Dynamical Mass of the Young Substellar Companion HD 984 B

Model-independent masses of substellar companions are critical tools to validate models of planet and brown dwarf cooling, test their input physics, and determine the formation and evolution of these objects. In this work, we measure the dynamical mass and orbit of the young substellar companion HD 984 B. We obtained new high-contrast imaging of the HD 984 system with Keck/NIRC2 that expands the baseline of relative astrometry from 3 to 8 yr. We also present new radial velocities of the host star with the Habitable-Zone Planet Finder spectrograph at the Hobby-Eberly Telescope. Furthermore, HD 984 exhibits a significant proper motion difference between Hipparcos and Gaia EDR3. Our joint orbit fit of the relative astrometry, proper motions, and radial velocities yields a dynamical mass of 61 ± 4 M Jup for HD 984 B, placing the companion firmly in the brown dwarf regime. The new fit also reveals a higher eccentricity for the companion (e = 0.76 ± 0.05) compared to previous orbit fits. Given the broad age constraint for HD 984, this mass is consistent with predictions from evolutionary models. HD 984 B’s dynamical mass places it among a small but growing list of giant planet and brown dwarf companions with direct mass measurements.

HST/WFC3 Complete Phase-resolved Spectroscopy of White-dwarf-brown-dwarf Binaries WD 0137 and EPIC 2122

Brown dwarfs in close-in orbits around white dwarfs offer an excellent opportunity to investigate properties of fast-rotating, tidally locked, and highly irradiated atmospheres. We present Hubble Space Telescope Wide Field Camera 3 G141 phase-resolved observations of two brown-dwarf-white-dwarf binaries: WD 0137-349 and EPIC 212235321. Their 1.1–1.7 μm phase curves demonstrate rotational modulations with semi-amplitudes of 5.27% ± 0.02% and 29.1% ± 0.1%; both can be fit well by multi-order Fourier series models. The high-order Fourier components have the same phase as the first-order and are likely caused by hot spots located at the substellar points, suggesting inefficient day/night heat transfer. Both brown dwarfs’ phase-resolved spectra can be accurately represented by linear combinations of their respective day- and nightside spectra. Fitting the irradiated brown dwarf model grids to the dayside spectra require a filling factor of ∼50%, further supporting a hot spot dominating the dayside emission. The nightside spectrum of WD 0137-349B is fit reasonably well by non-irradiated substellar models, and the one of EPIC 21223521B can be approximated by a Planck function. We find strong spectral variations in the brown dwarfs’ day/night flux and brightness temperature contrasts, highlighting the limitations of band-integrated measurements in probing heat transfer in irradiated objects. On the color–magnitude diagram, WD 0137-349B evolves along a cloudless model track connecting the early-L and mid-T spectral types, suggesting that clouds and disequilibrium chemistry have a negligible effect on this object. A full interpretation of these high-quality phase-resolved spectra calls for new models that couple atmospheric circulation and radiative transfer under high-irradiation conditions.

Mapping the Pressure-dependent Day–Night Temperature Contrast of a Strongly Irradiated Atmosphere with HST Spectroscopic Phase Curve

Many brown dwarfs are on ultrashort-period and tidally locked orbits around white dwarf hosts. Because of these small orbital separations, the brown dwarfs are irradiated at levels similar to hot Jupiters. Yet, they are easier to observe than hot Jupiters because white dwarfs are fainter than main-sequence stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore, ideal hot Jupiter analogs for studying the atmospheric response under strong irradiation and fast rotation. We present the 1.1–1.67 μm spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the SDSS J141126.20 + 200911.1 brown dwarf–white dwarf binary with the near-infrared G141 grism of the Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a 50M Jup brown dwarf with an irradiation temperature of 1300 K and has an orbital period of 2.02864 hr. Our best-fit model suggests a phase-curve amplitude of 1.4% and places an upper limit of 11° for the phase offset from the secondary eclipse. After fitting the white dwarf spectrum, we extract the phase-resolved brown dwarf emission spectra. We report a highly wavelength-dependent day–night spectral variation, with a water-band flux variation of about 360% ± 70% and a comparatively small J-band flux variation of 37% ± 2%. By combining the atmospheric modeling results and the day–night brightness temperature variations, we derive a pressure-dependent temperature contrast. We discuss the difference in the spectral features of SDSS1411-B and hot Jupiter WASP-43b, as well as the lower-than-predicted day–night temperature contrast of J4111-BD. Our study provides the high-precision observational constraints on the atmospheric structures of an irradiated brown dwarf at different orbital phases.

