Can Planets Influence the Horizontal Branch Morphology?

Abstract

I examine the influence of planets on the location of stars on the Hertzsprung-Russel diagram as the stars turn to the horizontal branch. As stars that have planetary systems evolve along the red giant branch and expand, they interact with the close planets, with orbital separations of ≲5 AU. The planets deposit angular momentum and energy into the red giant stars' envelopes, both of which are likely to enhance mass loss on the red giant branch. The enhanced mass loss causes the star to become bluer as it turns to the horizontal branch. I propose that the presence of planetary systems, through this mechanism, can explain some anomalies in horizontal branch morphologies. In particular, planetary systems may be related to the "second parameter," which determines the distribution of horizontal branch stars on the Hertzsprung-Russel diagram. The presence of planets, though, cannot be the only factor that influences the "second parameter." The distribution of planets' properties (e.g., mass, orbital separation, prevalence) in a specific globular cluster depends on several properties of the globular cluster itself (e.g., shape, density). This dependence may explain some of the anomalies and variations in the horizontal branch morphologies between different globular clusters. The proposed scenario predicts that surviving massive planets or brown dwarfs orbit some of the extreme blue horizontal branch stars, at orbital periods of ∼ 10 days.