The GAPS Programme with HARPS-N at TNG ^⋆,⋆⋆

III: The retrograde orbit of HAT-P-18b

¹ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 – La Laguna, Tenerife, Spain
e-mail: mesposito@iac.es
² Dep. de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
³ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Naples, Italy
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Fundación Galileo Galilei - INAF, Rambla José Ana Fernandez Pérez, 738712 Breña Baja, Tenerife, Spain
⁶ Astrophysics Group, Keele University, Keele ST5 5BG, UK
⁷ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁸ Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁹ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁰ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lecco), Italy
¹¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 90134 Palermo, Italy
¹³ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy
¹⁴ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy

Received: 28 February 2014
Accepted: 20 March 2014

Abstract

The measurement of the Rossiter-McLaughlin effect for transiting exoplanets places constraints on the orientation of the orbital axis with respect to the stellar spin axis, which can shed light on the mechanisms shaping the orbital configuration of planetary systems. Here we present the interesting case of the Saturn-mass planet HAT-P-18b, which orbits one of the coolest stars for which the Rossiter-McLaughlin effect has been measured so far. We acquired a spectroscopic time-series, spanning a full transit, with the HARPS-N spectrograph mounted at the TNG telescope. The very precise radial velocity measurements delivered by the HARPS-N pipeline were used to measure the Rossiter-McLaughlin effect. Complementary new photometric observations of another full transit were also analysed to obtain an independent determination of the star and planet parameters. We find that HAT-P-18b lies on a counter-rotating orbit, the sky-projected angle between the stellar spin axis and the planet orbital axis being λ = 132 ± 15 deg. By joint modelling of the radial velocity and photometric data we obtain new determinations of the star (M_⋆ = 0.770 ± 0.027 M_⊙; R_⋆ = 0.717 ± 0.026 R_⊙; VsinI_⋆ = 1.58 ± 0.18 km s^-1) and planet (M_p = 0.196 ± 0.008 M_J; R_p = 0.947 ± 0.044 R_J) parameters. Our spectra provide for the host star an effective temperature T_eff = 4870 ± 50 K, a surface gravity of log g_⋆ = 4.57 ± 0.07 cm s^-2, and an iron abundance of [Fe/H] = 0.10 ± 0.06. HAT-P-18b is one of the few planets known to transit a star with T_eff ≲ 6250 K on a retrograde orbit. Objects such as HAT-P-18b (low planet mass and/or relatively long orbital period) most likely have a weak tidal coupling with their parent stars, therefore their orbits preserve any original misalignment. As such, they are ideal targets to study the causes of orbital evolution in cool main-sequence stars.