Hydrogen-burning Mass Limit Research Articles

Three low-mass companions around aged stars discovered by TESS

ABSTRACT We report the discovery of three transiting low-mass companions to aged stars: a brown dwarf (TOI-2336b) and two objects near the hydrogen burning mass limit (TOI-1608b and TOI-2521b). These three systems were first identified using data from the Transiting Exoplanet Survey Satellite (TESS). TOI-2336b has a radius of 1.05 ± 0.04 RJ, a mass of 69.9 ± 2.3 MJ and an orbital period of 7.71 d. TOI-1608b has a radius of 1.21 ± 0.06 RJ, a mass of 90.7 ± 3.7 MJ and an orbital period of 2.47 d. TOI-2521b has a radius of 1.01 ± 0.04 RJ, a mass of 77.5 ± 3.3 MJ, and an orbital period of 5.56 d. We found all these low-mass companions are inflated. We fitted a relation between radius, mass, and incident flux using the sample of known transiting brown dwarfs and low-mass M dwarfs. We found a positive correlation between the flux and the radius for brown dwarfs and for low-mass stars that is weaker than the correlation observed for giant planets. We also found that TOI-1608 and TOI-2521 are very likely to be spin-orbit synchronized, leading to the unusually rapid rotation of the primary stars considering their evolutionary stages. Our estimates indicate that both systems have much shorter spin-orbit synchronization time-scales compared to their ages. These systems provide valuable insights into the evolution of stellar systems with brown dwarf and low-mass stellar companions influenced by tidal effects.

TOI-5375 B: A Very Low Mass Star at the Hydrogen-burning Limit Orbiting an Early M-type Star* *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby

The Transiting Exoplanet Survey Satellite (TESS) mission detected a companion orbiting TIC 71268730, categorized it as a planet candidate, and designated the system TOI-5375. Our follow-up analysis using radial-velocity data from the Habitable-zone Planet Finder, photometric data from Red Buttes Observatory, and speckle imaging with NN-EXPLORE Exoplanet Stellar Speckle Imager determined that the companion is a very low mass star near the hydrogen-burning mass limit with a mass of 0.080 ± 0.002M ☉ (83.81 ± 2.10M J ), a radius of (1.0841), and brightness temperature of 2600 ± 70 K. This object orbits with a period of 1.721553 ± 0.000001 days around an early M dwarf star (0.62 ± 0.016M ☉). TESS photometry shows regular variations in the host star’s TESS light curve, which we interpreted as an activity-induced variation of ∼2%, and used this variability to measure the host star’s stellar rotation period of days. The TOI-5375 system provides tight constraints on stellar models of low-mass stars at the hydrogen-burning limit and adds to the population in this important region.

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

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 MJup , and radii between 0.81 and 1.66 RJup . These targets have masses near the uncertain lower limit of hydrogen core fusion (~73-96 MJup ), 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.

TOI 694b and TIC 220568520b: Two Low-mass Companions near the Hydrogen-burning Mass Limit Orbiting Sun-like Stars

TOI 694b and TIC 220568520b: Two Low-mass Companions near the Hydrogen-burning Mass Limit Orbiting Sun-like Stars

TOI 694b and TIC 220568520b: Two Low-mass Companions near the Hydrogen-burning Mass Limit Orbiting Sun-like Stars

Abstract 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 ± 0.00019 days and eccentricity of 0.51946 ± 0.00081, and we derive a mass of 89.0 ± 5.3 (0.0849 ± 0.0051 ) and radius of 1.111 ± 0.017 (0.1142 ± 0.0017 ). TIC 220568520 b has an orbital period of 18.55769 ± 0.00039 days and eccentricity of 0.0964 ± 0.0032, and we derive a mass of 107.2 ± 5.2 (0.1023 ± 0.0050 ) and radius of 1.248 ± 0.018 (0.1282 ± 0.0019 ). 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 2σ above the canonical mass threshold of 0.075 . 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.

