External photoevaporation of protoplanetary discs in sparse stellar groups: the impact of dust growth

TL;DR

This study models photoevaporative winds from protoplanetary discs in sparse stellar groups, highlighting the significant impact of dust growth on mass loss rates and observable wind features, with implications for ALMA observations.

Contribution

It demonstrates that dust growth substantially increases mass loss rates in protoplanetary discs under moderate radiation fields, a factor previously underestimated.

Findings

Mass loss rates > 1e-8 M_sun/yr for discs > 150 AU with dust growth.

Dust growth reduces FUV cross section, enhancing wind vigor.

Observable wind features include dust-depleted outer regions and non-Keplerian rotation.

Abstract

We estimate the mass loss rates of photoevaporative winds launched from the outer edge of protoplanetary discs impinged by an ambient radiation field. We focus on mild/moderate environments (the number of stars in the group/cluster is N ~ 50), and explore disc sizes ranging between 20 and 250 AU. We evaluate the steady-state structures of the photoevaporative winds by coupling temperature estimates obtained with a PDR code with 1D radial hydrodynamical equations. We also consider the impact of dust dragging and grain growth on the final mass loss rates. We find that these winds are much more significant than have been appreciated hitherto when grain growth is included in the modelling: in particular, mass loss rates > 1e-8 M_sun/yr are predicted even for modest background field strengths ( ~ 30 G_0) in the case of discs that extend to R > 150 AU. Grain growth significantly affects the final mass loss rates by reducing the average cross section at FUV wavelengths, and thus allowing a much more vigorous flow. The radial profiles of observable quantities (in particular surface density, temperature and velocity patterns) indicate that these winds have characteristic features that are now potentially observable with ALMA. In particular, such discs should have extended gaseous emission that is dust depleted in the outer regions, characterised by a non-Keplerian rotation curve, and with a radially increasing temperature gradient.