Abstract Planetary engulfment events have long been proposed as a lithium (Li) enrichment mechanism contributing to the population of Li-rich giants ( A (Li) ≥ 1.5 dex). Using MESA stellar models and A (Li) abundance measurements obtained by the GALAH survey, we calculate the strength and observability of the surface Li enrichment signature produced by the engulfment of a hot Jupiter (HJ). We consider solar-metallicity stars in the mass range of 1–2 M ⊙ and the Li supplied by a HJ of 1.0 M J . We explore engulfment events that occur near the main-sequence turn-off (MSTO) and out to orbital separations of R ⋆ ∼ 0.1 au = 22 R ⊙ . We map our results onto the Hertzsprung–Russell Diagram, revealing the statistical significance and survival time of Li enrichment. We identify the parameter space of masses and evolutionary phases where the engulfment of a HJ can lead to Li enrichment signatures at a 5 σ confidence level and with meteoritic abundance strengths. The most compelling strengths and survival times of engulfment-derived Li enrichment are found among host stars of 1.4 M ⊙ near the MSTO. Our calculations indicate that planetary engulfment is not a viable enrichment pathway for stars that have evolved beyond the subgiant branch. For these sources, observed Li enhancements are likely to be produced by other mechanisms, such as the Cameron–Fowler process or the accretion of material from an asymptotic giant branch companion. Our results do not account for second-order effects, such as extra mixing processes, which can further dilute Li enrichment signatures.
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Long-term lithium abundance signatures following planetary engulfment
Planetary engulfment events can occur while host stars are on the main sequence. The addition of rocky planetary material during engulfment will lead to refractory abundance enhancements in the host star photosphere, but the level of enrichment and its duration will depend on mixing processes that occur within the stellar interior, such as convection, diffusion, and thermohaline mixing. We examine engulfment signatures by modelling the evolution of photospheric lithium abundances. Because lithium can be burned before or after the engulfment event, it produces unique signatures that vary with time and host star type. Using mesa stellar models, we quantify the strength and duration of these signatures following the engulfment of a 1, 10, or 100 M⊕ planetary companion with bulk Earth composition, for solar-metallicity host stars with masses ranging from 0.5 to 1.4 M⊙. We find that lithium is quickly depleted via burning in low-mass host stars ($\lesssim 0.7 \, {\rm M}_\odot$) on a time-scale of a few hundred Myrs, but significant lithium enrichment signatures can last for Gyrs in G-type stars ($\sim \! 0.9 \, {\rm M}_{\odot }$). For more massive stars (1.3−1.4 M⊙), engulfment can enhance internal mixing and diffusion processes, potentially decreasing the surface lithium abundance. Our predicted signatures from exoplanet engulfment are consistent with observed lithium-rich solar-type stars and abundance enhancements in chemically inhomogeneous binary stars.
more » « less- NSF-PAR ID:
- 10371554
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 516
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3354-3365
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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 ∼5–45 Myr post-engulfment, weakening signatures by a factor of ∼2 before giving way to depletion via gravitational settling on longer time-scales. Solar metallicity stars in the 0.5–1.2 M⊙ mass range have observable signature time-scales of ∼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⊙) maintain observable signatures (>0.05 dex) over time-scales of ∼20 Myr–1.7 Gyr for nominal 10 M⊕ engulfment events, with longer-lived signatures occurring for low-metallicity and/or hotter stars (1 M⊙, ∼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⊙, ∼1.5 Gyr). These results indicate that it may be difficult to observe engulfment signatures in solar-like stars that are several Gyr old.
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ABSTRACT Dynamical evolution within planetary systems can cause planets to be engulfed by their host stars. Following engulfment, the stellar photosphere abundance pattern will reflect accretion of rocky material from planets. Multistar systems are excellent environments to search for such abundance trends because stellar companions form from the same natal gas cloud and are thus expected to share primordial chemical compositions to within 0.03–0.05 dex. Abundance measurements have occasionally yielded rocky enhancements, but a few observations targeted known planetary systems. To address this gap, we carried out a Keck-HIRES survey of 36 multistar systems, where at least one star is a known planet host. We found that only HAT-P-4 exhibits an abundance pattern suggestive of engulfment but is more likely primordial based on its large projected separation (30 000 ± 140 au) that exceeds typical turbulence scales in molecular clouds. To understand the lack of engulfment detections among our systems, we quantified the strength and duration of refractory enrichments in stellar photospheres using mesa stellar models. We found that observable signatures from 10 M⊕ engulfment events last for ∼90 Myr in 1 M⊙ stars. Signatures are largest and longest lived for 1.1–1.2 M⊙ stars, but are no longer observable ∼2 Gyr post-engulfment. This indicates that engulfment will rarely be detected in systems that are several Gyr old.
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