The GAPS Programme with HARPS-N at TNG

XIII. The orbital obliquity of three close-in massive planets hosted by dwarf K-type stars: WASP-43, HAT-P-20 and Qatar-2^⋆,⋆⋆

¹ INAF–Osservatorio Astronomico di Capodimonte, via Moiariello, 16, 80131 Naples, Italy
e-mail: mesposito@na.astro.it
² INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
⁶ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025, Pino Torinese, Italy
⁷ Instituto de Astrofísica de Canarias, C/ vía Láctea, s/n, 38205 La Laguna (Tenerife), Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, s/n, 38206 La Laguna (TF), Spain
⁹ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹⁰ Dipartimento di Fisica e Astronomia Galileo Galilei – Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
¹¹ INAF–Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
¹² INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ INAF–Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁴ INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
¹⁵ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
¹⁶ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy

Received: 14 September 2016
Accepted: 8 February 2017

Abstract

Context. The orbital obliquity of planets with respect to the rotational axis of their host stars is a relevant parameter for the characterization of the global architecture of planetary systems and a key observational constraint to discriminate between different scenarios proposed to explain the existence of close-in giant planets.

Aims. In the framework of the GAPS project, we conduct an observational programme aimed at determinating the orbital obliquity of known transiting exoplanets. The targets are selected to probe the obliquity against a wide range of stellar and planetary physical parameters.

Methods. We exploit high-precision radial velocity (RV) measurements, delivered by the HARPS-N spectrograph at the 3.6 m Telescopio Nazionale Galileo, to measure the Rossiter-McLaughlin (RM) effect in RV time-series bracketing planet transits, and to refine the orbital parameters determinations with out-of-transit RV data. We also analyse new transit light curves obtained with several 1−2 m class telescopes to better constrain the physical fundamental parameters of the planets and parent stars.

Results. We report here on new transit spectroscopic observations for three very massive close-in giant planets: WASP-43 b, HAT-P-20 b and Qatar-2 b (M_p = 2.00, 7.22, 2.62 M_J; a = 0.015, 0.036, 0.022 AU, respectively) orbiting dwarf K-type stars with effective temperature well below 5000 K (T_eff = 4500 ± 100, 4595 ± 45, 4640 ± 65 K respectively). These are the coolest stars (except for WASP-80) for which the RM effect has been observed so far. We find λ = 3.5 ± 6.8 deg for WASP-43 b and λ = −8.0 ± 6.9 deg for HAT-P-20 b, while for Qatar-2, our faintest target, the RM effect is only marginally detected, though our best-fit value λ = 15 ± 20 deg is in agreement with a previous determination. In combination with stellar rotational periods derived photometrically, we estimate the true spin-orbit angle, finding that WASP-43 b is aligned while the orbit of HAT-P-20 b presents a small but significant obliquity (Ψ = 36_-12⁺¹⁰ deg). By analyzing the CaII H&K chromospheric emission lines for HAT-P-20 and WASP-43, we find evidence for an enhanced level of stellar activity that is possibly induced by star-planet interactions.