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

TL;DR

This paper introduces a new mechanism where large dust grains in star-forming clouds create significant abundance fluctuations, leading to the formation of nearly 'totally metal' stars and impacting galaxy evolution and planet formation.

Contribution

It reveals a novel aerodynamic dust fluctuation mechanism in molecular clouds that explains abundance spreads and the formation of 'totally metal' stars, a new stellar evolution pathway.

Findings

Large dust grains cause significant abundance fluctuations in star-forming cores.

Predicted existence of rare 'totally metal' stars with near-complete metal enrichment.

Implications for galaxy evolution, stellar populations, and planet formation processes.

Abstract

Dust grains in neutral gas behave as aerodynamic particles, so they can develop large density fluctuations independent of gas density fluctuations. Specifically, gas turbulence can drive order-of-magnitude 'resonant' fluctuations in the dust on scales where the gas stopping/drag timescale is comparable to the turbulent eddy turnover time. Here we show that for large grains (size >0.1 micron, containing most grain mass) in sufficiently large molecular clouds (radii >1-10 pc, masses >10^4 M_sun), this scale becomes larger 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 significant abundance spreads in the elements preferentially found in large grains. 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 necessarily visible in extinction mapping. We also predict the shape of the abundance distribution -- when these fluctuations occur, a small fraction of the cores may actually be seeded with abundances ~100 times the 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.