Minimum mass for D and H burning during slow accretion

Astronomy and Astrophysics – Astrophysics

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Accretion Disks, Brown Dwarf Stars, Nuclear Fusion, Stellar Interiors, Stellar Mass, Dark Matter, Deuterium, Entropy, Galactic Halos, Stellar Mass Accretion, Stellar Models

Scientific paper

A recent paper by Lenzuni et al. (1992) calculated atmosphere models for slow accretion onto the surface of a zero-temperature brown dwarf, giving the specific entropy of newly laid-down material as a function of the present mass M and mass accretion rate. The present paper gives order-of-magnitude estimates for the evolution of the interior as M increases during continuing but slow Bondi accretion at a rate of mass accretion rate less than about 10 exp -9 solar mass/yr. The interior electrons remain highly degenerate, so that the burning of primordial deuterium is delayed until M of about 0.05 solar mass. The deuterium burning after an initial 'nuclear flash' keeps the interior somewhat less degenerate than before. The central temperature is nevertheless smaller than for a nondegenerate interior, and there is an increase in the minimum mass for hydrogen burning but only by a factor of 2 or slightly less, to M of about 0.15 solar mass. These estimates are for metal-free primordial material; if dark matter in outer galactic halos contains brown dwarfs formed by slow accretion, an enhancement in the mass distribution near M of about 0.05 solar mass is suggested.

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