The EBLM Project

Measurements of the physical properties of stars at the lower end of the main sequence are scarce. In this context we report masses, radii and surface gravities of ten very-low-mass stars in eclipsing binary systems, with orbital periods of the order of several days. The objects probe the stellar mass-radius relation in the fully convective regime, M⋆ ≲ 0.35 M⊙, down to the hydrogen burning mass-limit, MHB ∼ 0.07 M⊙. The stars were detected by the WASP survey for transiting extra-solar planets, as low-mass, eclipsing companions orbiting more massive, F- and G-type host stars. We use eclipse observations of the host stars, performed with the TRAPPIST, Leonhard Euler and SPECULOOS telescopes, and radial velocities of the host stars obtained with the CORALIE spectrograph, to determine the physical properties of the low-mass companions. Surface gravities of the low-mass companions are derived from the eclipse and orbital parameters of each system. Spectroscopic measurements of the host star effective temperature and metallicity are used to infer the host star mass and age from stellar evolution models for solar-type stars. Masses and radii of the low-mass companions are then derived from the eclipse and orbital parameters of the binary systems. The objects are compared to stellar evolution models for low-mass stars, to test for an effect of the stellar metallicity and orbital period on the radius of low-mass stars in close binary systems. Measurements are found to be in good agreement with stellar evolution models; a systematic inflation of the radius of low-mass stars with respect to model predictions is limited to 1.6 ± 1.2%, in the fully convective low-mass regime. The sample of ten objects indicates a scaling of the radius of low-mass stars with the host star metallicity. No correlation between stellar radii and the orbital periods of the binary systems is determined. A combined analysis with thirteen comparable objects from the literature is consistent with this result.

Kepler-503b: An Object at the Hydrogen Burning Mass Limit Orbiting a Subgiant Star

Abstract Using spectroscopic radial velocities with the Apache Point Observatory Galaxy Evolution Experiment (APOGEE) instrument and Gaia distance estimates, we demonstrate that Kepler-503b, currently considered a validated Kepler planet, is in fact a brown-dwarf/low-mass star in a nearly circular 7.2-day orbit around a subgiant star. Using a mass estimate for the primary star derived from stellar models, we derive a companion mass and radius of 0.075 ± 0.003 M ⊙ (78.6 ± 3.1 M Jup) and ( R Jup), respectively. Assuming that the system is coeval, the evolutionary state of the primary indicates the age is ∼6.7 Gyr. Kepler-503b sits right at the hydrogen-burning mass limit, straddling the boundary between brown dwarfs and very low-mass stars. More precise radial velocities and secondary eclipse spectroscopy with the James Webb Space Telescope (JWST) will provide improved measurements of the physical parameters and age of this important system to better constrain and understand the physics of these objects and their spectra. This system emphasizes the value of radial velocity observations to distinguish a genuine planet from astrophysical false positives, and is the first result from the Sloan Digital Sky Survey (SDSS)-IV monitoring of Kepler planet candidates with the multi-object APOGEE instrument.

The Pan-Pacific Planet Search III: five companions orbiting giant stars

We report a new giant planet orbiting the K giant HD 155233, as well as four stellar-mass companions from the Pan-Pacific Planet Search, a southern hemisphere radial velocity survey for planets orbiting nearby giants and subgiants. We also present updated velocities and a refined orbit for HD 47205b (7 CMa b), the first planet discovered by this survey. HD 155233b has a period of 885$\pm$63 days, eccentricity e=0.03$\pm$0.20, and m sin i=2.0$\pm$0.5 M_jup. The stellar-mass companions range in m sin i from 0.066 M_sun to 0.33 M_sun. Whilst HD 104358B falls slightly below the traditional 0.08 M_sun hydrogen-burning mass limit, and is hence a brown dwarf candidate, we estimate only a 50% a priori probability of a truly substellar mass.

