, there are some other factors needed to be taken into account like the need of very massive cloud cores and a higher accretion rate. There are different scenarios which shall help us to simulate the formation stages. But for computing a realistic scenario and comparing the results with real observations, one has to know the initial conditions of star-forming regions. My studies concentrate on the early stages of infrared dark clouds (IRDCs) which are supposed to be star-forming regions, but do not contain any protostellar objects, yet. To learn more about the initial conditions of IRDCs, 220 candidates with strong contrast profile in the infrared were chosen. Based on the infrared observations of the Midcourse Space Experiment (MSX), a catalogue of possible IRDCs have been created containing all regions with a significant constrast - being defined as constrast = ( background - image ) / background - to the bright background. The 220 candidate IRDCs have been observed in ammonia with the 100m-telescope in Effelsberg. With these data, I was able to calculate the temperature and column density of ammonia, and the distances and virial masses of the IRDCs. I used the Atacama Pathfinder Experiment (APEX) Telescope Large Area Survey of the Galaxy (ATLASGAL) in dust emission at 870 μm for deriving the gas masses and the virial parameter of the sample. This survey observed a great part of the Galactic Midplane in submillimeter wavelengths and is, thus, helpful for estimating initial conditions like gas masses, column densities, density structures, as well as for studying large-scale morphologies. The IRDCs' rotation temperatures are averaged about 15 K, linewidths between 0.5 and 2.5 km s^−1, column densities in order of 10^15 cm^2 g^-1. Thus, they are colder and less turbulent than more evolved regions of high-mass star formation. The virial masses are between 100 and a few 1000_sun being sufficient for forming high-mass stars. The virial parameter is defined as ratio between the gravitional and kinetic energy of an source. The parameters of the sample IRDCs are in order of 1. This indicates that the sources are approximately in virial equilibrium. In my thesis, I want to present these results in more detail, interpret them in the astrophysical context and compare the parameters with previous observations of high-mass protostellar objects (HMPOs) supposing to be the next evolutionary stage in high-mass star formation.

Thesis  [ 18.1MB pdf ]

Proceedings