The highly inflated giant planet WASP-174b^★

¹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: lmancini@roma2.infn.it
² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ INAF – Turin Astrophysical Observatory, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Department of Astrophysical Sciences, Princeton University, NJ 08544, USA
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁷ Center of Astro-Engineering UC, Pontificia Université Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁸ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁹ Millenium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁰ Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Av. Diagonal las Torres 2640, Peñalolén, Santiago, Chile
¹¹ Department of Physics, University of Texas at Dallas, Richardson, TX 75080, USA
¹² Las Cumbres Observatory Global Telescope, 6740 Cortona Dr., Suite 102, Goleta, CA 93111, USA
¹³ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹⁴ Astrochemistry Laboratory, Goddard Space Flight Center, NASA, 8800 Greenbelt Rd., Greenbelt, MD 20771, USA
¹⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
¹⁶ Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
¹⁷ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁸ Astrophysics Group, Keele University, Keele ST5 5BG, UK
¹⁹ Perth Exoplanet Survey Telescope, Perth, Australia

Received: 20 August 2019
Accepted: 18 September 2019

Abstract

Context. The transiting exoplanetary system WASP-174 was reported to be composed by a main-sequence F star (V = 11.8 mag) and a giant planet, WASP-174b (orbital period P_orb = 4.23 days). However only an upper limit was placed on the planet mass (<1.3 M_Jup), and a highly uncertain planetary radius (0.7−1.7 R_Jup) was determined.

Aims. We aim to better characterise both the star and the planet and precisely measure their orbital and physical parameters.

Methods. In order to constrain the mass of the planet, we obtained new measurements of the radial velocity of the star and joined them with those from the discovery paper. Photometric data from the HATSouth survey and new multi-band, high-quality (precision reached up to 0.37 mmag) photometric follow-up observations of transit events were acquired and analysed for getting accurate photometric parameters. We fit the model to all the observations, including data from the TESS space telescope, in two different modes: incorporating the stellar isochrones into the fit, and using an empirical method to get the stellar parameters. The two modes resulted to be consistent with each other to within 2σ.

Results. We confirm the grazing nature of the WASP-174b transits with a confidence level greater than 5σ, which is also corroborated by simultaneously observing the transit through four optical bands and noting how the transit depth changes due to the limb-darkening effect. We estimate that ≈76% of the disk of the planet actually eclipses the parent star at mid-transit of its transit events. We find that WASP-174b is a highly-inflated hot giant planet with a mass of M_p = 0.330 ± 0.091 M_Jup and a radius of R_p = 1.435 ± 0.050 R_Jup, and is therefore a good target for transmission-spectroscopy observations. With a density of ρ_p = 0.135 ± 0.042 g cm⁻³, it is amongst the lowest-density planets ever discovered with precisely measured mass and radius.