The Collapse of SelfGravitating Clouds of Pure Hydrogen
Abstract
Exploratory models of the collapse of spherical selfgravitating clouds are studied in relation to the problem of the formation of first generation starsystems. The masses which were considered are in the range of 83 to 5.2×10^{10} M _{⊙}. For simplicity, the assumed composition includes hydrogen only, which could be in the form of H, H_{2}, H^{+} or H^{}. Since the physical conditions that might have prevailed in a primeval nebula are not well known, rather simple initial conditions were chosen: The gas starts from rest and has initially a uniform temperature. We consider the case of rather cool (T _{0}∼100 K) neutral clouds with different initial ionization degrees. Some of the initial densitydistributions here considered are uniform while others are decreasing from the center outwards. The assumed initial values for the densities are ∼10^{24} g cm^{3}, except for one of the models, for which it is ∼10^{26} g cm^{3}. Several atomic processes within the gas, including physicalchemical reactions and the evaluation of radiative emission coefficients are considered. A system of differential equations is set up in order to evaluate the concentrationsn _{H},n _{H} _{2},n _{H} ^{+},n _{H} ^{} andn _{e} as a function of time. The treatment makes possible the study of the cooling and heating properties of the gas. Furthermore, the dynamical, thermal and chemical evolution of the cloud can be followed during the collapse. The computations apply only to the optically thin stages. The models show the importance of a correct evaluation of the chemical reactions and dissipative mechanisms, which cannot be ignored in a realistic treatment of the collapse of selfgravitating clouds. The influence of the initial conditions on the dynamical and thermal properties during evolution are also analysed.
 Publication:

Astrophysics and Space Science
 Pub Date:
 January 1975
 DOI:
 10.1007/BF00646224
 Bibcode:
 1975Ap&SS..32..175P
 Keywords:

 Astronomical Models;
 Chemical Reactions;
 Gravitational Collapse;
 Hydrogen Clouds;
 Interstellar Gas;
 Radiative Transfer;
 Density Distribution;
 Differential Equations;
 Gas Cooling;
 Gas Density;
 Gas Heating;
 Nebulae;
 Astrophysics