Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars

TL;DR

This paper presents evidence that a solar-type star, Kronos, accreted a significant amount of rocky material, leading to notable chemical abundance differences with its comoving twin, indicating planetary system formation and evolution.

Contribution

It provides the first detailed chemical analysis of a wide binary pair showing evidence of planetary accretion in one star, highlighting the impact of rocky material ingestion on stellar surface composition.

Findings

Kronos shows a 0.2 dex enhancement in refractory elements.

Kronos has a 0.5 dex higher lithium abundance than its companion.

The pair likely formed together but diverged chemically due to planetary accretion.

Abstract

We report and discuss the discovery of a comoving pair of bright solar-type stars, HD 240430 and HD 240429, with a significant difference in their chemical abundances. The two stars have an estimated 3D separation of $\approx 0.6$ pc ($\approx 0.01$ pc projected) at a distance of $r\approx 100$ pc with nearly identical three-dimensional velocities, as inferred from Gaia TGAS parallaxes and proper motions, and high-precision radial velocity measurements. Stellar parameters determined from high-resolution Keck HIRES spectra indicate that both stars are $\sim 4$ Gyr old. The more metal-rich of the two, HD 240430, shows an enhancement of refractory ($T_C>1200$ K) elements by $\approx 0.2$ dex and a marginal enhancement of (moderately) volatile elements ($T_C<1200$ K, C, N, O, Na, and Mn). This is the largest metallicity difference found in a wide binary pair yet. Additionally, HD 240430 shows an anomalously high surface lithium abundance ($A(\mathrm{Li})=2.75$), higher than its companion by $0.5$ dex. The proximity in phase-space and ages between the two stars suggests that they formed together with the same composition, at odds with the observed differences in metallicity and abundance patterns. We therefore suggest that the star HD~240430, "Kronos", accreted 15 $M_\oplus$ of rocky material after birth, selectively enhancing the refractory elements as well as lithium in its surface and convective envelope.