AmFm and lithium gap stars: Stellar evolution models with mass loss

TL;DR

This study models the effects of mass loss on A and F stars, comparing predictions with observations to understand surface and internal abundances, and explores implications for stellar phenomena like the Hyades lithium gap.

Contribution

It introduces self-consistent stellar evolution models including atomic diffusion and mass loss, constraining mass loss rates that match observed star abundances.

Findings

Mass loss rates between 5x10^-14 and 10^-13 M_sun/yr fit many observed stars.

Mass loss reduces surface abundance anomalies similarly to turbulent mixing.

Models cannot fully explain the Hyades lithium gap without separated winds.

Abstract

A thorough study of the effects of mass loss on internal and surface abundances of A and F stars is carried out in order to constrain mass loss rates for these stars, as well as further elucidate some of the processes which compete with atomic diffusion. Self-consistent stellar evolution models of 1.3 to 2.5 M_sun stars including atomic diffusion and radiative accelerations for all species within the OPAL opacity database were computed with mass loss and compared to observations as well as previous calculations with turbulent mixing. Models with unseparated mass loss rates between 5 x 10^-14 and 10^-13 M_sun/yr reproduce observations for many cluster AmFm stars as well as Sirius A and o Leonis. These models also explain cool Fm stars, but not the Hyades lithium gap. Like turbulent mixing, these mass loss rates reduce surface abundance anomalies; however, their effects are very different with respect to internal abundances. For most of the main sequence lifetime of an A or F star, surface abundances in the presence of such mass loss depend on separation which takes place between log(Delta M/M_star)= -6 and -5. The current observational constraints do not allow us to conclude that mass loss is to be preferred over turbulent mixing (induced by rotation or otherwise) in order to explain the AmFm phenomenon. Internal concentration variations which could be detectable through asteroseismic tests should provide further information. If atomic diffusion coupled with mass loss are to explain the Hyades Li gap, the wind would need to be separated.