External photoevaporation of protoplanetary discs: does location matter?

TL;DR

This study uses N-body simulations to investigate whether external photoevaporation by massive stars influences protoplanetary disc mass and distribution, finding little evidence for a direct correlation when considering projection effects and dynamical evolution.

Contribution

The paper provides new insights by showing that projection effects and dynamical evolution can obscure or negate the expected correlation between disc mass and proximity to massive stars.

Findings

Projection effects can be significant in observational data.

No strong trend of increasing disc mass with distance from massive stars.

Dynamical evolution disperses photoevaporated discs away from ionising sources.

Abstract

Many theoretical studies have shown that external photoevaporation from massive stars can severely truncate, or destroy altogether, the gaseous protoplanetary discs around young stars. In tandem, several observational studies report a correlation between the mass of a protoplanetary disc and its distance to massive ionising stars in star-forming regions, and cite external photoevaporation by the massive stars as the origin of this correlation. We present N-body simulations of the dynamical evolution of star-forming regions and determine the mass-loss in protoplanetary discs from external photoevaporation due to far ultraviolet (FUV) and extreme ultraviolet (EUV) radiation from massive stars. We find that projection effects can be significant, in that low-mass disc-hosting stars that appear close to the ionising sources may be fore- or background stars in the star-forming region. We find very little evidence in our simulations for a trend in increasing disc mass with increasing distance from the massive star(s), even when projection effects are ignored. Furthermore, the dynamical evolution of these young star-forming regions moves stars whose discs have been photoevaporated to far-flung locations, away from the ionising stars, and we suggest that any correlation between disc mass and distance the ionising star is either coincidental, or due to some process other than external photoevaporation.