Overluminous Blue Horizontal‐Branch Stars Formed by Low‐Mass Companions

We construct a speculative scenario for rotation-induced extra helium mixingto the envelope of horizontal branch (HB) stars. This scenario differs fromprevious ones in that the mixing occurs after the star has left the red giantbranch (RGB). We follow the evolution of a low metallicity star from the RGB tothe HB, and examine the density profile and radius in the core-envelopeboundary region. In the transition from the RGB to the HB the envelope shrinksby two orders of magnitude in size and the core swells, such that anynon-negligible rotation on the RGB will result in a strong rotational shear atthe core-envelope boundary. For a non-negligible rotation to exist on the RGBthe star has to be spun up by a companion spiraling inside its envelope (acommon envelope evolution). We speculate that shear instabilities on the HBmight mix helium-rich core material to the envelope. The shallow densityprofile on the HB is less likely to prevent mixing. As previously shown, extrahelium mixing can account for the overluminous blue HB stars found in someglobular clusters. Although being speculative, this study supports the ideathat the presence of low mass companions, from planets to low mass mainsequence stars, influence the evolution of stars, and can explain someproperties of the color-magnitude (Herzsprung-Russel) diagram of globularclusters. Namely, low mass companions can be an ingredient in the so called`second parameter' of globular clusters.Comment: ApJ, in pres