The Chemical Imprint of Silicate Dust on the Most Metal-Poor Stars

Abstract

We investigate the impact of dust-induced gas fragmentation on the formation of the first lowmass, metal-poor stars (< 1M ) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H] crit ). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H] crit = −4.5 ± 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H] crit = −5.3 ± 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < −4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].

The Formation of the Primitive Star SDSS J102915+172927: Effect of the Dust Mass and the Grain-Size Distribution

“…This was also shown by Ji et al (2014) within a simple onezone framework. Schneider et al (2012b) postulated that to have fragmentation at the metallicity of Caffau's star f dep should be larger than 0.01.…”

METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. II. DISCOVERY OF FOUR STARS WITH [Fe/H] ⩽ –3.5

“…Hence, there should be a threshold value (D trans = −3.5) below which C and O fine-structure line cooling would not be able to drive low-mass star formation ("Forbidden Zone"-see also Frebel et al (2007b). Ji et al (2013) consider an alternative fragmentation mode, the impact of thermal cooling from silicon-based dust on the formation of low-mass stars in the early universe. This channel could account for the formation of low-mass EMP stars in cases where an insufficient amount of C and O were present to induce fragmentation of the gas cloud.…”

“…The lower panel of Figure 8 shows the behavior of the [Si/H] abundance as a function of the metallicity for the program stars and stars from the literature. The solid line represents [Si/Fe] = 0, and the shaded areas show the lower limits on the Si abundances that would allow fragmentation (see Figure 5 of Ji et al 2013 for further details on the models). One can see that our program stars, as well as the stars from Cayrel et al (2004) and Yong et al (2013a), lie above the limits using both criteria, so their formation can be explained by either one of these processes.…”

Formation of Carbon-Enhanced Metal-Poor Stars in the Presence of Far-Ultraviolet Radiation

“…Simplified theoretical models (Frebel et al 2007;Safranek-Shrader et al 2010;Ji et al 2014) suggested possible conditions to form low mass CEMP stars that can be observed today by evaluating the amount of metals needed to induce cooling and subsequent fragmentation. Salvadori et al (2007Salvadori et al ( , 2010 explored the metallicity distribution function to probe the stellar population history of the Milky Way providing a critical metallicity of Z crit /Z ⊙ = 10 −4 .…”