The observable impact of runaway OB stars on protoplanetary discs

TL;DR

Runaway OB stars can cause significant, observable truncation of protoplanetary discs through external photoevaporation, with potential signatures detectable in specific stellar regions, influencing disc evolution and planet formation.

Contribution

This study demonstrates how high-velocity runaway OB stars impact protoplanetary discs differently from cluster-bound stars, highlighting observable signatures of external photoevaporation.

Findings

Discs exposed to short UV irradiation are significantly truncated.

Runaway OB stars create observable asymmetries in disc populations.

Faster runaway stars produce more distinct and lasting disc signatures.

Abstract

UV radiation from OB stars can drive ``external'' photoevaporative winds from discs in clusters, that have been shown to be important for disc evolution and planet formation. However, cluster dynamics can complicate the interpretation of this process. A significant fraction of OB stars are runaways, propagating at high velocity which might dominate over the wider cluster dynamics in setting the time variation of the UV field in part of the cluster. We explore the impact of a runaway OB star on discs and the observational impact that may have. We find that discs exposed to even short periods of strong irradiation are significantly truncated, and only rebound slightly following the ``flyby'' of the UV source. This is predicted to leave an observable imprint on a disc population, with those downstream of the OB star vector being more massive and extended than those upstream. Because external photoevaporation acts quickly, this imprint is less susceptible to being washed out by cluster dynamics for faster runaway OB stars. The Gaia proper motion vector of the B star 42 Ori in NGC 1977 is transverse to the low mass stellar population and so may make a good region to search for this signature in resolved disc observations.