A new L dwarf member of the moderately metal poor triple system HD 221356

We report on the discovery of a fourth component in the HD 221356 star system, previously known to be formed by an F8V, slightly metal-poor primary ([Fe/H]=-0.26), and a distant M8V+L3V pair. In our ongoing common proper motion search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have detected a faint (J=13.76+/-0.04 mag) co-moving companion of the F8 star located at angular separation of 12.13+/-0.18 arcsec (position angle of 221.8+/-1.7), corresponding to a projected distance of ~312 AU at 26 pc. Near-infrared spectroscopy of the new companion, covering the 1.5-2.4 micron wavelength range with a resolving power of R~600, indicates an L1+/-1 spectral type. Using evolutionary models the mass of the new companion is estimated at ~0.08 solar masses, which places the object close to the stellar-substellar borderline. This multiple system provides an interesting example of objects with masses slightly above and below the hydrogen burning mass limit. The low mass companions of HD 221356 have slightly bluer colours than field dwarfs with similar spectral type, which is likely a consequence of the sub-solar metallicity of the system.

DISCOVERY OF A VERY LOW MASS TRIPLE WITH LATE-M AND T DWARF COMPONENTS: LP 704-48/SDSS J0006–0852AB

We report the identification of the M9 dwarf SDSS J000649.16-085246.3 as a spectral binary and radial velocity variable with components straddling the hydrogen burning mass limit. Low-resolution near-infrared spectroscopy reveals spectral features indicative of a T dwarf companion, and spectral template fitting yields component types of M8.5\pm0.5 and T5\pm1. High-resolution near-infrared spectroscopy with Keck/NIRSPEC reveals pronounced radial velocity variations with a semi-amplitude of 8.2\pm0.4 km/s. From these we determine an orbital period of 147.6\pm1.5 days and eccentricity of 0.10\pm0.07, making SDSS J0006-0852AB the third tightest very low mass binary known. This system is also found to have a common proper motion companion, the inactive M7 dwarf LP 704-48, at a projected separation of 820\pm120 AU. The lack of H-alpha emission in both M dwarf components indicates that this system is relatively old, as confirmed by evolutionary model analysis of the tight binary. LP 704-48/SDSS J0006-0852AB is the lowest-mass confirmed triple identified to date, and one of only seven candidate and confirmed triples with total masses below 0.3 Msun currently known. We show that current star and brown dwarf formation models cannot produce triple systems like LP 704-48/SDSS J0006-0852AB, and we rule out Kozai-Lidov perturbations and tidal circularization as a viable mechanism to shrink the inner orbit. The similarities between this system and the recently uncovered low-mass eclipsing triples NLTT 41135AB/41136 and LHS 6343ABC suggest that substellar tertiaries may be common in wide M dwarf pairs.

YSOVAR: THE FIRST SENSITIVE, WIDE-AREA, MID-INFRARED PHOTOMETRIC MONITORING OF THE ORION NEBULA CLUSTER

We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

Lithium abundances of very low mass members of Chamaeleon I

Aims. We present the first study of the lithium abundances of very low mass objects in Chamaeleon I close to the hydrogen burning mass limit based on atmospheric models and high-resolution spectroscopic observations. The studied objects, Cha Hα 2, 3, 4, 5, 6 and 8, are very young brown dwarf candidates and very low mass stars on the verge of lithium depletion. Methods. For this analysis, we have computed a new “GAIA-cond” class model grid over effective temperatures from 2600 K to 3100 K, surface gravities from log(g) = 3.5 to 5.5, and lithium abundances from log � = 0. 0t o 3.7, for two different line profile setups introduced in previous work. Calculated synthetic spectra are compared with high-resolution UVES / VLT echelle spectra of the objects. Results. We find good descriptions of the lithium resonance doublet lines at 6708 A and of the surrounding pseudo-continuum and determine a consistent set of lithium abundances (log(� ) = 1.55). However, the derived lithium abundances are lower than the meteoritic one (log(� ) = 3.31) and that of higher mass stars in Cha I (log(� ) = 3.1/3.4 for LTE–/non-LTE–calculations). By modeling the TiO-line, we demonstrate that veiling does not make the lithium lines appear weaker. We can also rule out that the results are spoiled by the presence of spots. Conclusions. A possible explanation for these results would be that the objects are either more massive, or much older, than previously thought, so that the lithium depletion has already started. Although the uncertainties of the masses and ages are large, they are not large enough as to explain the observed lithium depletion. Therefore, the most likely explanation is either a lack of understanding of the details of the formation of the lithium line, or a lack of understanding of the internal structure of the very young low-mass objects.

