Drifters on the edge of town: $\lambda$ Bo\"otis stars in clusters

TL;DR

This study uses N-body simulations to demonstrate that λ Boötis stars can form in star clusters through accretion of pristine interstellar medium gas, even in the presence of ionising radiation from massive stars.

Contribution

It provides the first dynamical evidence that λ Boötis stars can develop in clusters despite radiation barriers, supporting the accretion hypothesis.

Findings

Stars can leave clusters and accrete pristine gas beyond the tidal radius.

λ Boötis abundance patterns can occur in stars that re-enter clusters.

Binary systems with different abundances in components are consistent with accretion scenario.

Abstract

$\lambda$ Bo\"otis stars are a subset of chemically peculiar A-stars that display Solar abundances in lighter elements (C, N, O, S, etc.) but a deficiency in Iron-peak elements. This difference has been attributed to the A-stars accreting pristine (metal deficient) gas from the Interstellar Medium. However, the recent discovery of $\lambda$ Bo\"otis stars in clusters challenges this theory, due to the presence of ionising radiation from intermediate/massive ($>$5 M$_\odot$) stars, which could prevent accretion of pristine ISM gas. We use $N$-body simulations to track the dynamical histories of A-stars during the evolution of a star cluster. We find that some stars leave the confines of the cluster and travel beyond the tidal radius, where they may be able to accrete pristine ISM gas. These A-stars then sometimes move back into the inner regions of the cluster, but the photoionising radiation flux they receive is not high enough to prevent $\lambda$ Bo\"otis abundances from occurring in these A-stars. We find that A-stars can develop $\lambda$ Bo\"otis abundances and subsequently form a wide ($>100$ au) binary system, meaning that observations of binary systems that have different abundances between the component stars would not rule out the ISM accretion scenario. Whilst we have shown that $\lambda$ Bo\"otis stars can reside in and around star clusters, further research is required to assess the validity of the accretion rates required to explain their abundance patterns.