Metal Stars & Their Implications for Planets

Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies

Authors:

Hopkins et al

Abstract:

Dust grains in neutral gas behave as aerodynamic particles, so they can develop large local density fluctuations entirely independent of gas density fluctuations. Specifically, gas turbulence can drive order-of-magnitude 'resonant' fluctuations in the dust density on scales where the gas stopping/drag timescale is comparable to the turbulent eddy turnover time. Here we show that for large grains (size greater than 0.1 micron, containing most grain mass) in sufficiently large molecular clouds (radii greater than 1-10 pc, masses greater than 10^4 solar), this scale becomes longer than the characteristic sizes of pre-stellar cores (the sonic length), so large fluctuations in the dust-to-gas ratio are imprinted on cores. As a result, star clusters and protostellar disks formed in large clouds should exhibit substantial abundance spreads in the elements preferentially found in large grains (C, O, Si). This naturally predicts populations of carbon-enhanced stars, certain highly unusual stellar populations observed in nearby open clusters, and may explain the 'UV upturn' in early-type galaxies. It will also dramatically change planet formation in the resulting protostellar disks, by preferentially seeding disks with an enhancement in large carbonaceous or silicate grains. The relevant threshold for this behavior scales simply with cloud densities and temperatures, making straightforward predictions for clusters in starbursts and high-redshift galaxies. Because of the selective sorting by size, this process is not visible in extinction mapping. We also predict the shape of the abundance distribution. When these fluctuations occur, a small fraction of the cores are actually seeded with abundances Z~100 Z_mean, such that they are almost 'totally metal' (Z~1)! Assuming the cores collapse, these totally metal stars would be rare (1 in 10^4 in clusters where this occurs), but represent a fundamentally new stellar evolution channel.