Spitzer IRS Spectra of Young Stars Near the Hydrogen-burning Mass Limit

We present Spitzer Infrared Spectrograph measurements for two young stars near the hydrogen-burning mass limit in the Taurus star-forming region. While one of the objects, V410 X-ray 3, displays no mid-infrared excess, the other one, V410 Anon 13, shows a clear excess at wavelengths longward of 10 mm, indicative of a circumstellar disk. Moreover, the disk surrounding V410 Anon 13 is reminiscent of flared accretion disks around classical T Tauri stars; small dust grains in the disk photosphere generate the broad 10 mm silicate emission feature, whose structure suggests the presence of crystalline components. This demonstrates that very low mass objects, like their more massive counterparts, experience dust processing in their disks.

Rotational Evolution of Intermediate and Low Mass Main Sequence Stars

Bob Kraft (1967) showed that there is a break in the mean rotational velocity of stars at about spectral type F5, with more massive stars generally being rapid rotators and less massive stars generally being slow rotators. He also showed that in the late F spectral range at least, there is an evolution with time on the main sequence, with younger F stars being more rapidly rotating. Kraft's observational database extended only to about one solar mass due to the sensitivy limitations of photographic plates. Modern observations of low mass stars in open clusters, extending down in mass to nearly the hydrogen burning mass limit in a few clusters, have since been used to show that rotational spindown is the common feature of stars less massive than the sun but that there is a wide spread in rotational velocities when stars arrive on the ZAMS. I will review what is known empirically concerning the rotational velocities of intermediate and low mass field stars, using the open cluster data to place the field star observations in context.

The Quiescent Corona of VB 10

We present results from a Chandra ACIS observation of the M8 dwarf star VB 10, a star near the hydrogen-burning mass limit. Until now, VB 10 has only been detected to flare at X-ray wavelengths. We can now report that nonflare, quiescent, X-ray emission has been detected with a luminosity, LX = (2.4 ± 0.05) × 1025 ergs s-1 and log(LX/Lbol) = -4.9. This is consistent with the previous ROSAT nondetections of quiescent emission from VB 10. We discuss the implications of this discovery for the nature of coronae in ultracool dwarfs.

The First Substellar Subdwarf? Discovery of a Metal‐poor L Dwarf with Halo Kinematics

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H2 absorption, resulting in blue near-infrared (NIR) colors (J-Ks = 0.26 ± 0.16). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205-1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, μ = 260 ± 015 yr-1, and a substantial radial velocity, vrad = -195 ± 11 km s-1, and its probable proximity to the Sun (d = 10-30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 M 0.085 M☉ for ages 10-15 Gyr and metallicities Z = 0.1-0.01 Z☉. The discovery of this object clearly indicates that star formation occurred below the hydrogen burning mass limit at early times, consistent with prior results indicating a flat or slightly rising mass function for the lowest mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper-motion surveys.

Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry

Based on 14 data points obtained with near-infrared speckle interferometry and covering an almost entire revolution, we present a first visual orbit for the low-mass binary system Gliese 22 AC. The quality of the orbit is largely improved with respect to previous astrometric solutions. The dynamical system mass is $0.592\pm 0.065\,M_{\odot}$, where the largest part of the error is due to the Hipparcos parallax. A comparison of this dynamical mass with mass-luminosity relations on the lower main sequence and theoretical evolutionary models for low-mass objects shows that both probably underestimate the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates that this object is a very low-mass star with a mass close to the hydrogen burning mass limit.

