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



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 Li-rich 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 12C/13C – a proxy for mixing. Lower 12C/13C 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. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)