TOI 560 : Two Transiting Planets Orbiting a K Dwarf Validated with iSHELL, PFS and HIRES RVs

We validate the presence of a two-planet system orbiting the 0.2--1.4 Gyr K4 dwarf TOI 560 (HD 73583). The system consists of an inner moderately eccentric transiting mini-Neptune (TOI 560 b, P = 6.397438 ± 0.000037 days, e = 0.294^(0.13)_(0.062)) initially discovered in the Sector 8 TESS mission observations, and a transiting mini-Neptune (TOI 560 c, P = 18.8779 ± 0.0016 days) discovered in the Sector 34 observations, in a rare 1:3 orbital resonance. We utilize photometric data from TESS Spitzer, and ground-based follow-up observations to confirm the ephemerides and period of the transiting planets and vet false positive scenarios. We obtain follow-up spectroscopy and corresponding precise radial velocities (RVs) with the iSHELL spectrograph at the NASA Infrared Telescope Facility and the HIRES Spectrograph at Keck Observatory to validate the planetary nature of these signals, which we combine with published PFS RVs from Magellan Observatory. We place upper limits on the masses of both planets of <2.1 and <4.1 M_(Nep) for b and c, respectively. We apply a Gaussian Processes (GP) model to the TESS light curves to place priors on a chromatic radial velocity GP model to constrain the stellar activity of the TOI 560 host star. TOI 560 is a nearby moderately young multi-planet system with two planets suitable for atmospheric characterization with James Webb Space Telescope (JWST) and other upcoming missions. In particular, it will undergo six transit pairs separated by <6 hours before June 2027.