Computer Science
Scientific paper
Oct 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........36b&link_type=abstract
PhD Thesis, Leiden Observatory, 2008, 167 pages.
Computer Science
Scientific paper
Using a hydrodynamical simulation of a gravitational collapse and subsequent disk formation, we calculate a time-resolved synthetic data set with a sophisticated molecular excitation and radiation transfer code. These synthetic data consist of a number of molecular gas emission lines that contains information about the density, temperature, and the velocity field. We use this simulated data set to asses how accurately we can extract information about the underlying velocity field from the lines with a simple parameterized velocity model. This model has only two free parameters, the central stellar mass and a geometric angle that describes the ratio of infall to rotation. We find that, by modeling the spectral lines, we can reliably and uniquely describe the underlying velocity field as given by the hydrodynamical simulation and we then assume that by applying the same parameterized model to real data, we can equally well determine the velocity field of observed young stellar objects.
We observe two young sources, L1489 IRS in the Taurus star forming region and IRAS2A in NGC-1333. Both sources are observed with single dish telescopes (JCMT, OSO) and with the Submilimeter Array. For L1489~IRS, the interferometric observations reveal a kinematically distinct region on a scale of a few hundred AUs, dominated by rotation, which is still surrounded by some envelope material. Contrary to this, IRAS2A shows no sign of rotation despite the fact that a compact (disk) component is needed in order to interpret the continuum measurements. We do not detect this component in the velocity field and we conclude that IRAS2A is a considerably younger source than L1489 IRS. While this result is based on the gas flow alone, it is entirely consistent with the current classification of IRAS2A as a Class 0 object and L1489~IRS as a Class I object.
This thesis also contains a treatment of CO depletion in the disk and envelope. Under certain temperature and density conditions, CO may freeze-out onto dust grains and no longer contribute to the radiation field. This may happen in regions which are kinematically important (e.g., the disk) and can mask the rotation, resulting in the circumstellar material appearing more infall dominated than it actually is. We show how this effect can be taken into account when modeling the velocity field in order to avoid misinterpretation.
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