Nitrogen depletion in field red giants: mixing during the He flash?

TL;DR

This study combines asteroseismology and spectroscopic data to track nitrogen surface abundances in low-mass stars, revealing evidence of nitrogen depletion possibly due to burning during the He flash.

Contribution

It provides new observational evidence of nitrogen depletion during the He flash, suggesting mixing processes not fully accounted for in current stellar models.

Findings

Nitrogen surface abundances increase after the first dredge-up as expected.

Evidence of non-canonical extra-mixing on the RGB for low-mass stars.

Detection of nitrogen depletion between the RGB tip and the red clump.

Abstract

We combine simultaneous constraints on stellar evolutionary status from asteroseismology, and on nitrogen abundances derived from large spectroscopic surveys, to follow nitrogen surface abundances all along the evolution of a low-mass star, comparing model expectations with data. After testing and calibrating the observed yields from the APOGEE survey, we first show that nitrogen surface abundances follow the expected trend after the first dredge-up occurred, i.e. that the more massive is the star the more nitrogen is enhanced. Moreover, the behaviour of nitrogen data along the evolution confirms the existence of non-canonical extra-mixing on the RGB for all low-mass stars in the field. But more surprisingly, the data indicate that nitrogen has been depleted between the RGB tip and the red clump. This may suggest that some nitrogen has been burnt near or at the He flash episode.