Stellar Chromospheric Flux and the Surface Gravity of Close-in Planets

On the correlation between stellar chromospheric flux and the surface gravity of close-in planets

Author:

Lanza

Abstract:

The chromospheric emission of stars with close-by transiting planets has been found to correlate with the surface gravity of their planets. Stars with low-gravity planets have an average lower chromospheric flux. We propose that such a correlation is due to the absorption by circumstellar matter that comes from the evaporation of the planets. Planets with a lower gravity have a greater mass loss rate that leads to a higher column density of circumstellar absorption thus explaining the lower level of chromospheric emission observed in their host stars. We estimate the required column density and find that planetary evaporation can account for it. A theoretical relationship between the chromospheric emission as measured in the core of the Ca II H&K lines and the planet gravity is derived. We apply our relationship to a sample of transiting systems for which both the stellar Ca II H&K emission and the planetary surface gravity are known and find a good agreement, given the various sources of uncertainties and the intrinsic variability of the stellar emissions and planetary evaporation rates. We consider implications for the radial velocity jitter applied to fit the spectroscopic orbits and for the age estimates of planetary systems based on the chromospheric activity level of their host stars.