Ignition of Deuterium in Low-mass Brown Dwarfs

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Session 90 -- Planetary Nebula and Degenerate Stars
Oral presentation, Wednesday, 11, 1995, 10:00am - 11:30am

[90.06] Ignition of Deuterium in Low-mass Brown Dwarfs

W.B. Hubbard, J.I. Lunine, D. Saumon (LPL, U of AZ), A. Burrows (Depts. of Phys. \& Astron., U of AZ)

Deuterium is converted to ${}^3$He in brown dwarfs at significantly lower temperatures than those required to make the p-p reaction proceed. This lowest thermonuclear threshold in astrophysics is reached by objects only about one order of magnitude more massive than Jupiter.

We have carried out high-resolution evolutionary calculations for our standard brown dwarf models with solar metallicity and D/H = $2 \times 10^{-5}$, for masses ranging from 0.010 $M_{\odot}$ to 0.020 $M_{\odot}$ in steps of 0.001 $M_{\odot}$ (one Jupiter mass). There is a phase lasting $\sim 4 \times 10^7$ years when brown dwarfs in this mass range stay at luminosities about an order of magnitude larger than normal while they consume their initial stores of deuterium. We have also considered the evolution of an ensemble of zero-metallicity BDs with the same masses and elevated deuterium fractions (D/H = $2 \times 10^{-4}$). The increased deuterium abundance is meant to correspond to an elevated primordial value reported by Songaila et al. (1994), and these zero-metallicity BDs thus correspond to hypothetical Population III objects, which might have formed out of low-metallicity, high-deuterium primordial Big Bang material not previously processed in stars. The effect of the extra deuterium and reduced opacity is to increase luminosities in the deuterium-burning phase and to prolong lifetimes in this phase to values approaching $10^8$ years.

We have computed sequences of models with extremely small mass steps in order to accurately determine the lowest mass brown dwarf which can burn deuterium. The limit is insensitive to metallicity and is found to be 13 Jupiter masses.

This research was supported by NSF grants AST92-17322 and AST93-18970, and by NASA grant NAGW-2145.

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