Nadine Soliman

Stars form within dense cores composed of both gas and dust within molecular clouds. However, despite the crucial role that dust plays in the star formation process, its dynamics is frequently overlooked, with the common assumption being a constant, spatially uniform dust-to-gas ratio and grain size spectrum. In this study, we introduce a set of radiation-dust-magnetohydrodynamic simulations of star forming molecular clouds from the \small STARFORGE project. These simulations expand upon the earlier radiation MHD models, which included cooling, individual star formation, and feedback. Notably, they explicitly address the dynamics of dust grains, considering radiation, drag, and Lorentz forces acting on a diverse size spectrum of live dust grains. We find that interactions between radiation and dust significantly influence the properties of gas surrounding and accreting onto massive stars. Specifically, we find that once stars exceed a certain mass threshold (∼2M⊙), their emitted radiation can evacuate dust grains from their vicinity, giving rise to a dust-suppressed zone of size ∼100 AU. Commencing during the early accretion stages and preceding the Main-sequence phase, this process results in a mass-dependent depletion in the accreted dust-to-gas (ADG) mass ratio within both the circumstellar disc and the star. We predict massive stars (≳10M⊙) would exhibit ADG ratios that are approximately one order of magnitude lower than that of their parent clouds. Consequently, stars, their discs, and circumstellar environments would display notable deviations in the abundances of elements commonly associated with dust grains, such as carbon and oxygen.

The dust grain size distribution (GSD) likely varies significantly across different star-forming environments in the Universe, but the overall impact of this variation on star formation remains unclear. This ambiguity arises because the GSD interacts non-linearly with processes like heating/cooling, radiation, and chemistry, which have competing effects and different environmental dependencies. In this study, we investigate the effects of GSD variation on the thermochemistry and evolution of giant molecular clouds (GMCs). To achieve this, we conducted radiation-dust-magnetohydrodynamic simulations spanning a range of cloud masses and grain sizes, which explicitly incorporate the dynamics of dust grains within the full-physics framework of the STARFORGE project. We find that differences in grain size significantly alter the thermochemistry of GMCs. Specifically, we show that the leading-order effect is that larger grains, under fixed dust mass and dust-to-gas ratio conditions, result in lower dust opacities. This reduced opacity permits ISRF photons to penetrate more deeply and allows internal radiation field photons to permeate more extensively into the cloud, resulting in rapid gas heating and the inhibition of star formation. We find that star formation efficiency is highly sensitive to grain size, with an order of magnitude reduction in efficiency when grain size increases from 0.1 μm to 10 μm. Additionally, we note that warmer gas suppresses the formation of low-mass stars. Moreover, as a consequence of the decreased opacities, we observe a greater proportion of gas residing in diffuse ionized structures.

Numerous stars exhibit surprisingly large variations in their refractory element abundances, often interpreted as signatures of planetary ingestion events. In this study, we propose that differences in the dust-to-gas ratio near stars during their formation can produce similar observational signals. We investigate this hypothesis using a suite of radiation-dust-magnetohydrodynamic STARFORGE simulations of star formation. Our results show that the distribution of refractory abundance variations (Δ[X/H]) has extended tails, with about 10-30% of all stars displaying variations around ∼0.1 dex. These variations are comparable to the accretion of 2−5M⊕ of planetary material into the convective zones of Sun-like stars. The width of the distributions increases with the incorporation of more detailed dust physics, such as radiation pressure and back-reaction forces, as well as with larger dust grain sizes and finer resolutions. Furthermore, our simulations reveal no correlation between Δ[X/H] and stellar separations, suggesting that dust-to-gas fluctuations likely occur on scales smaller than those of wide binaries. These findings highlight the importance of considering dust dynamics as a potential source of the observed chemical enrichment in stars.

We append two additional black hole (BH) accretion models, namely viscous disc and gravitational torque-driven accretion, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. We show that these accretion models, characterized by a weaker dependence on the BH mass compared to the commonly used Bondi-Hoyle accretion, naturally create a common evolutionary track (co-existence) between the mass of the BH and the stellar mass of the galaxy, even without any direct coupling via feedback (FB). While FB is indeed required to control the final BH and stellar mass of the galaxies, our results suggest that FB might not be the leading driver of the cosmic co-evolution between these two quantities; in these models, co-evolution is simply determined by the shared central gas supply. Conversely, simulations using Bondi-Hoyle accretion show a two-step evolution, with an early growth of stellar mass followed by exponential growth of the central supermassive black hole (SMBH). Our results show that the modelling of BH accretion (sometimes overlooked) is an extremely important part of BH evolution and can improve our understanding of how scaling relations emerge and evolve, and whether SMBH and stellar mass co-exist or co-evolve through cosmic time.

Partial dust obscuration in active galactic nuclei (AGNs) has been proposed as a potential explanation for some cases of AGN variability. The dust–gas mixture present in AGN tori is accelerated by radiation pressure, leading to the launching of an AGN wind. Dust under these conditions has been shown to be unstable to a generic class of fast-growing resonant drag instabilities (RDIs). In this work, we present the first numerical simulations of radiation-driven outflows that explicitly include dust dynamics in conditions resembling AGN winds. We investigate the implications of RDIs on the torus morphology, AGN variability, and the ability of radiation to effectively launch a wind. We find that the RDIs rapidly develop, reaching saturation at times much shorter than the global time-scales of the outflows, resulting in the formation of filamentary structure on box-size scales with strong dust clumping and super-Alfvénic velocity dispersions. The instabilities lead to fluctuations in dust opacity and gas column density of 10–20  per cent when integrated along mock observed lines of sight to the quasar accretion disc. These fluctuations occur over year to decade time-scales and exhibit a red-noise power spectrum commonly observed for AGNs. Additionally, we find that the radiation effectively couples with the dust–gas mixture, launching highly supersonic winds that entrain 70–90  per cent of the gas, with a factor of ≲3 photon momentum loss relative to the predicted multiple-scattering momentum loading rate. Therefore, our findings suggest that RDIs play an important role in driving the clumpy nature of AGN tori and generating AGN variability consistent with observations.