Sirius A: turbulence or mass loss?

Abstract

Context. Abundance anomalies observed in a fraction of A and B stars of both Pop I and II are apparently related to internal particle transport. Aims. Using available constraints from Sirius A, we wish to determine how well evolutionary models including atomic diffusion can explain observed abundance anomalies when either turbulence or mass loss is used as the main competitor to atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative accelerations, have been computed from the zero age main-sequence of 2.1M\odot stars for metallicities of Z0 = 0.01 \pm 0.001 and shown to agree with the observed parameters of Sirius A. Surface abundances were predicted for three values of the mass loss rate and for four values of the mixed surface zone. Results. A mixed mass of ~ 10^-6 M\odot or a mass loss rate of 10^-13 M\odot/yr were determined through comparison with observations. Of the 17 abundances determined observationally which are included in our calculations, up to 15 can be predicted within 2 sigmas and 3 of the 4 determined upper limits are compatible. Conclusions. While the abundance anomalies can be reproduced slightly better using turbulence as the process competing with atomic diffusion, mass loss probably ought to be preferred since the mass loss rate required to fit abundance anomalies is compatible with the observationally determined rate. A mass loss rate within a factor of 2 of 10^-13 M\odot/yr is preferred. This restricts the range of the directly observed mass loss rate.