MuSCAT2 multicolour validation of TESS candidates: an ultra-short-period substellar object around an M dwarf

¹ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
e-mail: hannu@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Centro de Astrobiologia (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejon de Ardoz, Madrid, Spain
⁴ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁵ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Japan Science and Technology Agency, PRESTO, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan
⁹ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Abteilung Planetenforschung, Aachener Str. 209, 50931 Köln, Germany
¹⁰ European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹¹ Department of Physics, Kyoto Sangyo University, Kyoto, Japan
¹² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹³ Center for Astrophysics ∣ Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
¹⁵ American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138, USA
¹⁶ Astronomy and Physics, Wesleyan University, Middletown, CT, USA, MIT, Cambridge, MA, USA
¹⁷ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹⁸ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁹ SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
²⁰ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²¹ Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²² Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

Received: 25 May 2019
Accepted: 7 November 2019

Abstract

Context. We report the discovery of TOI 263.01 (TIC 120916706), a transiting substellar object (R = 0.87 R_Jup) orbiting a faint M3.5 V dwarf (V = 18.97) on a 0.56 d orbit.

Aims. We setout to determine the nature of the Transiting Exoplanet Survey Satellite (TESS) planet candidate TOI 263.01 using ground-based multicolour transit photometry. The host star is faint, which makes radial-velocity confirmation challenging, but the large transit depth makes the candidate suitable for validation through multicolour photometry.

Methods. Our analysis combines three transits observed simultaneously in r′, i′, and z_s bands usingthe MuSCAT2 multicolour imager, three LCOGT-observed transit light curves in g′, r′, and i′ bands, a TESS light curve from Sector 3, and a low-resolution spectrum for stellar characterisation observed with the ALFOSC spectrograph. We modelled the light curves with PYTRANSIT using a transit model that includes a physics-based light contamination component, allowing us to estimate the contamination from unresolved sources from the multicolour photometry. Using this information we were able to derive the true planet–star radius ratio marginalised over the contamination allowed by the photometry.Combining this with the stellar radius, we were able to make a reliable estimate of the absolute radius of the object.

Results. The ground-based photometry strongly excludes contamination from unresolved sources with a significant colour difference to TOI 263. Furthermore, contamination from sources of the same stellar type as the host is constrained to levels where the true radius ratio posterior has a median of 0.217 and a 99 percentile of0.286. The median and maximum radius ratios correspond to absolute planet radii of 0.87 and 1.41 R_Jup, respectively,which confirms the substellar nature of the planet candidate. The object is either a giant planetor a brown dwarf (BD) located deep inside the so-called “brown dwarf desert”. Both possibilities offer a challenge to current planet/BD formation models and make TOI 263.01 an object that merits in-depth follow-up studies.