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Register a new userTwo close binaries across the hydrogen-burning limit in the Praesepe open cluster
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We present Keck I/OSIRIS and Keck II/NIRC2 adaptive optics imaging of two member candidates of the Praesepe stellar cluster (d=186.18$pm$0.11 pc; 590-790 Myr), UGC J08451066+2148171 (L1.5$pm$0.5) and UGCS J08301935$+$2003293 (no spectroscopic classification). We resolved UGCS J08451066$+$2148171 into a binary system in the near-infrared, with a $K$-band wavelength flux ratio of 0.89$pm$0.04, a projected separation of 60.3$pm$1.3 mas (11.2$pm$0.7 au; 1$sigma$). We also resolved UGCS J08301935$+$2003293 into a binary system with a flux ratio of 0.46$pm$0.03 and a separation of 62.5$pm$0.9 mas. Assuming zero eccentricity, we estimate minimum orbital periods of $sim$100 years for both systems. According to theoretical evolutionary models, we derive masses in the range of 0.074-0.078 M$_{odot}$ and 0.072-0.076 M$_{odot}$ for the primary and secondary of UGCS J08451066$+$2148171 for an age of 700$pm$100 Myr. In the case of UGCS J08301935$+$2003293, the primary is a low-mass star at the stellar/substellar boundary (0.070-0.078 M$_{odot}$) while the companion candidate might be a brown dwarf (0.051-0.065 M$_{odot}$). These are the first two binaries composed of L dwarfs in Praesepe. They are benchmark systems to derive the location of the substellar limit at the age and metallicity of Praesepe, determine the age of the cluster based on the lithium depletion boundary test, derive dynamical masses, and improve low-mass stellar and substellar evolutionary models at a well-known age and metallicity.
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We report the discovery of TOI 694 b and TIC 220568520 b, two low-mass stellar companions in eccentric orbits around metal-rich Sun-like stars, first detected by the Transiting Exoplanet Survey Satellite (TESS). TOI 694 b has an orbital period of 48.05131$pm$0.00019 days and eccentricity of 0.51946$pm$0.00081, and we derive a mass of 89.0$pm$5.3 $M_J$ (0.0849$pm$0.0051 $M_odot$) and radius of 1.111$pm$0.017 $R_J$ (0.1142$pm$0.0017 $R_odot$). TIC 220568520 b has an orbital period of 18.55769$pm$0.00039 days and eccentricity of 0.0964$pm$0.0032, and we derive a mass of 107.2$pm$5.2 $M_J$ (0.1023$pm$0.0050 $M_odot$) and radius of 1.248$pm$0.018 $R_J$ (0.1282$pm$0.0019 $R_odot$). Both binary companions lie close to and above the Hydrogen burning mass threshold that separates brown dwarfs and the lowest mass stars, with TOI 694 b being 2-$sigma$ above the canonical mass threshold of 0.075 $M_odot$. The relatively long periods of the systems mean that the magnetic fields of the low-mass companions are not expected to inhibit convection and inflate the radius, which according to one leading theory is common in similar objects residing in short-period tidally-synchronized binary systems. Indeed we do not find radius inflation for these two objects when compared to theoretical isochrones. These two new objects add to the short but growing list of low-mass stars with well-measured masses and radii, and highlight the potential of the TESS mission for detecting such rare objects orbiting bright stars.
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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 $mathrm{M_{Jup}}$, and radii between 0.81 and 1.66 $mathrm{R_{Jup}}$. These targets have masses near the uncertain lower limit of hydrogen core fusion ($sim$73-96 $mathrm{M_{Jup}}$), which separates brown dwarfs and low-mass stars. We constrained young ages for TOI-587 (0.2 $pm$ 0.1 Gyr) and TOI-681 (0.17 $pm$ 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 $pm$ 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.
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