Close to the Dredge: Precise X-Ray C and N Abundances in λ Andromeda and Its Precocious Red Giant Branch Mixing Problem

Abstract

Chandra LETG+HRC-S and XMM-Newton RGS spectra of H-like C and N lines formed in the corona of the primary star of the RS CVn-type binary λ And, a mildly metal-poor G8 III-IV first ascent giant that completed dredge-up ~50 Myr ago, have been used to make a precise measurement of its surface C/N ratio. We obtain the formal result [C/N] = 0.03 ± 0.07, which is typical of old disk giants and in agreement with standard dredge-up theory for stars <~ 1 M _sun. In contrast, these stars as a group, including λ And, have ¹²C/¹³C lsim 20, which is much lower than standard model predictions. We show that the abundances of the old disk giants are consistent with models including thermohaline mixing that begins at the red giant branch luminosity function "bump." Instead, λ And indicates that the ¹²C/¹³C anomaly can be present immediately following dredge-up, contrary to current models of extra mixing on the red giant branch. In the context of other recent C and N abundance results for RS CVn-type binaries it seems likely that the anomaly is associated with either strong magnetic activity, fast rotation, or both, rather than close binarity itself.