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

TL;DR

This paper investigates the correlation between stellar chromospheric flux and the surface gravity of close-in transiting planets, proposing planetary evaporation and circumstellar absorption as key factors influencing observed stellar activity levels.

Contribution

It introduces a theoretical model linking chromospheric emission to planetary gravity and demonstrates its consistency with observational data.

Findings

Lower gravity planets induce higher circumstellar absorption.

Planetary evaporation can explain the observed flux reduction.

The model aligns well with measured stellar activity and planet surface gravity.

Abstract

The chromospheric emission of stars with close-in transiting planets has been found to correlate with the surface gravity of their planets. Stars with low-gravity planets have on average a lower chromospheric flux. We propose that this 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 which leads to a higher column density of circumstellar absorption and this in turn explains the lower level of chromospheric emission observed in their host stars. We estimated the required column density and found that planetary evaporation can account for it. We derived a theoretical relationship between the chromospheric emission as measured in the core of the Ca II H&K lines and the planet gravity. We applied this 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 found 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.