Precise masses for the transiting planetary system HD 106315 with HARPS^⋆

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: susana.barros@astro.up.pt
² Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13013 Marseille, France
³ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁴ Observatoire Astronomique de l’Universite de Geneve, 51 chemin des Maillettes, 1290 Versoix, Switzerland
⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Institute for Astronomy, University of Hawaii, 34 Ohia Ku Street, Pukalani, Maui, HI 96790, USA
⁷ Depto. de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC campus 28692 Villanueva de la Cañada ( Madrid), Spain
⁸ Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
⁹ INAF–Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
¹⁰ McDonald Observatory and Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
¹¹ Centre for Exoplanet Science, SUPA School of Physics & Astronomy, University of St. Andrews, North Haugh St. Andrews, Fife, KY16 9SS, UK
¹² Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, 1053 Buenos Aires, Argentina
¹³ CONICET – Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), 1053 Buenos Aires, Argentina
¹⁴ Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹⁵ Millennium Institute of Astrophysics, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹⁶ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
¹⁷ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁸ Aix-Marseille Univ, CNRS, OHP, Observatoire de Haute Provence, 04870 Saint-Michel l’ Observatoire, France
¹⁹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
²⁰ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
²¹ Division of Geological and PlanetarySciences, California Institute of Technology, Pasadena, CA 91125, USA
²² Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0 HA Cambridge, UK
²³ Université de Toulouse, UPS-OMP, IRAP, 31013 Toulouse, France

Received: 29 May 2017
Accepted: 9 August 2017

Abstract

Context. The multi-planetary system HD 106315 was recently found in K2 data. The planets have periods of P_b ~ 9.55 and P_c ~ 21.06 days, and radii of r_b = 2.44 ± 0.17 R_⊕ and r_c = 4.35 ± 0.23 R_⊕ . The brightness of the host star (V = 9.0 mag) makes it an excellent target for transmission spectroscopy. However, to interpret transmission spectra it is crucial to measure the planetary masses.

Aims. We obtained high precision radial velocities for HD 106315 to determine the mass of the two transiting planets discovered with Kepler K2. Our successful observation strategy was carefully tailored to mitigate the effect of stellar variability.

Methods. We modelled the new radial velocity data together with the K2 transit photometry and a new ground-based partial transit of HD 106315c to derive system parameters.

Results. We estimate the mass of HD 106315b to be 12.6 ± 3.2 M_⊕ and the density to be 4.7 ± 1.7 g cm^-3, while for HD 106315c we estimate a mass of 15.2 ± 3.7 M_⊕ and a density of 1.01 ± 0.29 g cm^-3. Hence, despite planet c having a radius almost twice as large as planet b, their masses are consistent with one another.

Conclusions. We conclude that HD 106315c has a thick hydrogen-helium gaseous envelope. A detailed investigation of HD 106315b using a planetary interior model constrains the core mass fraction to be 5–29%, and the water mass fraction to be 10–50%. An alternative, not considered by our model, is that HD 106315b is composed of a large rocky core with a thick H–He envelope. Transmission spectroscopy of these planets will give insight into their atmospheric compositions and also help constrain their core compositions.