WASP-23b: a transiting hot Jupiter around a K dwarf and its Rossiter-McLaughlin effect

TL;DR

This paper reports the discovery and characterization of WASP-23b, a hot Jupiter around a K dwarf, including its transit, mass, radius, and Rossiter-McLaughlin effect, highlighting challenges in constraining spin-orbit alignment.

Contribution

First detailed analysis of WASP-23b's properties and Rossiter-McLaughlin effect, revealing complexities in stellar rotation measurements and spin-orbit alignment.

Findings

WASP-23b is a gas giant with 0.88 Mj and 0.96 Rj.

Detected Rossiter-McLaughlin effect during transit.

Uncertain spin-orbit alignment due to conflicting stellar rotation data.

Abstract

We report the discovery of a new transiting planet in the Southern Hemisphere. It has been found by the WASP-south transit survey and confirmed photometrically and spectroscopically by the 1.2m Swiss Euler telescope, LCOGT 2m Faulkes South Telescope, the 60 cm TRAPPIST telescope and the ESO 3.6m telescope. The orbital period of the planet is 2.94 days. We find it is a gas giant with a mass of 0.88 \pm 0.10 Mj and a radius estimated at 0.96 \pm 0.05 Rj . We have also obtained spectra during transit with the HARPS spectrograph and detect the Rossiter-McLaughlin effect despite its small amplitude. Because of the low signal to noise of the effect and of a small impact parameter we cannot place a constraint on the projected spin-orbit angle. We find two confiicting values for the stellar rotation. Our determination, via spectral line broadening gives v sin I = 2.2 \pm 0.3 km/s, while another method, based on the activity level using the index log R'HK, gives an equatorial rotation velocity of only v = 1.35 \pm 0.20 km/s. Using these as priors in our analysis, the planet could either be misaligned or aligned. This should send strong warnings regarding the use of such priors. There is no evidence for eccentricity nor of any radial velocity drift with time.