The Origin of Hot Subdwarf Stars and the Far-UV Excess in Elliptical Galaxies

摘要

In this talk, we review the formation scenarios of hot subdwarf stars and their importance to the far-ultraviolet (far-UV) excess in elliptical galaxies. Hot sub dwarf stars play an important role in many aspects of astrophysics, e.g., stellar evo lution, distance indicators, Galactic structure, the long-standing problem of far-Uv excess in early-type galaxies. Han et al (2002, 2003) proposed a binary model for the formation of hot subdwarf stars and the model well reproduces field hot subdwarf stars. Han (2008) applied the model to globular clusters and found that the extreme horizontal branch stars can also be reproduced. By applying the model to the study of evolutionary population synthesis, Han et al. (2007) obtained an "a priori" model for the far-UV excess of elliptical galaxies, and showed that the far-UV excess is most likely resulted from 'oinary interactions, which has major implications for understand ing the evolution of the far-UV excess and elliptical galaxies in general. The far-UV excess model showed that the far-UV excess is expected to be universal and its magni tude does not depend much on metallicity or redshift. The far-UV excess model of Han et al. is supported by recent GALEX observations and was applied to naturally explain the colours of both dwarf ellipticals and giant ellipticals without the requirement of dichotomy between their stellar population properties.