Wednesday, July 10, 2013

1307.2239 (Alexander P. Ji et al.)

The Chemical Imprint of Dust on the Most Metal-Poor Stars    [PDF]

Alexander P. Ji, Anna Frebel, Volker Bromm
We investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (< 1M_sun) in the early universe. Previous work has posited the existence of a critical dust-to-gas ratio, below which dust thermal cooling is unable to cause fragmentation. Using silicon-based (rather than carbon-based) dust compositions, we compute such critical dust-to-gas ratios and associated critical silicon abundances. We evaluate the robustness of these critical values by considering variations in the dust chemical composition, grain size distribution, and star formation environment. Variations in the dust chemical composition are less important than variations in the size distribution, and the most likely environment where dust cooling becomes significant is in a rotationally supported protostellar disk. We test the dust cooling theory by comparing to silicon abundances observed in metal-poor stars. Several stars have silicon abundances low enough to rule out fragmentation induced by dust which follows a standard Milky Way grain size distribution. Moreover, two of the most iron-poor stars have such low silicon abundances that even dust with a shocked grain size distribution cannot easily explain their formation. We see evidence that stars with [Fe/H] < -4.0 exhibit either high carbon and low silicon abundances or the reverse. This suggests that the earliest low-mass star formation in the most metal-poor regime likely proceeded through two distinct pathways, one that relied on fine structure cooling and one that relied on dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].
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