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Mathematisch-Naturwissenschaftliche Fakultät - Jahrgang 2014


Titel Shocks in the interstellar medium
Autor Sibylle Anderl
Publikationsform Dissertation
Abstract Shocks are ubiquitous in the interstellar medium (ISM), occurring whenever large pressure gradients lead to fluid-dynamical disturbances that move at a velocity that exceeds the local sound speed. As shocks dissipate kinetic energy into heat, they give rise to strong cooling radiation that constitutes excellent diagnostics for the study of the conditions in the shocked gas. The interpretation of this radiation requires the application of detailed numerical shock models.
Grain-grain processing has been shown to be an indispensable ingredient of shock modelling in high-density environments. However, an analysis of the effects of shattering and vaporization on molecular line emission had remained open. I have developed a new method for implementing grain-grain processing into a 2-fluid magnetohydrodynamic (MHD) shock model, which includes a self-consistent treatment of the molecular line transfer. Using this combined model, it was shown that shattering has a strong influence on continuous MHD shocks ("C-type shocks") for a broad range of shock parameters: the shocks become hotter and thinner. Predictions were made for the emission of H2, CO, OH and H2O. The main focus of the study lay on SiO, which is a prominent indicator of shock processing in dense clouds and is released into the gas-phase by the vaporization of grain cores. The release by vaporization already early in the shock changes the excitation characteristics of the SiO line radiation, although it does not change the width of SiO rotational lines. This study has significantly improved our understanding of shock emission in high-density environments. The method that was developed will make it possible to easily implement the effect of grain-grain processing in other numerical shock models.
MHD shock models were applied in the interpretation of observations of supernova remnants (SNRs) interacting with molecular clouds. New CO rotational line observations with the APEX telescope from shocked regions in two of these SNRs, W28 and W44, were presented. Towards W28, data was also taken with the SOFIA telescope. The integrated CO intensities observed towards positions of shock interaction were compared with a large grid of MHD shock models. Towards W28, it was found that only stationary C-type shock models were compatible with the observed emission. These shocks could satisfactorily account for the pure rotational H2 emission as observed with Spitzer. In W44, however, only models of much younger, non-stationary shocks could reproduce the observations. The preshock densities found in both SNRs were too low for grain-grain processing to be significant. Based on our modelling, we were able to constrain the physical and chemical conditions in the shocked regions, give predictions for H2O and the full ladder of CO rotational transitions, and quantify the momentum and energy injection of the SNR into the ISM. The results are important for a proper understanding of the local characteristics of SNR-cloud interactions, as well as for the study of the global energetics and dynamics of the ISM and the study of cosmic rays. The developed method enables a systematic comparison of a large grid of detailed MHD shock models with observations of shocked molecular gas and will be further applied in future studies.
I conclude with a critical reflection of research on astrophysical shocks within the framework of recent discussions in the philosophy of science.
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© Universitäts- und Landesbibliothek Bonn | Veröffentlicht: 09.01.2014