HATS-18 b: An Extreme Short--Period Massive Transiting Planet Spinning Up Its Star

TL;DR

This paper reports the discovery of HATS-18 b, a massive, ultra-short-period planet that significantly influences its host star's rotation, providing insights into star-planet tidal interactions and system evolution.

Contribution

It presents the first detailed analysis of a very short-period massive planet's impact on stellar spin and constrains the stellar tidal quality factor using observational data.

Findings

HATS-18 b has a mass of approximately 1.98 Mj and orbits in 0.8378 days.

The system shows evidence of stellar spin-up due to tidal interactions.

The tidal quality factor for Sun-like stars is constrained to 6.5 <= lg(Q*/k_2) <= 7.

Abstract

We report the discovery by the HATSouth network of HATS-18 b: a 1.980 +/- 0.077 Mj, 1.337 +0.102 -0.049 Rj planet in a 0.8378 day orbit, around a solar analog star (mass 1.037 +/- 0.047 Msun, and radius 1.020 +0.057 -0.031 Rsun) 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 6.5 <= lg(Q*/k_2) <= 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.