Are Lithium-Rich Giants Binaries? A Radial Velocity Variability Analysis of 1,400 Giants

Abstract

Low-mass giants with large amounts of lithium (Li) have challenged stellar evolution for decades. One of the possibilities usually discussed to explain them involves the interaction with a close binary companion. This predicts that when compared against their non-enriched counterparts, Li-rich giants should preferentially be found as part of binary systems. In order to test this scenario, we assemble a sample of 1418 giants with radial velocities (RVs) from RAVE, GALAH, and Gaia, as well as stellar parameters and Li abundances from GALAH. Evolutionary states can be determined for 1030 of these giants. We develop a method that quantifies the degree of RV variability, which we use as a proxy for close binary companions. The method is tested and calibrated against samples of known RV standard stars and known spectroscopic binaries. We also compare the results of our RV variability analysis with binarity indicators from Gaia. We find that the accuracy of the classification is controlled by the precision of the RVs, which for the set of RVs available for the giants is 80-85%. Consistent with seismic studies, the resulting sample of giants contains a fraction of Li-rich objects in the red clump (RC) that is twice as large as that for first-ascent giants (RGB). Among RC giants, the fractions of Li-rich objects with high RV variability and with no RV variability are the same as those for Li-normal objects, which argues against a binary interaction scenario for the genesis of the bulk of Li-rich giants at that evolutionary stage. On the other hand, Li-rich giants in the RGB appear to have a small but detectable preference for higher RV variability, and thus possibly a larger close binary fraction, than the Li-normal giants at that stage. Additional measurements of the RVs of these giants at higher RV precision would greatly help confirm and more robustly quantify these results.

arXiv commentPublished in A&A

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