Externally Fed Accretion onto Protostars

TL;DR

This paper revisits the classical model of spherical gas accretion onto protostars, incorporating self-gravity and subsonic boundary conditions, to explain observed low accretion rates and slow infall in dense cores.

Contribution

It introduces a modified accretion model with subsonic outer flow, explaining low luminosities and extended infall regions observed in star-forming cores.

Findings

Accretion rate is reduced by the Mach number of the outer flow.

Infall region expands more slowly, near the subsonic inflow speed.

Supports the idea that dense cores' contraction is not purely hydrodynamic.

Abstract

The asymmetric molecular emission lines from dense cores reveal slow, inward motion in the clouds' outer regions. This motion is present both before and after the formation of a central star. Motivated by these observations, we revisit the classic problem of steady, spherical accretion of gas onto a gravitating point mass, but now include self-gravity of the gas and impose a finite, subsonic velocity as the outer boundary condition. We find that the accretion rate onto the protostar is lower than values obtained for isolated, collapsing clouds, by a factor that is the Mach number of the outer flow. Moreover, the region of infall surrounding the protostar spreads out more slowly, at a speed close to the subsonic, incoming velocity. Our calculation, while highly idealized, provides insight into two longstanding problems -- the surprisingly low accretion luminosities of even the most deeply embedded stellar sources, and the failure so far to detect spatially extended, supersonic infall within their parent dense cores. Indeed, the observed subsonic contraction in the outer regions of dense cores following star formation appears to rule out a purely hydrodynamic origin for these clouds.