F stars: A challenge to stellar evolution

TL;DR

This paper investigates the overluminosity of certain F stars, revealing that their unusual properties challenge standard stellar evolution models and suggesting nonuniform interior rotation as a key factor.

Contribution

It identifies the limitations of current models in explaining overluminous F stars and proposes nonuniform interior rotation as a potential explanation.

Findings

Overluminous F stars have different age-velocity relations from normal stars.

X-ray luminosity of overluminous F stars increases with age, opposite to normal stars.

Standard models fail to accurately predict the properties of overluminous F stars.

Abstract

Many main-sequence F and early G stars are too luminous for their effective temperature, surface gravity, and chemical composition. These {\it overluminous stars} have two curious properties. First, their kinematics as a function of age from stellar evolution modeling (isochrone fitting) is very different from that of normal stars. Second, while X-ray luminosity of normal stars declines with age, the X-ray luminosity of overluminous F stars changes in the opposite direction, being on average higher for older stars. These properties imply that, in defiance of standard models of stellar evolution, F stars of a given mass and chemical composition can evolve very differently. Assuming that the models correctly describe normal stars, for overluminous F stars they predict too young age and the X-ray emission evolving in the direction opposite to the actually observed trend. This discrepancy between modeling results and observational data suggests that standard stellar evolution models and models of stellar activity are missing some important factors, which makes stellar age and predictions for stellar activity from these models problematic. The data and literature analysis presented in this paper point to a nonuniform rotation of the stellar interior as a plausible key factor able to reconcile the divergent trends in age-velocity relationships of normal and overluminous F stars and explain in a coherent and self-consistent way the overluminosity phenomenon.