Connecting planets around horizontal branch stars with known exoplanets

Abstract

We study the distribution of exoplanets around main sequence (MS) stars and apply our results to the binary model for the formation of extreme horizontal branch (EHB; sdO; sdB; hot subdwarfs) stars. By Binary model we refer both to stellar and substellar companions that enhance the mass loss rate, where substellar companions stand for both massive planets and brown dwarfs. We conclude that sdB (EHB) stars are prime targets for planet searches. We reach this conclusion by noticing that the bimodal distribution of planets around stars with respect to the parameter M_p*a^2, is most prominent for stars in the mass range 1Mo < M < 1.5Mo; 'a' is the orbital separation, 'M' is the stellar mass and 'M_p' the planet mass. This is also the mass range of the progenitors of EHB stars that are formed through the interaction of their progenitors with planets (assuming the EHB formation mechanism is the binary model). In the binary model for the formation of EHB stars interaction with a binary companion or a substellar object (a planet or a brown dwarf), causes the progenitor to lose most of its envelope mass during its red giant branch (RGB) phase. As a result of that the descendant HB star is hot, i.e., an EHB (sdB) star. The bimodal distribution suggests that even if the close-in planet that formed the EHB star did not survive its RGB common envelope evolution, one planet or more might survive at a>1AU. Also, if a planet or more are observed at a>1AU, it is possible that a closer massive planet did survive the common envelope phase, and it is orbiting the EHB with an orbital period of hours to days.