Hats-18B: An extreme short-period massive transiting planet spinning up its star

We report the discovery by the HATSouth network of HATS-18b: a 1.980+/- 0.077 M<SUB>J</SUB>, 1.337<SUB>-0.049</SUB><SUP>+0.102</SUP> R<SUB>J</SUB> planet in a 0.8378 day orbit, around a solar analog star (mass 1.037+/- 0.047 M<SUB>☉ </SUB> and radius 1.020<SUB>-0.031</SUB><SUP>+0.057</SUP> R<SUB>☉ </SUB>) with V=14.067+/- 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range of 6.5≲ log<SUB>10</SUB>(Q<SUP>* </SUP>/k<SUB>2</SUB>)≲ 7 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star-planet interactions, be they gravitational, magnetic, or radiative, as well as planet formation and migration theories. 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 the Pontificia Universidad Católica de Chile (PUC). The station at Las Campanas Observatory (LCO) of the Carnegie Institute is operated by PU in conjunction with PUC, the station at the High Energy Spectroscopic Survey (H.E.S.S.) site is operated in conjunction with MPIA, and the station at Siding Spring Observatory (SSO) is operated jointly with ANU. This paper includes data gathered with the MPG 2.2 m telescope at the ESO Observatory in La Silla. This paper uses observations obtained with facilities of the Las Cumbres Observatory Global Telescope.