HATS-37Ab and HATS-38b: Two Transiting Hot Neptunes in the Desert

We report the discovery of two transiting Neptunes by the HATSouth survey. The planet HATS-37Ab has a mass of $0.099\pm 0.042$ ${M}_{{\rm{J}}}$ (31.5 ± 13.4 M_⊕) and a radius of $0.606\pm 0.016$ ${R}_{{\rm{J}}}$ , and is on a $P=4.3315$ day orbit around a $V=12.266\pm 0.030$ mag, ${0.843}_{-0.012}^{+0.017}$ ${M}_{\odot }$ star with a radius of ${0.877}_{-0.012}^{+0.019}$ ${R}_{\odot }$ . We also present evidence that the star HATS-37A has an unresolved stellar companion HATS-37B, with a photometrically estimated mass of $0.654\pm 0.033$ ${M}_{\odot }$ . The planet HATS-38b has a mass of $0.074\pm 0.011$ ${M}_{{\rm{J}}}$ (23.5 ± 3.5 M_⊕) and a radius of $0.614\pm 0.017$ ${R}_{{\rm{J}}}$ , and is on a $P=4.3750$ day orbit around a $V=12.411\pm 0.030$ mag, ${0.890}_{-0.012}^{+0.016}$ ${M}_{\odot }$ star with a radius of $1.105\pm 0.016$ ${R}_{\odot }$ . Both systems appear to be old, with isochrone-based ages of ${11.46}_{-1.45}^{+0.79}$ Gyr, and $11.89\pm 0.60$ Gyr, respectively. Both HATS-37Ab and HATS-38b lie in the Neptune desert and are thus examples of a population with a low occurrence rate. They are also among the lowest-mass planets found from ground-based wide-field surveys to date. * The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and Universidad Adolfo Ibáñez (UAI). The station at Las Campanas Observatory of the Carnegie Institution for Science is operated by PU in conjunction with UAI, the station at the High Energy Spectroscopic Survey site is operated in conjunction with MPIA, and the station at Siding Spring Observatory (SSO) is operated jointly with ANU. This work is based in part on observations made with the MPG 2.2 m Telescope at the ESO Observatory in La Silla and based on observations collected at the European Southern Observatory under ESO programmes 094.C-0428(A), 095.C-0367(A), 097.C-0571(A), 098.C-0292(A), 099.C-0374(A), 0100.C-0406(A), 0100.C-0406(B). This paper includes data gathered with the 6.5 m Magellan Telescopes at Las Campanas Observatory, Chile, and is based in part on observations made with the Anglo-Australian Telescope operated by the Australian Astronomical Observatory.