Substellar Mass Function of Young Open Clusters as Determined through a Statistical Approach Using Two Micron All Sky Survey and Guide Star Catalog Data

In this paper we present the mass functions in the substellar regime of three young open clusters, IC 348, σ Orionis, and Pleiades, as derived using the data from the Two Micron All Sky Survey catalog, which has a limiting magnitude of Ks ~ 15, and the latest version of the Guide Star Catalog, which has a limiting magnitude of F ~ 21. Based on recent evolutionary models for low-mass stars, we have formulated the selection criteria for stars with masses below 0.5 M☉. Using a statistical approach to correct for the background contamination, we derive the mass function of objects with masses ranging from 0.5 M☉ down to the substellar domain, well below the hydrogen-burning mass limit. The lowest mass bins in our analysis are 0.025, 0.045, and 0.055 M☉ for IC 348, σ Orionis, and Pleiades, respectively. The resultant slopes of the mass functions are 0.8 ± 0.2 for IC 348, 1.2 ± 0.2 for σ Orionis, and 0.5 ± 0.2 for Pleiades, which are consistent with the previous results. The contribution of objects below 0.5 M☉ to the total mass of the cluster is ~40%, and the contribution of objects below 0.08 M☉ to the total mass is ~4%.

Young Low‐Mass Stars and Brown Dwarfs in IC 348

I present new results from a continuing program to identify and characterize the low-mass stellar and substellar populations in the young cluster IC 348 (1-10~Myr). Optical spectroscopy has revealed young objects with spectral types as late as M8.25. The intrinsic J-H and H-K colors of these sources are dwarf-like, whereas the R-I and I-J colors appear intermediate between the colors of dwarfs and giants. Furthermore, the spectra from 6500 to 9500 A are reproduced well with averages of standard dwarf and giant spectra, suggesting that such averages should be used in the classification of young late-type sources. An H-R diagram is constructed for the low-mass population in IC 348 (K6-M8). The presumably coeval components of the young quadruple system GG~Tau (White et al.) and the locus of stars in IC 348 are used as empirical isochrones to test the theoretical evolutionary models. For the models of Baraffe et al., an adjustment of the temperature scale to progressively warmer temperatures at later M types, intermediate between dwarfs and giants, brings all components of GG~Tau onto the same model isochrone and gives the population of IC 348 a constant age and age spread as a function of mass. When other observational constraints are considered, such as the dynamical masses of GM~Aur, DM~Tau, and GG~Tau~A, the models of Baraffe et al. are the most consistent with observations of young systems. With compatible temperature scales, the models of both D'Antona & Mazzitelli and Baraffe et al. suggest that the hydrogen burning mass limit occurs near M6 at ages of <10 Myr. Thus, several likely brown dwarfs are discovered in this study of IC 348, with masses down to ~20-30 M_J.

Results of a Deep Imaging Survey of One Square Degree of the Pleiades for Low‐Luminosity Cluster Members

We have obtained relatively deep imaging in two colors, with limiting magnitudes of V ~ 22.5 and Ic ~ 21, of about one square degree of the Pleiades open cluster. Our primary goal was to identify new candidate brown dwarf members of this ~100 Myr-old open cluster. In the process, we have also obtained V and Ic photometry for a large number of brighter proper-motion members of the cluster for which only photographic photometry had been available up until now. We identify six candidate objects whose locations in a V versus (V - I)c color-magnitude diagram are consistent with their being Pleiades members near or below the hydrogen-burning mass limit. We have obtained near-infrared photometry for several of these brown dwarf candidates, as well as for a selection of previously identified very low mass Pleiades members, and we use this new photometry to help determine whether the objects identified in the visual are indeed Pleiades members or not. Finally, we have obtained a moderate-resolution spectrum of one of the new brown dwarf candidates with the Keck LRIS spectrograph. The spectrum shows that the star has a spectral type of M7, which is compatible with its photometric colors, and that it has an Hα equivalent width of about 14 A and a marginally detected lithium 6708 A absorption equivalent width of about 0.4 A. These spectroscopic characteristics suggest that this object (MHObd1) is indeed a Pleiades member with a mass at or slightly above the hydrogen-burning mass limit.