Giants eating giants: Mass loss and giant planets modifying the luminosity of the Tip of the Giant Branch

Abstract

During the red giant phase, stars loose mass at the highest rate since birth. The mass-loss rate is not fixed, but varies from star-to-star by up to 5\%, resulting in variations of the star's luminosity at the tip of the red giant branch (TRGB). Also, most stars, during this phase, engulf part of their planetary system, including their gas giant planets and possibly brown dwarfs. Gas giant planet masses range between 0.1 to 2\% of the host star mass. The engulfing of their gas giants planets can modify their luminosity at the TRGB, i.e. the point at which the He-core degeneracy is removed. We show that the increase in mass of the star by the engulfing of the gas giant planets only modifies the luminosity of a star at the TRGB by less than 0.1\%, while metallicity can modify the luminosity of a star at the TRGB by up to 0.5\%. However, the increase in turbulence of the convective envelope of the star, has a more dramatic effect, on the star's luminosity, which we estimate could be as large as 5\%. The effect is always in the direction to increase the turbulence and thus the mixing length which turns into a systematic decrease of the luminosity of the star at the TRGB. We find that the star-to-star variation of the mass-loss rate will dominate the variations in the luminosity of the TRGB with a contribution at the 5\% level. If the star-to-star variation is driven by environmental effects, the same effects can potentially create an environmentally-driven mean effect on the luminosity of the tip of the red giant branch of a galaxy. Engulfment of a brown dwarf will have a more dramatic effect. Finally, we touch upon how to infer the frequency, and identify the engulfment, of exoplanets in low-metallicity RGB stars through high resolution spectroscopy as well as how to quantify mass loss rate distributions from the morphology of the horizontal branch.