Overluminous Blue Horizontal Branch Stars Formed by Low Mass Companions

Abstract

We construct a speculative scenario for rotation-induced extra helium mixing to the envelope of horizontal branch (HB) stars. This scenario differs from previous ones in that the mixing occurs after the star has left the red giant branch (RGB). We follow the evolution of a low metallicity star from the RGB to the HB, and examine the density profile and radius in the core-envelope boundary region. In the transition from the RGB to the HB the envelope shrinks by two orders of magnitude in size and the core swells, such that any non-negligible rotation on the RGB will result in a strong rotational shear at the core-envelope boundary. For a non-negligible rotation to exist on the RGB the star has to be spun up by a companion spiraling inside its envelope (a common envelope evolution). We speculate that shear instabilities on the HB might mix helium-rich core material to the envelope. The shallow density profile on the HB is less likely to prevent mixing. As previously shown, extra helium mixing can account for the overluminous blue HB stars found in some globular clusters. Although being speculative, this study supports the idea that the presence of low mass companions, from planets to low mass main sequence stars, influence the evolution of stars, and can explain some properties of the color-magnitude (Herzsprung-Russel) diagram of globular clusters. Namely, low mass companions can be an ingredient in the so called `second parameter' of globular clusters.