The GAPS Programme at TNG. XLII. A characterisation study of the multi-planet system around the 400 Myr-old star HD63433 (TOI-1726)

For more than two years, we monitored with the HARPS-N spectrograph 400 Myr-old star HD\,63433, which hosts close-in (orbital periods $P_b\sim7.1$ and $P_c\sim20.5$ days) sub-Neptunes detected by TESS space telescope, it was announced in 2020. Using radial velocities additional photometry, aim to provide first measurement of their masses, improve measure size orbital parameters, study evolution atmospheric mass-loss rate due photoevaporation. We tested state-of-the-art analysis techniques different models mitigate dominant signals stellar activity that are velocity time series. used a hydro-based analytical description rate, coupled core-envelope model evolutionary tracks, past future planetary masses radii. derived new measurements radii ($P_b=7.10794\pm0.000009$ d, $r_b=2.02^{+0.06}_{-0.05}$ $R_{\oplus}$; $P_c=20.54379\pm0.00002$ $r_c=2.44\pm0.07$ $R_{\oplus}$), determined mass upper limits ($m_b\lesssim$11 $M_{\oplus}$; $m_c\lesssim$31 95$\%$ confidence level), evidence at 2.1--2.7$\sigma$ significance level HD\,63433\,c might be dense mini-Neptune Neptune-like mass. grid test below our dynamical limits, found HD\,63433\,b has very likely lost any gaseous H-He envelope, supporting HST-based observations indicative there being no ongoing evaporation. will keep evaporating over next $\sim$5 Gyr if its current is $m_c\lesssim$15 $M_{\oplus}$, while should hydrodynamically stable for higher masses.