Weak G-band stars on the H-R diagram: clues to the origin of the Li anomaly

Aims. Weak G-band (WGB) stars are a rare class of cool luminous stars that present a strong depletion in carbon, but also lithium abundance anomalies that have been little explored in the literature since the first discovery of these peculiar objects in the early 1950s. Here we focus on the Li-rich WGB stars and report on their evolutionary status. We explored different paths to propose a tentative explanation for the lithium anomaly. Methods. Using archive data, we derived the fundamental parameters of WGB (T-eff, log g, log(L/L-circle dot)) using Hipparcos parallaxes and recent temperature scales. From the equivalent widths of Li resonance line at 6707 angstrom, we uniformly derived the lithium abundances and applied NLTE corrections when possible following the procedure described by Lind et al. (2009). We also computed dedicated stellar evolution models in the mass range 3.0 to 4.5 M-circle dot, exploring the effects of rotation-induced and thermohaline mixing. These models are used to locate the WGB stars in the H-R diagram and to explore the origin of the abundance anomalies. Results. The location of WGB stars in the Hertzsprung-Russell diagram shows that these are intermediate mass stars of masses ranging from 3.0 to 4.5 M-circle dot located at the clump, which implies a degeneracy of their evolutionary status between subgiant/red giant branch and core helium burning phases. The atmospheres of a large proportion of WGB stars (more than 50%) exhibit lithium abundances A(Li) >= 1.4 dex similar to Li-rich K giants. However, the position of WGB stars along with the Li-rich K giants in the H-R diagram, indicates that both are well-separated groups. The combined and tentatively consistent analysis of the abundance pattern for lithium, carbon, and nitrogen of WGB stars seems to indicate that carbon underabundance could be decorrelated from the lithium and nitrogen overabundances.