The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure

Charles J. Lada, A.A. Muench, & J. Rathborne
Harvard-Smithsonian Center for Astrophysics

J.F. Alves
Calar Alto Observatory

M. Lombardi
European Southern Observatory

In this paper we present the results of a systematic investigation of an entire population of dust cores within a single molecular cloud. This population of predominately starless cores was previously identified in an infrared extinction survey of the Pipe Nebula, a nearby molecular cloud with a well-established distance but negligible star forming activity. Analysis of extinction data shows the cores to be dense objects characterized by a narrow range of density with a median value of n(H_2) = 7000 cm^{-3}, assuming a normal gas-to-dust ratio. Analysis of C18O and NH_3 molecular-line observations reveals very narrow lines. The non-thermal velocity dispersions measured in both these tracers are found to be subsonic for the large majority of the cores and show no correlation with core mass (or size). The bulk gas motions are thus acoustic in nature and thermally dominated. Thermal pressure is thus the dominate source of internal gas pressure and support for most of the core population. The total internal gas pressures of the cores are found to be roughly independent of core mass over the entire (0.2-20 solar masses) range of the core mass function (CMF) indicating that the cores are in pressure equilibrium with an external source of pressure. This external pressure is most likely provided by the weight of the surrounding Pipe cloud within which the cores are embedded. Most of the cores appear to be pressure confined, gravitationally unbound entities whose nature, structure and future evolution are determined by only a few physical factors which include self-gravity, the fundamental processes of thermal physics (i.e., heating and cooling) and the simple requirement of pressure equilibrium with the surrounding environment. The observed core properties likely constitute the initial conditions for star formation in dense gas. The entire core population is found to be characterized by a single critical Bonnor-Ebert mass of approximately 2 solar masses. This mass coincides with the characteristic mass of the Pipe CMF indicating that most cores formed in the cloud are near critical stability. This suggests that the mass function of cores (and ultimately the stellar IMF) has its origin in the physical process of thermal fragmentation in a pressurized medium.

Astrophysical Journal 2008, 672, (January 1)


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