The Origin of Extreme Horizontal Branch Stars

Strong mass loss on the red giant branch (RGB) can result in the formation of extreme horizontal branch (EHB) stars. The EHB stars spend most of their He core and shell-burning phase at high temperatures and produce copious ultraviolet flux. They have very small hydrogen envelopes and occupy a small range in mass. It has been suggested that the "ultraviolet excess phenomenon, seen in elliptical galaxies and spiral bulges, is largely due to the ultraviolet flux from EHB stars and their progeny. The main motivation behind this work is to understand how EHB stars can be produced over a wide range of metallicities without fine-tuning the mass-loss process.

We have computed evolutionary RGB models with mass loss for stars with main-sequence ages between 12.5 and 14.5 Gyr, initial masses less than 1.1 M_sun, and metallicities of [Fe/H] = - 2.26, -1.48, -0.78,0.0, and 0.37. We used the Reimers formula to characterize mass loss, but investigated a larger range of the mass-loss efficiency parameter, η_R than is common. We adopted values of η_R from 0 to 1.2 for the compositions considered. Sufficiently rapid RGB mass loss causes stars to "peel-off" the giant branch, evolve to high temperatures, and settle on the white dwarf cooling curve. Some such stars have adequate helium core mass to undergo the helium flash, but only later at high temperatures: we call these "hot He-flashers." After helium ignition, the hot-flashers lie at the very blue end of the horizontal branch, forming a "blue hook." The blue-hook stars are a part of the EHB population and evolve into AGB-manqué stars. The rest of the peel-off stars form helium white dwarfs, which we call "flashmanqué" stars. To understand how the number of EHB stars varies with metallicity in a stellar population we considered how the zero-age horizontal branch (ZAHB) is populated. We assumed the distribution of ZAHB stars to be driven by a distribution in the mass-loss efficiency (parameterized by η_R) rather than by a distribution in mass. Our results show that at low metallicity, EHB stars are produced if η_R is 2-3 times larger than the value producing normal "mid-HB stars." However, the range in η_R producing EHB stars is comparable to that producing mid-HB stars. Somewhat surprisingly, neither the range nor magnitude of η_R producing EHB stars varies much with metallicity. In contrast, the range of η_R producing mid-HB stars decreases with increasing metallicity. Hence, the HB of populations with solar metallicity and higher, such as those expected in elliptical galaxies and spiral bulges, will be bimodal if the distribution covers a sufficiently large range in η_R.