New determination of abundances and stellar parameters for a set of weak G-band stars

TL;DR

This study provides a detailed spectroscopic analysis of weak G-band stars, revealing their fundamental parameters, chemical compositions, and evolutionary status, but the origin of their peculiar abundance patterns remains unresolved.

Contribution

First homogeneous spectroscopic analysis of 19 weak G-band stars with stellar modeling, clarifying their masses, evolutionary phases, and chemical peculiarities.

Findings

Weak G-band stars are 3.2 to 4.2 solar masses.

Most are mildly evolved on the SGB undergoing 1st DUP.

They exhibit strong N enrichment and C depletion, not due to self-enrichment.

Abstract

Weak G-band (wGb) stars are very peculiar red giants almost devoided of carbon and often mildly enriched in lithium. Despite their very puzzling abundance patterns, very few detailed spectroscopic studies existed up to a few years ago, preventing any clear understanding of the wGb phenomenon. We recently proposed the first consistent analysis of published data for 28 wGb stars and identified them as descendants of early A-type to late B-type stars, without being able to conclude on their evolutionary status or the origin of their peculiar abundance pattern. We used newly obtained high-resolution and high SNR spectra for 19 wGb stars in the southern and northern hemisphere to homogeneously derive their fundamental parameters, metallicities, as well as the spectroscopic abundances for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution models that we used to determine the masses and to investigate the evolutionary status and chemical history of the stars in our sample. We confirm that the wGb stars are stars in the mass range 3.2 to 4.2 M$_\odot$. We suggest that a large fraction could be mildly evolved stars on the SGB currently undergoing the 1st DUP, while a smaller number of stars are more probably in the core He burning phase at the clump. After analysing their abundance pattern, we confirm their strong N enrichment anti-correlated with large C depletion, characteristic of material fully processed through the CNO cycle to an extent not known in other evolved intermediate-mass stars. However, we demonstrate here that such a pattern is very unlikely due to self-enrichment. In the light of the current observational constraints, no solid self-consistent pollution scenario can be presented either, leaving the wGb puzzle largely unsolved.