Li‐enrichment in red giant rapid rotators: Planet engulfment versus extra mixing

Abstract

Low mass stars undergo dramatic changes during the post main sequence evolution as the nuclear energy source shifts from the stellar core to a thin shell of active hydrogen burning. The outer convection zones of these stars deepen, dredging up nuclear-processed material and altering the stellar surface abundances. Some light elements, such as lithium, are easily destroyed in the stellar interior, and dredge-up depletes the surface abundances of these elements. The red giant stars' expanding radii also pose a threat to the stability of close orbiting planets, which can be tidally engulfed by the star. Planet engulfment may be able to account for two of the well known classes of atypical red giant stars: the rapid rotators and Lirich red giants. Alternatively, internal Li regeneration combined with "extra-mixing" in red giants may account for Li-rich giants, but this mechanism may not be able to explain rapid rotation. We have recently completed an observational study of field red giant stars, targeting both slow and rapid rotators, to compare the light element distribution between these two classes of rotators. We find that the rapid rotators have enriched Li abundances compared to the slow rotators; however, both classes of stars have similar distributions of 12 C/ 13 C -a proxy for mixing. Lower 12 C/ 13 C ratios are expected in stars that have regenerated Li internally. The peculiar red giant stars in our sample (both Li-rich and rapid rotators) are not easily explained with either planet engulfment or Li regeneration alone. Both processes are likely at work.

3D Hydrodynamical Simulations of a Brown Dwarf Accretion by a Main-Sequence Star and its Impact on the Surface Li Abundance

“…This is encouraging since there is observational evidence that planet engulfment may induce a significant increase of the rotational velocity of the host star (at least in red giants, e.g. Carlberg et al 2013), although not as large as we find here. Note, however, that cases discussed here are probably realistic engulfment scenarios; case D (in progress) simulates a smoother merging between both stars, being a more realistic process.…”

Lithium in Stellar Atmospheres: Observations and Theory

“…It has been pointed out [65] that the absorption of a large planet by a cold giant can increase the amount of lithium and also increase the star's rotation velocity. This could explain the fact, noted above, that some Li-rich giants have high rotation velocities that are utterly atypical of normal cold giants.…”

Lithium-Rich Giants in Globular Clusters*

“…In the engulfment scenario (e.g., Siess & Livio 1999;Denissenkov & Weiss 2000;Melo et al 2005;Villaver & Livio 2009;Adamów et al 2012), a red giant expands into the orbit of a rocky planet, a hot Jupiter, or a companion brown dwarf. The destroyed companion could potentially enrich the star with Li and other volatile elements that concentrate in planets (Carlberg et al 2013). Even if the engulfed companion does not donate Li to its host star, it would provide angular momentum.…”