An eclipsing double-line spectroscopic binary at the stellar/substellar boundary in the Upper Scorpius OB association^⋆

¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
e-mail: nlodieu@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Department of Astronomy, University of California, Berkeley, CA 94720, USA
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁵ Astrobiology Center, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁶ SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, 240-0193 Kanagawa, Japan
⁷ Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Bunkyo-ku, 113-0033 Tokyo, Japan
⁸ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan

Received: 28 September 2015
Accepted: 9 November 2015

Abstract

Aims. We aim at constraining evolutionary models at low mass and young ages by identifying interesting transiting system members of the nearest OB association to the Sun, Upper Scorpius (USco), which has been targeted by the Kepler mission.

Methods. We produced light curves for M-dwarf members of the USco region that has been surveyed during the second campaign of the Kepler K2 mission. We identified by eye a transiting system, USco J161630.68−251220.1 (=EPIC 203710387) with a combined spectral type of M5.25, whose photometric, astrometric, and spectroscopic properties makes it a member of USco. We conducted an extensive photometric and spectroscopic follow-up of this transiting system with a suite of telescopes and instruments to characterise the properties of each component of the system.

Results. We calculated a transit duration of about 2.42 h that occurs every 2.88 days with a slight difference in transit depth and phase between the two components. We estimated a mass ratio of 0.922 ± 0.015 from the semi-amplitudes of the radial velocity curves for each component. We derived masses of 0.091 ± 0.005M_⊙ and 0.084 ± 0.004M_⊙, radii of 0.388 ± 0.008R_⊙ and 0.380 ± 0.008R_⊙, luminosities of log (L/L_⊙) = −2.020_-0.121^+0.099 dex and −2.032_-0.121^+0.099 dex, and effective temperatures of 2901_-172⁺¹⁹⁹ K and 2908_-172⁺¹⁹⁹ K for the primary and secondary, respectively.

Conclusions. We present a complete photometric and radial velocity characterisation of the least massive double-line eclipsing binary system in the young USco association with two components close to the stellar/substellar limit. This system falls in a gap between the least massive eclipsing binaries in the low-mass and substellar regimes at young ages and represents an important addition to constraining evolutionary models at young ages.