An Alternative Formation Scenario for Uranium-rich Giants: Engulfing a Earth-like Planet

TL;DR

This paper proposes that uranium-rich giant stars may form when red giants engulf Earth-like planets, explaining observed uranium abundances without significantly altering stellar evolution.

Contribution

It introduces an accretion model showing how planetary engulfment can account for uranium enrichment in giant stars, a novel formation scenario for U-rich giants.

Findings

Engulfment significantly increases heavy elements on metal-poor stars.

The process explains observed uranium abundances in U-rich giants.

Stellar structure and evolution remain largely unaffected by engulfment.

Abstract

The actinides, such as the uranium (U) element, are typically synthesized through the rapid neutron-capture process (r-process), which can occur in core-collapse supernovae or double neutron star mergers. There exist nine r-process giant stars exhibiting conspicuousUabundances, commonly referred to as U-rich giants. However, the origins of these U-rich giants remain ambiguous. We propose an alternative formation scenario for these U-rich giants whereby a red giant (RG) engulfs an Earth-like planet. To approximate the process of a RG engulfing an Earth-like planet, we employ an accretion model wherein the RG assimilates materials from said planet. Our findings demonstrate that this engulfment event can considerably enhance the presence of heavy elements originating from Earth-like planets on the surfaces of very metal-poor stars (Z = 0.00001), while its impact on solar-metallicity stars is comparatively modest. Importantly, the structural and evolutionary properties of both very metalpoor and solar-metallicity stars remain largely unaffected. Notably, our engulfment model effectively accounts for the observed U abundances in known U-rich giants. Furthermore, the evolutionary trajectories of U abundances on the surfaces of RGs subsequent to the engulfment of Earth-like planets encompass all known U-rich giants. Therefore, it is plausible that U-rich giants are formed when a RG engulfs an Earth-like planet.