The GAPS programme with HARPS-N at TNG

XI. Pr 0211 in M 44: the first multi-planet system in an open cluster^⋆,⋆⋆

¹ Dipartimento di Fisica e Astronomia Galileo Galilei – Università di Padova, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
e-mail: luca.malavolta@unipd.it
² INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Department of Physics & Astronomy – Georgia State University, Atlanta, GA 30303, USA
⁴ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁶ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁷ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁸ Fundación Galileo Galilei–INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁹ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ INAF–Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹¹ INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
¹² INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹³ INAF–Osservatorio Astronomico di Roma – Sede di Monteporzio Catone, via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹⁴ Instituto de Astrofísica de Canarias, Calle vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
¹⁶ Dipartimento di Fisica, Universitá di Torino, via P. Giuria 1, 10125 Torino, Italy
¹⁷ Landessternwarte Königstuhl, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany

Received: 9 December 2015
Accepted: 26 January 2016

Abstract

Context. Open cluster (OC) stars share the same age and metallicity, and, in general, their age and mass can be estimated with higher precision than for field stars. For this reason, OCs are considered an importantlaboratory to study the relation between the physical properties of the planets and those of their host stars, and the evolution of planetary systems. However, only a handful of planets have been discovered around OC main-sequence stars so far, all of them in single-planet systems. For this reason we started an observational campaign within the GAPS collaboration to search for and characterize planets in OCs

Aims. We monitored the Praesepe member Pr 0211 to improve our knowledge of the eccentricity of the hot Jupiter (HJ) that is already known to orbit this star and search for additional intermediate-mass planets. An eccentric orbit for the HJ would support a planet-planet scattering process rather than a disk-driven migration after its formation.

Methods. From 2012 to 2015, we collected 70 radial velocity (RV) measurements with HARPS-N and 36 with TRES of Pr 0211. Simultaneous photometric observations were carried out with the robotic STELLA telescope to characterize the stellar activity. We discovered a long-term trend in the RV residuals that we show as being due to the presence of a second, massive, outer planet. Orbital parameters for the two planets are derived by simultaneously fitting RVs and photometric light curves, with the activity signal modelled as a series of sinusoids at the rotational period of the star and its harmonics.

Results. We confirm that Pr 0211b has a nearly circular orbit (e = 0.02 ± 0.01), with an improvement of a factor two with respect to the previous determination of its eccentricity, and estimate that Pr 0211c has a mass M_p sin i = 7.9 ± 0.2 M_J, a period P> 3500 days and a very eccentric orbit (e> 0.60). This kind of peculiar system may be typical of open clusters if the planet-planet scattering phase, which lead to the formation of HJs, is caused by stellar encounters rather than by unstable primordial orbits. Pr 0211 is the first multi-planet system discovered around an OC star.