Fractal structure of H2 gas Observatoire e Paris分形结构的H2气体巴黎天文台.pptVIP

Fractal structure of H2 gas SAAS-FEE Lecture 3 Fran?oise COMBES Definition of a fractal Interstellar Clouds Clouds mapped in HI 21cm absorption Fractal Structure -- Scaling laws Filamentary structure Turbulence Numerical simulations of turbulence Self-gravity Molecular Fractal and IMF Fractal structure in Galactic Star Fields Conclusions IMF slopes in different clusters from Scalo (1998) Salpeter -1.35 = dash MW solid LMC open Models from random sampling (Elmegreen 1999) (4) Observations favor stars forming in pre-formed clumps Therefore the IMF does not give any highlight in star formation but only in cloud formation! The IMF has two characteristic masses, and 3 slopes = why the characteristic mass of 0.3-1 Mo? By accretion and collapse of smaller clumps Stars grow until a self-limiting mass, a little larger than deuterium burning limit (stellar winds) (Larson 82, Shu et al 87) It is not the smallest fragments in clouds that determine the smallest star mass! Are the smallest clouds self-gravitating? (Elmegreen Falgarone 96, Walker Wardle 98) The smallest mass depends on temperature and local pressure (Bonnert-Ebert) M ~P1.5 n-2 Note a certain chaos between initial cloud masses and the final star mass, due to a series of dynamical events, unpredictable (accretion, fragmentation, turbulence, self-gravity..) Another model for a limiting smallest mass, opacity-limited clumps, of the order of 10-3 Mo Then accretion was to operate to reach the first stellar mass (and may be form brown dwarfs in between) Rees 1976, Yoshii Saio 85 Pure hierarchy of clouds n(M)dlogm ~m-1 dlogm this contains equal mass per logarithmic interval. Clouds are hierarchical, but it is difficult to know whether the hierarchy is pure The probability to have a random mass m is m-1 dlogm or m-2 dm This is what is observed, n(m) in m-1.85 (Heithausen et al 98) Stella