Atmospheric composition of weak G band stars: CNO and Li abundances

TL;DR

This study analyzes the chemical composition of weak G band stars, revealing significant CN-cycle processing, high lithium levels in some stars, and Na overabundance, suggesting complex internal mixing and nucleosynthesis processes.

Contribution

First detailed spectroscopic analysis of C, N, O, Li, and other elements in weak G band stars, highlighting their unique abundance patterns and potential internal mechanisms.

Findings

Weak G band stars show strong CN-cycle signatures with low C and high N.

Some stars exhibit high Li abundances, indicating Li production via the Cameron-Fowler mechanism.

Na overabundance aligns with the NeNa chain activity in these stars.

Abstract

We determined the chemical composition of a large sample of weak G band stars -- a rare class of G and K giants of intermediate mass with unusual abundances of C, N, and Li. We have observed 24 weak G band stars with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory and derived spectroscopic abundances for C, N, O, and Li, as well as for selected elements from Na - Eu. The results show that the atmospheres of weak G band stars are highly contaminated with CN-cycle products. The C underabundance is about a factor of 20 larger than for normal giants and the $^{12}$C/$^{13}$C ratio approaches the CN-cycle equilibrium value. In addition to the striking CN-cycle signature the strong N overabundance may indicate the presence of partially ON-cycled material in the atmospheres of the weak G band stars. The exact mechanism responsible for the transport of the elements to the surface has yet to be identified but could be induced by rapid rotation of the main sequence progenitors of the stars. The unusually high Li abundances in some of the stars are an indicator for Li production by the Cameron-Fowler mechanism. A quantitative prediction of a weak G band star's Li abundance is complicated by the strong temperature sensitivity of the mechanism and its participants. In addition to the unusual abundances of CN-cycle elements and Li we find an overabundance of Na that is in accordance with the NeNa chain running in parallel with the CN-cycle. Apart from these peculiarities the element abundances in a weak G band star's atmosphere are consistent with those of normal giants.