The young HD 73583 (TOI-560) planetary system: Two 10-M${}_{\oplus }$ mini-Neptunes transiting a 500-Myr-old, bright, and active K dwarf

Abstract

We present the discovery and characterisation of two transiting planets observed by \textit{TESS} in the light curves of the young and bright (V=9.67) star HD73583 (TOI-560). We perform an intensive spectroscopic and photometric space- and ground-based follow-up in order to confirm and characterise the system. We found that HD73583 is a young ($\sim 500$~Myr) active star with a rotational period of $12.08\pm 0.11$\,d, and a mass and radius of $0.73\pm 0.02M_{\odot }$ and $0.65\pm 0.02R_{\odot }$, respectively. HD73583 b ($P_b=6.3980420_{-0.0000062}^{+0.0000067}$ d) has a mass and radius of $10.2_{-3.1}^{+3.4}{M}_{\oplus }$ and$2.79\pm 0.10R_{\oplus }$, respectively, that gives a density of $2.58_{-0.81}^{0.95}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. HD73583 c ($P_c=18.87974_{-0.00074}^{+0.00086}$) has a mass and radius of $9.7_{-1.7}^{+1.8}{M}_{\oplus }$ and $2.39_{-0.09}^{+0.10}{R}_{\oplus }$, respectively, this translates to a density of $3.88_{-0.80}^{+0.91}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$. Both planets are consistent with worlds made of a solid core surrounded by a volatile envelope. Because of their youth and host star brightness, they both are excellent candidates to perform transmission spectroscopy studies. We expect ongoing atmospheric mass-loss for both planets caused by stellar irradiation. We estimate that the detection of evaporating signatures on H and He would be challenging, but doable with present and future instruments.

Accepted for publication in MNRAS