The strongly irradiated planets in Praesepe

Abstract

We present an analysis of XMM–Newton observations of four stars in the young (670 Myr) open cluster Praesepe. The planets hosted by these stars all lie close in radius–period space to the radius–period valley and/or the Neptunian desert, two features that photoevaporation by X-ray and extreme ultraviolet (EUV) photons could be driving. Although the stars are no longer in the saturated regime, strong X-ray and extreme ultraviolet irradiation is still ongoing. Based on EUV time evolution slopes, we derived in a previous paper, in all four cases, two-thirds of their EUV irradiation is still to come. We compare the XMM–Newton light curves to those simultaneously measured with K2 at optical wavelengths, allowing us to search for correlated variability between the X-ray and optical light curves. We find that the X-ray flux decreases and flattens off while the optical flux rises throughout for K2-100, something that could result from active regions disappearing from view as the star spins. Finally, we also investigate possible futures for the four planets in our sample with simulations of their atmosphere evolution still to come, finding that complete photoevaporative stripping of the envelope of three of the four planets is possible, depending on the current planet masses.