Computer Science
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........21w&link_type=abstract
PhD dissertation. Univ. Bonn, Bonn, Germany.153 pp. , (1997)
Computer Science
8
Scientific paper
This work demonstrates the feasibility of a study bringing together theoretical concepts of the earliest phase of low-mass star formation and its observational evidence. Thus, two aspects have been considered: Observational evidence: In order to detect protostellar collapse by virtue of kinematical features in spectral line profiles, both optically thick and optically thin tracers are needed. According to Leung & Brown (1977, ApJ 214, L73), a protostellar envelope undergoing collapse exhibits a red-shifted self-absorption in a molecular line transition if the excitation gradient is negative. Optically thin emission (e.g. from the corresponding isotopomere's line) corroborates the conclusions by ruling out the case of independent components filling the observing beam. The nearby (d~200 pc) globular filament L 1082 (no. 9 from a catalog assembled by Schneider & Elmegreen, 1979, ApJS 41, 87) provides at least three candidates showing unambigeous footprints of protostellar collapse. By means of millimeter-interferometry (with the iram and bima interferometers Asz well as single dish spectroscopy and continuum imaging (using the iram 30 m telescope), these candidates were identified and characterized. As moderately optically thick high-density tracers, the CS (2,1), (3,2) and (5,4) transitions have been observed. The optically thin (2,1) lines of C34S and C18O were measured to confirm the evidence for collapse. Preliminary results from observations with isophot and isocam were used to better constrain the luminosity of one of the collapse candidates, which subsequently has been classified as an extreme Class 0 protostar. Theoretical concepts: For reasons evidenced by the observed column density distributions and by systematic shifts of the molecular line emission across the sources, spherically-symmetric collapse has to be ruled out. Instead, scenarios such as core formation in sheet-like clouds (as proposed by Hartmann et al., 1994, ApJ 430, L49) and magnetic accretion shocks (Li & McKee, 1996, ApJ 464, 373) are discussed. Most models are calculated by semi-analytical methods. The CS emission is severely affected by non-LTE effects in the subcritical envelope regions. Therefore, I developed a code to solve the excitation conditions in cylindrical symmetry, leaving off simplifying assumptions such as microturbulent approaches or the limiting case of a large velocity gradient. It is entirely possible to solve for the non-LTE radiative transfer problem with a classical ray-tracing approach: even for non-local coupling between radiation and matter, only part of a cloud has to be considered. Using a multivariate quadrature rule published by Steinacker et al. (1995, JQSRT), and second-order acceleration of convergence together with a modified convergence criterion (as proposed by Dickel & Auer, 1994, ApJ 437, 222), it has been possible to perform a parameter study considering the effects of inclination, rotation, and turbulence on the line profiles. As an example, a toroidal protostellar envelope, as seen in the CS lines, shows a red-shifted self-absorption feature if it is viewed from the edge. The feature disappears in case of a face-on orientation. -- Finally, I tried to describe the observations in terms of the above models. At present, it is not possible to rule out either of the (simplifying) scenarios considered. Nevertheless, some features in the line profiles are reproduced, using models that satisfy constraints inferred from various mass estimates. Characteristic scale-heights are deduced that allow for a comparison with the period of the regularly spaced globules.
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