Close to the Dredge: Precise X-ray C and N Abundances in lambda Andromeda and its Precocious RGB Mixing Problem

TL;DR

This study measures surface C/N ratios in lambda Andromeda using X-ray spectra, confirming standard dredge-up predictions but revealing an unexpected low 12C/13C ratio immediately after dredge-up, challenging existing models.

Contribution

It provides the first precise surface C/N ratio measurement in lambda And and highlights discrepancies in 12C/13C ratios, questioning current models of red giant branch mixing.

Findings

C/N ratio consistent with standard dredge-up theory

12C/13C ratio is lower than predicted by models

Lambda And shows early 12C/13C anomaly after dredge-up

Abstract

Chandra LETG+HRC-S and XMM-Newton RGS spectra of H-like C and N formed in the corona of the lambda And primary star, 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 of about 1 M_sun or lower. In contrast, these stars as a group, including lambda And, have 12C/13C ~< 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, lambda 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.