Statistics – Computation
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........16m&link_type=abstract
Ph.D. thesis, Univ. Tokyo, (1999)
Statistics
Computation
5
Scientific paper
Investigation of star formation processes has been a significant subject both for theoretical astrophysics and for observational astronomy. Nevertheless, many problems are left unresolved still to date especially in an early phase in the evolution where a protostar is under dynamical growth. The lack of our knowledge for protostar formation is a ``missing link'' in the evolutionary picture of star formation. In order to construct a theoretical model for protostar formation which is capable to account for recent observations, we have developed a numerical code for radiation hydrodynamic calculations. The numerical code is designed to yield the evolution of spectral energy distributions as well as the dynamical evolution. We established in this work a theoretical model for the whole evolution of protostar formation in a consistent scheme which is accountable for observations. Numerical calculations are carried out to reveal physical processes in the formation of a 1 Modot protostar. The whole evolution are pursued from the beginning of the first collapse to the end of the main accretion phase. We confirm that the typical features in the evolution are in good agreement with previous studies. We consider two different initial conditions for the density distribution: homogeneous and hydrostatic cloud cores. For the initially homogeneous model, the accretion luminosity rapidly rises to the maximum value of 25 Lodot just after the formation of a protostar, and declines gradually as the mass accretion rate decreases. In contrast, the luminosity increases monotonically with time for the initially hydrostatic model. This discrepancy arises because the mass accretion rate varies depending on the inward acceleration in the initial condition, which affects the luminosity curve. We confirm that the SED evolves from a 10K greybody spectrum to hotter spectra typical for class I and II sources. The SED for the class 0 sources corresponds to the age of 2× 104yr, which is smaller by an order of magnitude than the typical age of class I objects. Considering possible non-spherically-symmetric effects, we suggest that observed class 0 sources should be the compound of the ``genuine'' class 0 that is as young as 104yr and more evolved protostars on edge-on view. The contamination of older protostars are not negligible because they are intrinsically abundant than genuine class 0 objects. Since observations indicate that the class 0 sources are typically more luminous than class I sources, we exclude the initially hydrostatic model where the luminosity increases monotonically with time. The initially homogeneous model, in contrast, is found to show the tendencies consistent with observations. We have also developed a numerical code for non-LTE line transfer problems and applied it to protostar formation. For dynamical models, results from the radiation hydrodynamical calculations are used. We confirm that the computational results show double-peaked profiles in a stronger blue peak for optically thick molecular lines. Optically thin lines show single-peaked profiles with a slight blue asymmetry. These qualitative features are consistent with past studies. On the contrary to the remarks by a previous work we do not find overestimation of line widths. Furthermore, the infall motion produces wings extending to v = +/- 2km/s in line spectra, while wings could not be produced in previous studies. These results imply that simplified infall models such as the isothermal self-similar solutions adopted by previous authors are not suitable to the detailed modeling of line spectra. On the basis of the results and other theoretical and observational evidence, we illustrate an evolutionary picture of protostar formation. In terms of the evolutionary time and the inclination to an observer, we find that protostellar objects are successfully categorized. http://www.ccsr.u-tokyo.ac.jp/~masunaga/index_prs.html
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