WASP-166b: a bloated super-Neptune transiting a V  = 9 star

Abstract

<jats:title>Abstract</jats:title> <jats:p>We report the discovery of WASP-166b, a super-Neptune planet with a mass of 0.1 MJup (1.9 MNep) and a bloated radius of 0.63 RJup. It transits a V = 9.36, F9V star in a 5.44-d orbit that is aligned with the stellar rotation axis (sky-projected obliquity angle λ = 3 ± 5 deg). Variations in the radial-velocity measurements are likely the result of magnetic activity over a 12-d stellar rotation period. WASP-166b appears to be a rare object within the ‘Neptune desert’.</jats:p>

Simultaneous TESS and NGTS transit observations of WASP-166 b

“…We fitted the limb darkening coefficients during the modelling, therefore any differences in measured R p /R * values will not be due to any depth difference caused by the differences between the TESS and NGTS pass bands. Hellier et al (2019) measure a value of R p /R * = 0.0530 ± 0.0007, which is consistent with the value measured from the NGTS transit and the first TESS transit, but is discrepant with the value from the TESS transit on 2020 February 25. During Sector 8 of the TESS mission, a few days prior to the 2020 February 25 transit, an instrument anomaly caused the heaters to switch on.…”

“…However, Hellier et al (2019) were able to very tightly constrain the period due to the very long time baseline afforded to them by the combination of WASP and TESS photometry, as well as their precise measurement of the Rossiter-McLaughlin effect for WASP-166. Therefore we place a 1σ Gaussian prior on the period, based on the period and uncertainty reported in Hellier et al (2019). We also utilize the stellar parameters from version 8 of the TIC (Stassun et al 2018) to place a Gaussian prior on ρ * with a mean and standard deviation of 0.58 ± 0.11ρ .…”

“…In this paper, we present a study into the photometric precision and noise properties of the NGTS multitelescope observations for a bright star hosting a transiting planet. WASP-166 is a V = 9.351 ± 0.002 F-type dwarf star that hosts a 0.64 ± 0.03 R J transiting exoplanet that orbits the star on a period of 5.4435 d (Hellier et al 2019). A summary of the main properties of WASP-166 is set out in Table 1.…”

WASP-South hot Jupiters: WASP-178b, WASP-184b, WASP-185b, and WASP-192b

“…Our 24 CORALIE RVs trace out an eccentric orbit, though there is clearly additional scatter of unknown origin. This could be magnetic activity of the host star, though no rotational modulation is seen in the WASP data to a limit of 1 mmag (WASP-166 is an example of a system showing RV variation owing to magnetic activity, but no rotational modulation; Hellier et al 2019b). The planet's orbit has a relatively long period for a hot Jupiter, at 9.39 d, and has an eccentricity of e = 0.24 ± 0.04.…”

“…We apply the Stassun & Torres (2018) correction to the parallax to produce a distance estimate, and then use the Infra-Red Flux Method (Blackwell & Shallis 1977) to arrive at the stellar radius. Before the GAIA DR2, getting the stellar radius wrong was one of the commonest sources of systematic error in transit analyses, and thus a prior on the radius improves the reliability of the solution and can make up for limitations in the transit photometry (see, e.g., Hellier et al 2019b).…”