Other
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........35o&link_type=abstract
PhD Thesis, Instituto de Astronomia, Universidad Nacional Autonoma de Mexico
Other
2
Massive Stars, Hot Molecular Cores, Dust, Ammonia
Scientific paper
The so-called ``Hot Molecular Cores'' (HMCs) are small condensations of hot molecular gas that are found in the proximity of ultra compact HII (UCHII) regions. It is believed that some HMCs harbor young embedded massive stars in a very early stage of their formation process, previous to the development of an observable HII region. Under this hypothesis, HMCs could be the precursors of UCHII regions, and they may represent the youngest phase yet observed in the life of a massive star. As a result of the work conducted during this thesis, a model to calculate both the continuum and line emission from these HMCs has been developed. A HMC has been modeled as a spherically symmetric envelope of gas and dust that is falling onto a recently formed massive central protostar. The central heating source has two components, the stellar luminosity and the accretion luminosity. The radiative transfer across the infalling envelope was calculated and the emerging dust continuum emission in the wavelength range from 3 micron to 3 cm was obtained, for density distributions resulting both from the collapse of a Singular Isothermal Sphere (SIS) and of a Singular Logatropic Sphere (SLS) and assuming dust an opacity of Draine & Lee (1984). The spectral energy distributions derived from the model, for different values of the mas acretion rate, spectral type of the central star, and size of the envelope, were compared with the available observations of the sources G34.24+0.13MM, W3(H_2O), Orion, IRAS 23385+6053, and G31.41+0.31, in order to obtain the physical parameters of the envelope and of the central star that provide the best fit to the data. It was found that the density distribution corresponding to the collapse of the SLS provides a good agreement with the observations. In general, early B-type central stars, with high mass acretion rates (of the order of 0.001 solar masser per year), resulting in accretion luminosities higher that the stellar luminosity, were required in order to fit the observational data. Also, it was found that the accretion rates obtained for the HMCs exceed the ``critical'' mass accretion rate required to quench the development of an UCHII region, making the free-free emission from the ionized material in these objects undetectable. For the opacities that we assumed, it was found that a central star with 30-50 Msun can be formed before the radiation pressure onto the dust grains stops the accretion flow. As a second part of this thesis, the emission from inversion transitions of the ammonia molecule in HMCs was derived. This calculation was performed by solving the transfer equation for the line emission across the envelope, adopting the density, temperature, infall velocity and velocity dispersion fields of the SLS model that provided the best fit to the observed dust continuum emission of each source. The emerging ammonia line spectra for different positions along the envelope were derived and were compared with the available observations. It was found that the best fit was obtained when adopting a variable ammonia abundance in the gas phase resulting from the release of ammonia molecules from grain mantles, after sublimation of water ice. The main features of the observed ammonia spectra were reproduced the model, although the line widths predicted appear to be larger than observed. However, observations with angular resolution high enough to allow a detailed comparison are still scarce and new data will be required to obtain definitive conclusions. Thesis Advisor: Susana Lizano
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