Abstract
We present the full results of our decade-long astrometric monitoring programs targeting 31 ultracool binaries with component spectral types M7–T5. Joint analysis of resolved imaging from ...Keck Observatory and
Hubble Space Telescope
and unresolved astrometry from CFHT/WIRCam yields parallactic distances for all systems, robust orbit determinations for 23 systems, and photocenter orbits for 19 systems. As a result, we measure 38 precise individual masses spanning 30–115
. We determine a model-independent substellar boundary that is ≈70
in mass (≈L4 in spectral type), and we validate Baraffe et al. evolutionary model predictions for the lithium-depletion boundary (60
at field ages). Assuming each binary is coeval, we test models of the substellar mass–luminosity relation and find that in the L/T transition, only the Saumon & Marley “hybrid” models accounting for cloud clearing match our data. We derive a precise, mass-calibrated spectral type–effective temperature relation covering 1100–2800 K. Our masses enable a novel direct determination of the age distribution of field brown dwarfs spanning L4–T5 and 30–70
. We determine a median age of 1.3 Gyr, and our population synthesis modeling indicates our sample is consistent with a constant star formation history modulated by dynamical heating in the Galactic disk. We discover two triple-brown-dwarf systems, the first with directly measured masses and eccentricities. We examine the eccentricity distribution, carefully considering biases and completeness, and find that low-eccentricity orbits are significantly more common among ultracool binaries than solar-type binaries, possibly indicating the early influence of long-lived dissipative gas disks. Overall, this work represents a major advance in the empirical view of very low-mass stars and brown dwarfs.
ABSTRACT
We present a large, uniform analysis of young (≈10–150 Myr) ultracool dwarfs, based on new high-precision infrared (IR) parallaxes for 68 objects. We find that low-gravity (
vl-g
) late-M ...and L dwarfs form a continuous sequence in IR color–magnitude diagrams, separate from the field population and from current theoretical models. These
vl-g
objects also appear distinct from young substellar (brown dwarf and exoplanet) companions, suggesting that the two populations may have a different range of physical properties. In contrast, at the L/T transition, young, old, and spectrally peculiar objects all span a relatively narrow range in near-IR absolute magnitudes. At a given spectral type, the IR absolute magnitudes of young objects can be offset from ordinary field dwarfs, with the largest offsets occurring in the
Y
and
J
bands for late-M dwarfs (brighter than the field) and mid-/late-L dwarfs (fainter than the field). Overall, low-gravity (
vl-g
) objects have the most uniform photometric behavior, while intermediate gravity (
int-g
) objects are more diverse, suggesting a third governing parameter beyond spectral type and gravity class. We examine the moving group membership for all young ultracool dwarfs with parallaxes, changing the status of 23 objects (including 8 previously identified planetary-mass candidates) and fortifying the status of another 28 objects. We use our resulting age-calibrated sample to establish empirical young isochrones and show a declining frequency of
vl-g
objects relative to
int-g
objects with increasing age. Notable individual objects in our sample include high-velocity (≳100 km s
−1
)
int-g
objects, very red late-L dwarfs with high surface gravities, candidate disk-bearing members of the MBM20 cloud and
β
Pic moving group, and very young distant interlopers. Finally, we provide a comprehensive summary of the absolute magnitudes and spectral classifications of young ultracool dwarfs, using a combined sample of 102 objects found in the field and as substellar companions to young stars.
The coolest known brown dwarfs are our best analogs to extrasolar gas-giant planets. The prolific detections of such cold substellar objects in the past 2 years have spurred intensive follow-up, but ...the lack of accurate distances is a key gap in our understanding; We present a large sample of precise distances based on homogeneous mid-infrared astrometry that robustly establishes absolute fluxes, luminosities, and temperatures. The coolest brown dwarfs have temperatures of 400 to 450 kelvin and masses almost equal to 5 to 20 times that of Jupiter, showing they bridge the gap between hotter brown dwarfs and gas-giant planets. At these extremes, spectral energy distributions no longer follow a simple correspondence with temperature, suggesting an increasing role of other physical parameters, such as surface gravity, vertical mixing, clouds, and metallicity.
We measure dynamical masses for five objects-three ultracool dwarfs, one low-mass star, and one white dwarf-by fitting orbits to a combination of the Hipparcos-Gaia Catalog of Accelerations, ...literature radial velocities, and relative astrometry. Our approach provides precise masses without any assumptions about the primary star, even though the observations typically cover only a small fraction of an orbit. We also perform a uniform re-analysis of the host stars' ages. Two of our objects, HD 4747B and HR 7672B, already have precise dynamical masses near the stellar/substellar boundary and are used to validate our approach. For Gl 758B, we obtain a mass of , the most precise mass measurement of this companion to date. Gl 758B is the coldest brown dwarf with a dynamical mass, and the combination of our low mass and slightly older host-star age resolves its previously noted discrepancy with substellar evolutionary models. HD 68017B, a late M-dwarf, has a mass of m = 0.149 0.002 M , consistent with stellar theory and previous empirical estimates based on its absolute magnitude. The progenitor of the white dwarf Gl 86B has been debated in the literature, and our dynamical measurement of m = 0.597 0.010 M is consistent with a higher progenitor mass and younger age for this planet-hosting binary system. Overall, these case studies represent only five of the thousands of accelerating systems identified by combining Hipparcos and Gaia. Our analysis could be repeated for many of them to build a large sample of companions with dynamical masses.
