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

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.

• The paper investigates the significant chemical abundance differences in the comoving binary Kronos and Krios, proposing that Kronos accreted rocky planetary material post-formation.
• The study uses Gaia astrometry and Keck spectroscopy to show Kronos is 0.2 dex more metal-rich in refractories than Krios, supporting a post-formation accretion event.
• These findings challenge the assumption that wide binary stars have identical chemical compositions, suggesting that post-formation processes like planetary accretion can significantly alter stellar characteristics.

Analysis of Elemental Abundance Differences in a Comoving Binary System: HD 240430 and HD 240429

The paper presents a detailed investigation of HD 240430 and HD 240429, a pair of solar-type stars exhibiting significant differences in chemical composition despite their proximity in space and inferred common origin. The study leverages astrometric data from the Gaia mission and high-resolution spectra from the Keck HIRES spectrograph to substantiate the hypothesis of post-formation accretion of planetary material by one of the stars, HD 240430, referred to as Kronos.

Key Findings

• Astrometric and Kinematic Analysis: The stars display nearly identical 3D velocities and a close physical separation of approximately 0.6 pc, consistent with them being a gravitationally bound pair. Isochrone fitting estimates suggests that both stars are around 4 Gyr old, supporting the idea that they formed together.
• Chemical Abundance Discrepancy: A remarkable difference in chemical abundances is observed between the two stars. Kronos is metal-rich by approximately 0.2 dex compared to HD 240429, named Krios. This difference is primarily observed in refractory elements and is a standout feature among known wide binary systems.
• Elemental Enhancements: Kronos exhibits a concurrent enhancement of refractory elements, with a significant lithium surface abundance that exceeds that of Krios by 0.5 dex. The presence of refractory elements predominantly suggests post-formation accretion of rocky material.

Theoretical Implications

The findings challenge the notion that stars in wide binaries share identical chemical compositions, even when born from the same molecular cloud. The paper posits that Kronos likely accreted rocky planetary debris post-formation, possibly through interactions such as planetary engulfment. This accretion after the star's thin convective envelope had formed effectively isolates the metallic contribution to the observable surface.

Practical Implications

From an observational perspective, these results underscore the importance of examining binary systems for deviations in chemical makeup, which can be indicative of dynamic processes like planetary migration or star-planet interactions. The investigation into the post-formation history of such stars offers insights into the complex processes of planetary system development and evolution.

Speculations for Future Research

The paper opens avenues for further exploration of planetary material accretion in solar-type stars and how these events might differ across varying stellar environments. Future Gaia data releases and complementary observational campaigns could enrich our understanding and potentially confirm the presence of surviving giant planets or their remnants around Kronos through precise radial velocity measurements.

In conclusion, the study of this peculiar star pair contributes to our understanding of stellar astrophysics, challenging assumptions of uniformity in birth chemical compositions within nascent binary systems. It suggests that post-formation processes notably alter observable stellar characteristics, prompting further investigation into how widely this phenomenon occurs across the galaxy.