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Title:
The Jeans mass and the origin of the knee in the IMF
Authors:
Bonnell, I. A.; Clarke, C. J.; Bate, M. R.
Affiliation:
AA(School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS), AB(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA), AC(School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 368, Issue 3, pp. 1296-1300. (MNRAS Homepage)
Publication Date:
05/2006
Origin:
MNRAS
Astronomy Keywords:
stars: formation: stars: luminosity function, mass function: open clusters and associations: general, stars: formation, stars: luminosity function, mass function, open clusters and associations: general
DOI:
10.1111/j.1365-2966.2006.10214.x
Bibliographic Code:
2006MNRAS.368.1296B

Abstract

We use numerical simulations of the fragmentation of a 1000Msolar molecular cloud and the formation of a stellar cluster to study how the initial conditions for star formation affect the resulting initial mass function (IMF). In particular, we are interested in the relation between the thermal Jeans mass in a cloud and the knee of the IMF, i.e. the mass separating the region with a flat IMF slope from that typified by a steeper, Salpeter-like, slope. In three isothermal simulations with MJeans= 1, 2 and 5Msolar, the number of stars formed, at comparable dynamical times, scales roughly with the number of initial Jeans masses in the cloud. The mean stellar mass also increases (though less than linearly) with the initial Jeans mass in the cloud. It is found that the IMF in each case displays a prominent knee, located roughly at the mass scale of the initial Jeans mass. Thus clouds with higher initial Jeans masses produce IMFs which are shallow to higher masses. This implies that a universal IMF requires a physical mechanism that sets the Jeans mass to be near 1Msolar. Simulations including a barotropic equation of state as suggested by Larson, with cooling at low densities followed by gentle heating at higher densities, are able to produce realistic IMFs with the knee located at ~1Msolar, even with an initial MJeans= 5Msolar. We therefore suggest that the observed universality of the IMF in the local Universe does not require any fine tuning of the initial conditions in star forming clouds but is instead imprinted by details of the cooling physics of the collapsing gas.

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