GTC/CanariCam Deep Mid-infrared Imaging Survey of Northern Stars within 5 pc

In this work we present the results of a direct imaging survey for brown dwarf companions around the nearest stars at the mid-infrared 10 micron range (λ c = 8.7 μm, Δλ = 1.1 μm) using the CanariCam instrument on the 10.4 m Gran Telescopio Canarias (GTC). We imaged the 25 nearest stellar systems within 5 pc of the Sun at declinations δ > −25° (at least half have planets from radial-velocity studies), reaching a mean detection limit of 11.3 ± 0.2 mag (1.5 mJy) in the Si-2 8.7 μm band over a range of angular separations from 1″ to 10″. This would have allowed us to uncover substellar companions at projected orbital separations between ∼2 and 50 au, with effective temperatures down to 600 K and masses greater than 30 M Jup assuming an average age of 5 Gyr and masses down to the deuterium-burning mass limit for objects with ages <1 Gyr. From the nondetection of such companions, we determined upper limits on their occurrence rate at depths and orbital separations yet unexplored by deep imaging programs. For the M dwarfs, the main component of our sample, we found with a 90% confidence level that fewer than 20% of these low-mass stars have L- and T-type brown dwarf companions with m ≳ 30 M Jup and T eff ≳ 600 K at ∼3.5–35 au projected orbital separations.

The Brown Dwarf Kinematics Project (BDKP). V. Radial and Rotational Velocities of T Dwarfs from Keck/NIRSPEC High-resolution Spectroscopy

We report multiepoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo forward-modeling method, we achieve median precisions of 0.5 and 0.9 km s−1 for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin-disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously unresolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 and 12 yr, respectively. We find a kinematic age of 3.5 ± 0.3 Gyr for local T dwarfs, consistent with nearby late M dwarfs (4.1 ± 0.3 Gyr). Removal of thick-disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2 ± 0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4–L6 subtypes, likely reflecting the terminus of the stellar main sequence. We provide a compilation of precise radial velocities for 172 late M, L, and T dwarfs within ∼20 pc of the Sun.

A Brown Dwarf Companion to the Nova-like Variable RW Tri

The orbital period of Nova-like variable RW Tri is expected to experience a long-term evolution due to a stable mass transfer from the red dwarf to the white dwarf. By adding 297 new eclipse timings obtained from our own observations and a cross-identification of many databases, we fully reinvestigated the variations in orbital period of RW Tri, based on a total of 658 data points spanning over 80 years. The new O-C diagram demonstrates a more complicate pattern than a pure sinusoidal modulation shown in the previous O-C analyses. The best fit of the O-C variations is a quadratic-plus-sinusoidal curve with a period of 22.66 (2) years and a typical decrease rate of P˙ = −2d.32(4) × 10−9 yr−1. To explain secular orbital period decrease, the magnetic braking effect is required to cause the orbital angular moment loss in RW Tri with a mass ratio less than unity, while a conserved mass transfer is also enough for RW Tri with a mass ratio larger than unity. No matter what the mass ratio is, a slightly enhanced mass transfer rate, 2.4–5.3 × 10−9 M⊙ yr−1, derived from our O-C diagram, providing an evidence supporting the disk instability model and the standard/revised models of cataclysmic variable evolution, is almost the same as that obtained from the light-curve modeling. This further confirms our observed orbital period decrease and the controversial system parameter, mass transfer rate. Our updated O-C analysis further verifies the claimed cyclical changes of orbital period with a period range of 21–24 years, which is approximately one half of the results in the literature. In accordance with the light-travel time effect, this periodical variation shown in our new O-C diagram indicates a brown dwarf hidden in RW Tri at a coplanar orbit. Note that the large scatter in the data range of 0–3 × 104 cycles requires the high-precision photometry in the longer base line in the future.

A proto-brown dwarf candidate in rho Ophiuchus

Brown dwarfs with masses below 0.075 solar masses are thought to form like low-mass stars (e.g., the Sun).However, it is still unclear how the physical formation processes occurin brown dwarfs at the ealiest stages (i.e., proto-brown dwarfs) of their formation.Up to date, only a few proto-brown dwarfs have been detected.The detection of proto-brown dwarfs offers us excellent benchmarks to studythe formation process of brown dwarfs, and thus understand their formation mechanism.In this paper, we present our identification of a proto-brown dwarf candidate in the star-forming regionrho Ophiuchus.The candidate shows a small-scale bipolar molecular outlfow that is similar to the outflows observed inother young brown dwarfs. The detection of this candidateprovides us with additional important implications for the formation mechanism of brown dwarfs.