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Fig. 2


Chromospheric emission index vs. the inverse of the surface planetary gravity g for our sample of close-in planets. The red dashed line is a linear regression computed by considering the 31 data points (filled circles) with M> 0.1MJ, a< 0.1 AU and stellar effective temperature 4200 <Teff< 6200 K. The red dot-dashed line is a linear regression computed with the entire set of 54 data points. Those not included in the restricted sample are indicated by open circles. Note that the point corresponding to WASP-17b (an open circle) is outside the scale of the plot, but it is included in the regression. The three-dots-dashed blue lines are the theoretical relationship (6) for the Sun (the line with the lower slope) and for the illustrative case of a star with twice the solar chromospheric and FEUV fluxes.

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