Hot horizontal branch stars: Predictions for mass loss

Winds, rotation, and the low gravity problem

¹ Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2BZ, UK
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ I.N.A.F. – Osservatorio Astronomico di Collurania, Via M. Maggini, 64100 Teramo, Italy

Corresponding author: J. S. Vink, jvink@ic.ac.uk

Received: 21 May 2002
Accepted: 17 June 2002

Abstract

We predict mass-loss rates for the late evolutionary phases of low-mass stars, with special emphasis on the consequences for the morphology of the Horizontal Branch (HB). We show that the computed rates, as predicted by the most plausible mechanism of radiation pressure on spectral lines, are too low to produce EHB/sdB stars. This invalidates the scenario recently outlined by Yong et al. (2000) to create these objects by mass loss on the HB. We argue, however, that mass loss plays a role in the distribution of rotational velocities of hot HB stars, and may – together with the enhancement of heavy element abundances due to radiative levitation – provide an explanation for the so-called “low gravity” problem. The mass loss recipe derived for hot HB (and extreme HB, sdB, sdOB) stars may also be applied to post-HB (AGB-manqué, UV-bright) stars over a range in effective temperatures between 12 500–40 000 K.