17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

Type: Oral

Topic: Evolved Stars and Hot-Cool Star Connection

Abstract
During the post main sequence evolution, low mass stars undergo dramatic changes as their nuclear energy sources shift from stellar cores to thin shells of active hydrogen burning. The outer convection zones of these stars deepen, which dredge up nuclear-processed material and alter the stellar surface abundances. Some light elements, such as lithium, are easily destroyed in the stellar interior, and dredge-up leads to depleted surface abundances of these fragile 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 some of the well known 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 mechanisms 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 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. At least one of our discovered Li-rich stars is difficult to explain with either Li-enrichment mechanism.