Dust-Evacuated Zones Near Massive Stars: Consequences of Dust Dynamics on Star-forming Regions

TL;DR

This study uses advanced radiation-dust-magnetohydrodynamic simulations to show that massive stars can create dust-free zones, significantly affecting local dust-to-gas ratios and element abundances in star-forming regions.

Contribution

It introduces detailed dust dynamics into star formation simulations, revealing how radiation from massive stars evacuates dust and alters chemical compositions.

Findings

Dust-free zones of ~100 AU form around stars >2 M_sun.

Massive stars (>10 M_sun) have dust-to-gas ratios an order of magnitude lower.

Dust depletion affects element abundances like carbon and oxygen.

Abstract

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 once stars exceed a certain mass threshold ($\sim 2 M_{\odot}$), their emitted radiation can evacuate dust grains from their vicinity, giving rise to a dust-suppressed zone of size $\sim 100$ AU. This removal of dust, which interacts with gas through cooling, chemistry, drag, and radiative transfer, alters the gas properties in the region. 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 ($\gtrsim 10 M_{\odot}$) 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.