A bimodal correlation between host star chromospheric emission and the surface gravity of hot-Jupiters

The chromospheric activity index log RH¢K of stars hosting transiting hot Jupiters appears to be correlated with the planets’ surface gravity. One of the possible explanations is based on the presence of condensations of planetaryevaporated material located in a circumstellar cloud that absorbs the Ca II H&K and Mg II h&k resonance lineemission flux, used to measure chromospheric activity. A larger column density in the condensations, orequivalently a stronger absorption in the chromospheric lines, is obtained when the evaporation rate of the planet is larger, which occurs for a lower gravity of the planet. We analyze here a sample of stars hosting transiting hotJupiters tuned in order to minimize systematic effects (e.g., interstellar medium absorption). Using a mixturemodel, we find that the data are best fit by a two-linear-regression model. We interpret this result in terms of theVaughan–Preston gap. We use a Monte Carlo approach to best take into account the uncertainties, finding that the two intercepts fit the observed peaks of the distribution of log RH¢K for main-sequence solar-like stars. We also find that the intercepts are correlated with the slopes, as predicted by the model based on the condensations of planetary evaporated material. Our findings bring further support to this model, although we cannot firmly exclude differentexplanations. A precise determination of the slopes of the two linear components would allow one to estimate the average effective stellar flux powering planetary evaporation, which can then be used for theoretical population and evolution studies of close-in planets.