On the Structure and Evolution of Massive Interstellar Clouds [chapter]

H. Kimura, Liu Cai-pin
1981 Fundamental Problems in the Theory of Stellar Evolution  
The evolution of massive clouds is discussed with emphasis on inhomogeneous and heterogeneous nature of the system. We start from the twophase model of interstellar medium and choose initial conditions of the clouds as being in the vicinity of critical state for gravitational instability. The equilibrium and stability problem for clouds is formu lated in terms of polytropic models. Combining this with the thermal and chemical balance problem, we find that, even before the commencement of
more » ... mencement of gravitational collapse, the fragmentation due to thermal instability can occur. The system begins to contract at a rate much slower than free fall, and thereafter, its evolution is governed mainly by colli sions between fragments. The collision dynamics of the N-fragment system should be examined carefully by numerical experiments. The general trend to be expected is that the system tends to develop into a core-halo structure, accompanying by sporadic local events of star formation. The core or a clustering of massive fragments near the center could grow up into a favourable formation site of star clusters. DISCUSSION Hasegawa: What is the typical size of the fragments? In what range of mass and density do thermal instabilities occur? Kimura: The mean size of fragments can be roughly estimated by diameter d = (Pc/^ c s T c' wnere c s denotes the sound speed and T the timescale of CO cooling. The degree of compression, (PJ/PQ) , may be°~5. Adopting typical values of c g < 1 km/s and T c = 10 5~1 0 fe years, we have d = 0.07-0.7 pc. With regard to your second question, it seems better to show the minimum values of local density n_ and effective column density N" from the surface that are required; the answer is N H £ 10 21 cm" 2 and n > 10 cm" , if the instability is due to CO cooling. Hasegawa: Recent analyses of the 21-cm line of HI show the existence of cold gas fragments with a total particle density of -.100 cm" 3 and a typical size of 20 pc in diameter. The formation of molecular clouds may occur via such a stage. 70
doi:10.1007/978-94-009-8477-6_8 fatcat:qmqhd7yhmfekzhhcgoghmw4y3y