  Atmospheric mass loss plays a major role in the evolution of exoplanets. This
process is driven by the stellar high-energy irradiation, especially in the
first hundreds of millions of years after dissipation of the proto-planetary
disk. A major source of uncertainty in modeling atmospheric photo-evaporation
and photo-chemistry is due to the lack of direct measurements of the stellar
flux at EUV wavelengths. Several empirical relationships have been proposed in
the past to link EUV fluxes to emission levels in X-rays, but stellar samples
employed for this aim are heterogeneous, and available scaling laws provide
significantly different predictions, especially for very active stars. We
present new UV and X-ray observations of V1298 Tau with HST/COS and XMM-Newton,
aimed to determine more accurately the XUV emission of this solar-mass pre-Main
Sequence star, which hosts four exoplanets. Spectroscopic data were employed to
derive the plasma emission measure distribution vs.\ temperature, from the
chromosphere to the corona, and the possible variability of this irradiation on
short and year-long time scales, due to magnetic activity. As a side result, we
have also measured the chemical abundances of several elements in the outer
atmosphere of V1298 Tau. We employ our results as a new benchmark point for the
calibration of the X-ray to EUV scaling laws, and hence to predict the time
evolution of the irradiation in the EUV band, and its effect on the evaporation
of exo-atmospheres.