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6-T9) in 49 ...systems, with a median uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17 systems, we significantly improve upon published results, with a median (best) improvement of 1.7 times (5 times). Three systems show astrometric perturbations indicative of orbital motion; two are known binaries and one is spectrally peculiar. In addition, we present here a large set of Keck adaptive optics imaging that more than triples the number of binaries with L6-T5 components that have both multi-band photometry and distances. Finally, we provide a comprehensive update to the absolute magnitudes of ultracool dwarfs as a function of spectral type using a combined sample of 314 objects.
ABSTRACT The dynamical influence of binary companions is expected to profoundly influence planetary systems. However, the difficulty of identifying planets in binary systems has left the magnitude of ...this effect uncertain; despite numerous theoretical hurdles to their formation and survival, at least some binary systems clearly host planets. We present high-resolution imaging of 382 Kepler Objects of Interest (KOIs) obtained using adaptive-optics imaging and nonredundant aperture-mask interferometry on the Keck II telescope. Among the full sample of 506 candidate binary companions to KOIs, we super-resolve some binary systems to projected separations of <5 au, showing that planets might form in these dynamically active environments. However, the full distribution of projected separations for our planet-host sample more broadly reveals a deep paucity of binary companions at solar-system scales. For a field binary population, we should have found 58 binary companions with projected separation < 50 au and mass ratio q > 0.4; we instead only found 23 companions (a 4.6 deficit), many of which must be wider pairs that are only close in projection. When the binary population is parametrized with a semimajor axis cutoff acut and a suppression factor inside that cutoff Sbin, we find with correlated uncertainties that inside au, the planet occurrence rate in binary systems is only times that of wider binaries or single stars. Our results demonstrate that a fifth of all solar-type stars in the Milky Way are disallowed from hosting planetary systems due to the influence of a binary companion.
Abstract
HR 8799 hosts four directly imaged giant planets, but none has a mass measured from first principles. We present the first dynamical mass measurement in this planetary system, finding that ...the innermost planet HR 8799 e has a mass of
9.6
−
1.8
+
1.9
M
Jup
. This mass results from combining the well-characterized orbits of all four planets with a new astrometric acceleration detection (5
σ
) from the Gaia EDR3 version of the Hipparcos-Gaia Catalog of Accelerations. We find with 95% confidence that HR 8799 e is below 13
M
Jup
, the deuterium-fusing mass limit. We derive a hot-start cooling age of
42
−
16
+
24
Myr for HR 8799 e that agrees well with its hypothesized membership in the Columba association but is also consistent with an alternative suggested membership in the
β
Pictoris moving group. We exclude the presence of any additional ≳5 −
M
Jup
planets interior to HR 8799 e with semimajor axes between ≈3–16 au. We provide proper motion anomalies and a matrix equation to solve for the mass of any of the planets of HR 8799 using only mass ratios between the planets.
Abstract
We present an open-source Python package, Orbits from Radial Velocity, Absolute, and/or Relative Astrometry (
orvara
), to fit Keplerian orbits to any combination of radial velocity, ...relative astrometry, and absolute astrometry data from the Hipparcos-Gaia Catalog of Accelerations. By combining these three data types, one can measure precise masses and sometimes orbital parameters even when the observations cover a small fraction of an orbit. The computational performance of
orvara
is achieved with an eccentric anomaly solver 5–10 times faster than commonly used approaches and low-level memory management to avoid Python overheads and by analytically marginalizing out parallax, barycenter proper motion, and instrument-specific radial velocity zero-points. Through its integration with the Hipparcos and Gaia intermediate astrometry package
htof
,
orvara
can properly account for the epoch astrometry measurements of Hipparcos and the measurement times and scan angles of individual Gaia epochs. We configure
orvara
with modifiable
.ini
configuration files tailored to any specific stellar or planetary system. We demonstrate
orvara
with a case study application to a recently discovered white dwarf/main-sequence system, HD 159062. By adding absolute astrometry to literature radial velocity and relative astrometry data, our comprehensive Markov Chain Monte Carlo analysis improves the precision of HD 159062B’s mass by more than an order of magnitude to
0.6083
−
0.0073
+
0.0083
M
☉
. We also derive a low eccentricity and large semimajor axis, establishing HD 159062AB as a system that did not experience Roche lobe overflow.
Abstract
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.
Abstract
We present comprehensive orbital analyses and dynamical masses for the substellar companions Gl 229 B, Gl 758 B, HD 13724 B, HD 19467 B, HD 33632 Ab, and HD 72946 B. Our dynamical fits ...incorporate radial velocities, relative astrometry, and, most importantly, calibrated Hipparcos-Gaia EDR3 accelerations. For HD 33632 A and HD 72946 we perform three-body fits that account for their outer stellar companions. We present new relative astrometry of Gl 229 B with Keck/NIRC2, extending its observed baseline to 25 yr. We obtain a <1% mass measurement of 71.4 ± 0.6
M
Jup
for the first T dwarf Gl 229 B and a 1.2% mass measurement of its host star (0.579 ± 0.007
M
⊙
) that agrees with the high-mass end of the M-dwarf mass–luminosity relation. We perform a homogeneous analysis of the host stars’ ages and use them, along with the companions’ measured masses and luminosities, to test substellar evolutionary models. Gl 229 B is the most discrepant, as models predict that an object this massive cannot cool to such a low luminosity within a Hubble time, implying that it may be an unresolved binary. The other companions are generally consistent with models, except for HD 13724 B, which has a host star activity age 3.8
σ
older than its substellar cooling age. Examining our results in context with other mass–age–luminosity benchmarks, we find no trend with spectral type but instead note that younger or lower-mass brown dwarfs are overluminous compared to models, while older or higher-mass brown dwarfs are underluminous. The presented mass measurements for some companions are so precise that the stellar host ages, not the masses, limit the analysis.
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