XUV irradiation of young planetary atmospheres. Results from a joint XMM-Newton and HST observation of HIP67522

Context. The evaporation and the chemistry of the atmospheres of warm and hot planets are strongly determined by the high-energy irradiation they receive from their parent stars. This is more crucial among young extra-solar systems because of the high activity of stars at early ages. In particular, the extreme-ultraviolet (EUV) part of the stellar spectrum drives significant processes of photochemical interaction, but it is not directly measurable because of strong interstellar absorption and a lack of sufficiently sensitive instrumentation. An alternative approach is to derive synthetic spectra from the analysis of far-ultraviolet (FUV) and X-ray emission lines, which allow us to estimate the missed flux in the EUV band. Aims. We performed joint and simultaneous spectroscopy of HIP 67522 with XMM-Newton and the Hubble Space Telescope in order to reconstruct the full high-energy spectrum of this 17 Myr-old solar-Type (G0) star, which is the youngest transiting multiplanet system known to date. Methods. We performed a time-resolved spectral analysis of the observations, including quiescent emission and flaring variability. We then derived the emission measure distribution (EMD) versus temperature of the chromospheric and coronal plasma from the high-resolution spectra obtained in X-rays with RGS and in FUV with COS. Results. We derived broad-band X-ray and EUV luminosities from the synthetic spectrum based on the EMD, which allowed us to test alternative EUV versus X-ray scaling laws available in the literature. We also employed the total X-EUV flux received by the inner planet of the system to estimate its instantaneous atmospheric mass-loss rate. Conclusions. We confirm that HIP 67522 is a very active star with a hot corona, reaching plasma temperatures above 20 MK even in quiescent state. Its EUV/X-ray flux ratio falls in between the predictions of the two scaling laws we tested, indicating an important spread in the stellar properties, which requires further investigation.