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The chromospheric activity index logR'HK of stars hosting transiting hotJupiters appears to be correlated with the planets' surface gravity. One of thepossible explanations is based on the presence of condensations of planetaryevaporated material located in a circumstellar cloud that absorbs the CaII H&Kand MgII h&k resonance line emission flux, used to measure chromosphericactivity. A larger column density in the condensations, or equivalently astronger absorption in the chromospheric lines, is obtained when theevaporation rate of the planet is larger, which occurs for a lower gravity ofthe planet. We analyze here a sample of stars hosting transiting hot Jupiterstuned in order to minimize systematic effects (e.g., interstellar mediumabsorption). Using a mixture model, we find that the data are best fit by atwo-linear-regression model. We interpret this result in terms of theVaughan-Preston gap. We use a Monte Carlo approach to best take into accountthe uncertainties, finding that the two intercepts fit the observed peaks ofthe distribution of logR'HK for main-sequence solar-like stars. We also findthat the intercepts are correlated with the slopes, as predicted by the modelbased on the condensations of planetary evaporated material. Our findings bringfurther support to this model, although we cannot firmly exclude differentexplanations. A precise determination of the slopes of the two linearcomponents would allow one to estimate the average effective stellar fluxpowering planetary evaporation, which can then be used for theoreticalpopulation and evolution studies of close-in planets.

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