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The formation of the first low-mass stars from gas with low carbon and oxygen abundances


The first stars in the Universe are predicted to have been much more massive than the Sun1,2,3. Gravitational condensation, accompanied by cooling of the primordial gas via molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass it undergoes a slow, quasi-hydrostatic contraction without further fragmentation1,2; lower-mass stars cannot form. Here we show that as soon as the primordial gas—left over from the Big Bang—is enriched by elements ejected from supernovae to a carbon or oxygen abundance as small as 0.01–0.1 per cent of that found in the Sun, cooling by singly ionized carbon or neutral oxygen can lead to the formation of low-mass stars by allowing cloud fragmentation to smaller clumps. This mechanism naturally accommodates the recent discovery4 of solar-mass stars with unusually low iron abundances (10-5.3 solar) but with relatively high (10-1.3 solar) carbon abundance. The critical abundances that we derive can be used to identify those metal-poor stars in our Galaxy with elemental patterns imprinted by the first supernovae. We also find that the minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background.

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Figure 1: Required carbon and oxygen abundances (relative to solar values) for cooling a clump of metal-poor gas faster than its free-fall (compressional heating) time.
Figure 2: Observed abundances in low-metallicity Galactic halo stars.


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We thank R. Barkana and particularly T. Beers for discussions, and are grateful to T. Beers and M. Pettini for making their data available prior to publication. This work was supported in part by NSF, NASA and the Guggenheim foundation (for A.L.).

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Correspondence to Volker Bromm or Abraham Loeb.

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Bromm, V., Loeb, A. The formation of the first low-mass stars from gas with low carbon and oxygen abundances. Nature 425, 812–814 (2003).

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