Are the Formation and Abundances of Metal-poor Stars the Result of Dust Dynamics?

Abstract

Large dust grains can fluctuate dramatically in their local density, relative to the gas, in neutralturbulent disks. Small, high-redshift galaxies (before reionization) represent ideal environments for this process. We show via simple arguments and simulations that order-of-magnitude fluctuations are expected in local abundances of large grains (>100 Å) under these conditions. This can have important consequences for star formation and stellar metal abundances in extremely metal-poor stars. Low-mass stars canform in dust-enhanced regions almost immediately after some dust formseven if the galaxy-average metallicity is too low for fragmentation to occur. We argue that the metal abundances of these "promoted" stars may contain interesting signaturesas the CNO abundances (concentrated in large carbonaceous grains and ices) and Mg and Si (in large silicate grains) can be enhanced and/or fluctuate almost independently. Remarkably, the otherwise puzzling abundance patterns of some metal-poor stars can be wellfit by standardIMF-averaged core-collapse SNe yieldsif we allow for fluctuating local dust-to-gas ratios. We also show that the observed log-normaldistribution of enhancements in these species agrees with our simulations. Moreover, we confirm that Mg and Si are correlated in these stars;theabundance ratios are similar to those in local silicate grains. Meanwhile [Mg/Ca], predicted to be nearly invariant from pure SNe yields, shows very large enhancements and variations up to factors of 100 as expected in the dust-promoted model, preferentially in the [C/Fe]-enhanced metal-poor stars. Together, this suggests that (1) dust exists in second-generation star formation, (2) local dust-to-gas ratio fluctuations occur inprotogalaxies and can be important for star formation, and (3) the light element abundances of these stars may be affected by the local chemistry of dust where they formed, rather than directly tracing nucleosynthesis from earlier populations.

The dynamics of charged dust in magnetized molecular clouds

“…Because high-density regions can have enhanced/suppressed dust-to-gas ratios in large grains (which contain a large fraction of the metal mass), this can have interesting implications for stellar abundances. Hopkins & Conroy (2015) use similar simulations, coupled to a specific dust chemistry model, to explore consequences for abundance patterns in metal-poor stars, and suggest that certain observed chemical signatures in these stars may demonstrate variable dust-to-gas ratios in their progenitor clouds. Hopkins (2014b) use a simple analytic model to further explore the consequences for stellar abundance variations across present-day star forming clouds.…”

“…More recently, several studies have suggested that dust grains in GMCs or neutral galactic disks should exhibit similar fluctu-E-mail:hlee2@mit.edu ations (Padoan et al 2006;Hopkins 2014b;Hopkins & Conroy 2015) -in terms of the aerodynamic drag equations, a grain of diameter ∼ 0.1−1 µm in a typical GMC is analogous to a meter-sized boulder in a protoplanetary disk. And observations have identified small-scale (∼ 0.01−1 pc) fluctuations in the local dust-to-gas ratio of large grains in a number of nearby molecular clouds (Thoraval et al 1997(Thoraval et al , 1999Abergel et al 2002;Flagey et al 2009;Boogert et al 2013).…”

Ubiquitous instabilities of dust moving in magnetized gas

“…30 As discussed in Paper II, the RDI could be an important part of observed phenomena ranging from the well-studied clumpiness, substructure, and turbulence in the dusty AGN 'torus' (see e.g. Krolik & Begelman 1988;Nenkova et al 2008;Mor, Netzer & Elitzur 2009;Hönig & Kishimoto 2010, and references therein), time variability in AGN dust obscuration (McKernan & Yaqoob 1998;Risaliti, Elvis & Nicastro 2002), AGN winds driven by radiation pressure on dust (Murray et al 2005;Elitzur & Shlosman 2006;Miller, Turner & Reeves 2008;Wada, Papadopoulos & Spaans 2009;Roth et al 2012), observed dust-gas segregation in GMCs (Padoan et al 2006) and abundance anomalies sourced by these (Hopkins 2014;Hopkins & Conroy 2017), dust growth and coagulation (believed to occur primarily in the cold, dense ISM; Draine 2003 and references therein), dust chemistry/cooling physics critical for star formation and formation of complex organic compounds and molecules (Goldsmith & Langer 1978;Dopcke et al 2013;Chiaki et al 2014;Ji, Frebel & Bromm 2014), radiation-pressure-driven outflows from massive stars (Murray et al 2005;Krumholz, Klein & McKee 2007;Hopkins, Quataert & Murray 2011;Grudić et al 2016), and thermal regulation of proto-star formation via heating dust in coupled dust-gas cores (Guszejnov, Krumholz & Hopkins 2016).…”

The resonant drag instability (RDI): acoustic modes

“…They may fundamentally alter the ability of radiation pressure from massive stars to drive outflows and source local turbulence (a subject of considerable interest and controversy; see Murray et al 2005;Thompson et al 2005;Krumholz et al 2007;Schartmann et al 2009;Hopkins et al 2011Hopkins et al , 2013Hopkins et al , 2014Guszejnov et al 2016;Grudić et al 2018). They will also directly source dust-togas fluctuations, which can in turn drive abundance anomalies in next-generation stars (Hopkins 2014;Hopkins & Conroy 2017), as well as altering the dust growth, chemistry, and cooling physics of the clouds (Goldsmith & Langer 1978;Dopcke et al 2013;Ji et al 2014;Chiaki et al 2014).…”