Planet Engulfment Signatures in Twin Stars

TL;DR

This study models how planet engulfment leaves detectable chemical signatures in stars, showing that these signatures vary with star type, age, and engulfment timing, and are influenced by stellar mixing processes.

Contribution

The paper provides a detailed quantification of the timescales and conditions under which planet engulfment signatures can be observed in different types of stars using MESA models.

Findings

Thermohaline mixing dominates for 5-45 Myr post-engulfment.

Observable signatures last from 1 Myr to 8 Gyr depending on star and engulfment details.

Signatures are more detectable in early type and low-metallicity stars.

Abstract

Planet engulfment can be inferred from enhancement of refractory elements in the photosphere of the engulfing star following accretion of rocky planetary material. Such refractory enrichments are subject to stellar interior mixing processes, namely thermohaline mixing induced by an inverse mean-molecular-weight gradient between the convective envelope and radiative core. Using MESA stellar models, we quantified the strength and duration of engulfment signatures following planet engulfment. We found that thermohaline mixing dominates during the first $\sim$5$-$45 Myr post-engulfment, weakening signatures by a factor of $\sim$2 before giving way to depletion via gravitational settling on longer timescales. Solar metallicity stars in the 0.5-1.2 $M_{\odot}$ mass range have observable signature timescales of $\sim$1 Myr$-$8 Gyr, depending on the engulfing star mass and amount of material engulfed. Early type stars exhibit larger initial refractory enhancements but more rapid depletion. Solar-like stars ($M$ = 0.9$-$1.1 $M_{\odot}$) maintain observable signatures ($>$0.05 dex) over timescales of $\sim$20 Myr$-$1.7 Gyr for nominal 10 $M_{\oplus}$ engulfment events, with longer-lived signatures occurring for low-metallicity and/or hotter stars (1 $M_{\odot}$, $\sim$2$-$3 Gyr). Engulfment events occurring well after the zero-age main sequence produce larger signals due to suppression of thermohaline mixing by gravitational settling of helium (1 $M_{\odot}$, $\sim$1.5 Gyr). These results indicate that it may be difficult to observe engulfment signatures in solar-like stars that are several Gyr old.