Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
e-mail: nolan.grieves@unige.ch
² Harvard University, Cambridge, MA 02138, USA
³ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd NE, Albuquerque, NM 87106, USA
⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Department of Astronomy, Wellesley College, Wellesley, MA 02481, USA
⁷ Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Av. Diagonal las Torres 2640, Peñalolén, Santiago, Chile
⁸ Millennium Institute for Astrophysics, Chile
⁹ Vanderbilt University, Department of Physics & Astronomy, 6301 Stevenson Center Lane, Nashville, TN 37235, USA
¹⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹¹ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹² Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08540, USA
¹³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
¹⁴ University of Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁵ School of Physics and Astronomy, University of Leicester, LE1 7RH, UK
¹⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁸ Institute for Advanced Study, Princeton, NJ 08540, USA
¹⁹ George Mason University, 4400 University Drive, Fairfax, VA, 22030 USA
²⁰ NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory (Code 667), Greenbelt, MD 20771, USA
²¹ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
²² School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
²³ El Sauce Observatory, Coquimbo Province, Chile
²⁴ Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD, 4350, Australia
²⁵ Dept. of Physics & Astronomy, Swarthmore College, Swarthmore PA 19081, USA
²⁶ Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA
²⁷ Concordia Station, IPEV/PNRA, Antarctica
²⁸ Ellinbank Observatory, Australia
²⁹ Brierfield Observatory, New South Wales, Australia
³⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
³¹ Royal Astronomical Society, Burlington House, Piccadilly, London W1J 0BQ, UK
³² Patashnick Voorheesville Observatory, Voorheesville, NY 12186, USA
³³ SETI Institute, Mountain View, CA 94043, USA
³⁴ Hazelwood Observatory, Australia
³⁵ Perth Exoplanet Survey Telescope, Perth, Western Australia
³⁶ Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA
³⁷ Mt. Stuart Observatory, New Zealand

Received: 21 April 2021
Accepted: 5 July 2021

Abstract

We report the discovery of five transiting companions near the hydrogen-burning mass limit in close orbits around main sequence stars originally identified by the Transiting Exoplanet Survey Satellite (TESS) as TESS objects of interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and TOI-1213. Using TESS and ground-based photometry as well as radial velocities from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the companions have orbital periods between 4.8 and 27.2 days, masses between 77 and 98 M_Jup , and radii between 0.81 and 1.66 R_Jup . These targets have masses near the uncertain lower limit of hydrogen core fusion (~73-96 M_Jup ), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 ± 0.1 Gyr) and TOI-681 (0.17 ± 0.03 Gyr) and found them to have relatively larger radii compared to other transiting companions of a similar mass. Conversely we estimated older ages for TOI-148 and TOI-746 and found them to have relatively smaller companion radii. With an effective temperature of 9800 ± 200 K, TOI-587 is the hottest known main-sequence star to host a transiting brown dwarf or very low-mass star. We found evidence of spin-orbit synchronization for TOI-148 and TOI-746 as well as tidal circularization for TOI-148. These companions add to the population of brown dwarfs and very low-mass stars with well measured parameters ideal to test formation models of these rare objects, the origin of the brown dwarf desert, and the distinction between brown dwarfs and hydrogen-burning main sequence stars.