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

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, due to the high activity of stars at early ages. In particular, the EUV part of the stellar spectra drives significant processes of photo-chemical interaction, but it is not directly measurable due to strong interstellar absorption and the lack of sufficiently sensitive instrumentation. An alternative approach is to derive synthetic spectra from the analysis of FUV and X-ray emission lines, that allow to estimate the missed flux in the EUV band. We performed joint and simultaneous spectroscopy of HIP 67522 with XMM-Newton and HST aimed at reconstructing the full high-energy spectrum of this 17 Myr old solar-type (G0) star, which is the youngest known transiting multi-planet system at present time. We performed a time-resolved spectral analysis of the observations, including quiescent emission and flaring variability. Then, we derived the Emission Measure Distribution (EMD) vs. temperature of the chromospheric and coronal plasma from the high-resolution spectra obtained in X-rays with RGS and in FUV with COS. We derived broad-band X-ray and EUV luminosities from the synthetic spectrum based on the EMD, that allowed us to test alternative EUV vs. X-ray scaling laws available in literature. We also employed the total XUV flux received by the inner planet of the system to estimate its instantaneous atmospheric mass loss rate. 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 have tested, indicating an important spread in the stellar properties, that requires further investigation.