Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies (1406.5509v2)

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.

• The paper identifies a novel turbulent mechanism that drives significant dust-to-gas ratio fluctuations in star-forming clouds.
• The paper shows that dust clustering can seed pre-stellar cores with unusual metal abundances, potentially explaining carbon-enhanced stars.
• The paper links enhanced dust concentration to modified planet formation, predicting distinct protostellar disk compositions.

The Role of Turbulent Dust-to-Gas Fluctuations in Star Formation and Its Astrophysical Consequences

Philip F. Hopkins' paper addresses a previously overlooked mechanism in star formation and the behavior of dust in star-forming molecular clouds. Dust grains in neutral gas, particularly large grains (diameter $\gtrsim0.1\,\mu{\rm m}$), can experience large local density fluctuations driven by gas turbulence, irrespective of changes in gas density. These "resonant" fluctuations are most significant when the gas stopping timescale for dust grains aligns with the turbulence eddy turnover time. For sufficiently large molecular clouds (radii $\gtrsim1-10\,$pc), these fluctuations exceed the average spatial scales of pre-stellar cores, which results in substantial variations in the dust-to-gas ratio within star clusters and protostellar disks formed in these clouds.

Key Findings and Implications

1. Mechanism of Dust Clustering: The paper argues that as dust grains behave like aerodynamic particles in a turbulent medium, they experience centrifugal forces that move them toward regions of high strain in the turbulent flow, leading to significant dust concentration. This clustering process can create notable fluctuations in the dust-to-gas ratio, particularly at scales larger than those typically considered in models of molecular clouds.
2. Variations in Stellar Abundances: The dust concentration mechanism has profound implications for the formation of stars, planets, and even entire galaxies. The resultant variations in the dust-to-gas ratio could lead to star clusters and protostellar disks exhibiting significant spreads in elemental abundances, particularly for elements found largely in grain form, such as carbon and oxygen. This provides a natural explanation for observed populations of carbon-enhanced stars, as well as other exceptional stellar populations with unusual abundance patterns.
3. Influence on Planet Formation: The enhancement of large grains in protostellar disks can dramatically alter the details of planet formation processes. A higher concentration of these grains can preferentially seed disks with carbonaceous or silicate materials, influencing the formation and composition of planetary bodies.
4. Thresholds and Observability: The paper posits a straightforward mathematical relationship between cloud properties (such as density and temperature) and the threshold conditions required for this process of turbulent concentration to occur. The mechanism is largely selective by grain size, implying it might not be detectable through conventional extinction mapping but could be observed by tuning into spectral features sensitive to large grains.
5. Rare 'Totally Metal' Stars: One fascinating prediction from this process is that under specific conditions, a small fraction of pre-stellar cores might be seeded with metal abundances much higher than typical cosmic values, resulting in stars that are predominantly metallic in composition. While rare, such stars present a new evolutionary channel that deserves further exploration.

Theoretical and Observational Implications

From a theoretical perspective, this mechanism challenges traditional star formation models that assume dust-to-gas ratios are uniform. It suggests that dust-induced abundance variations might be more widespread than previously recognized, especially in massive molecular clouds. Practically, this has implications for our understanding of the initial conditions of stellar and planetary systems, and the intricate interplay between turbulence, grain dynamics, and chemical evolution in the cosmos.

Observationally, verifying these predictions will require innovative approaches. Directly measuring dust density fluctuations using sub-mm wavelengths or studying stellar populations in areas of high metallicity scatter could provide supporting evidence. Moreover, linking the occurrence of peculiar stellar types and planetary systems with large clouds where this mechanism is active could illuminate new paths in stellar and planetary astrophysics.

Future Directions

The paper opens numerous avenues for future research. Accurate simulations of grain density fluctuations in turbulent environments, while methodologically challenging, could offer significant insights. Additionally, investigations into the stellar evolution of metal-rich stars, along with comprehensive spectroscopic surveys of large and massive star-forming regions, might yield critical empirical validation of Hopkins' theory.

In summary, Hopkins' work elucidates a compelling, yet underexplored, aspect of star and planet formation. It requires us to reconsider how dust grain dynamics contribute to the complex chemical tapestry of the universe, with potential repercussions across multiple domains of astronomical research.