Mathematics – Logic
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........3d&link_type=abstract
PhD Thesis, Queen Mary and Westfield College, University of London, UK, 2000.
Mathematics
Logic
Scientific paper
This thesis presents an investigation of the early stages of star formation using ground- and space-based observations of Young Stellar Objects (YSOs) in the far-infrared and in the submillimetre/millimetre regions of the spectrum. It represents an effort to make use of some of the latest available observational tools in this spectral domain to improve the observational constraints on some circumstellar parameters, and to look for trends that might be related to the evolutionary stage of the objects. Spectral energy distributions (SEDs) covering the 50 -- 190 micron range were obtained for a sample of five YSOs observed by the Infrared Space Observatory (ISO) and representing different stages of protostellar evolution. The SEDs were constructed by characterizing the continuum emission of the objects using data from the ISO Long Wavelength Spectrometer (LWS). A description of the instrument is given and the data reduction and calibration techniques are described in detail. A systematic study of the detector shifting problem is also presented. The ISO observations were complemented with IRAS data when available and with submillimetre observations done with the James Clerk Maxwell Telescope (JCMT). Data obtained with the single bolometer UKT14 were used together with 450 and 850 micron imaging data from SCUBA. The JCMT data reduction and calibration are described and the morphology of the objects is discussed. We also report high resolution 12CO J = 3--2 observations of a compact molecular outflow around VLA1, the powering source of the HH 1--2 system, and the only object of our sample for which a molecular outflow had previously not been discovered. These observations show that this outflow is one of the weakest, youngest and most compact ever seen. The continuum observations were modelled with a simple phenomenological model in order to characterize the circumstellar environment of the objects. The model consists of a spherically symmetric dusty envelope with power law density and temperature radial distributions. The role of each parameter in the model is illustrated and the uniqueness and degeneracy of the fits are discussed. An optically thick circumstellar disk embedded in the envelope was also included to account for the possible need for extra emission in the thermal infrared. We emphasise the importance of spatial information about the sources to distinguish between possible fits and to obtain reliable circumstellar parameters. The SCUBA images allowed to reveal the extension of the sources and to define their submillimetre intensity profiles. By comparing these profiles with those the model predicts, we were able to distinguish between multiple possible sets of parameters and select the ones that simultaneously fitted the SEDs and the intensity profiles. Envelope masses and bolometric and submillimetre luminosities are estimated for each object. Power law density distribution indices, p, are found in the range 1.1 -- 2.0. These are discussed in the light of current collapse theories. An anti-correlation between Lsubmm / Lbol and the submillimetre dust opacity index was found, and the use of this index as a possible age indicator is discussed.
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