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.
Abstract
The orbital eccentricities of directly imaged exoplanets and brown dwarf companions provide clues about their formation and dynamical histories. We combine new high-contrast imaging ...observations of substellar companions obtained primarily with Keck/NIRC2 together with astrometry from the literature to test for differences in the population-level eccentricity distributions of 27 long-period giant planets and brown dwarf companions between 5 and 100 au using hierarchical Bayesian modeling. Orbit fits are performed in a uniform manner for companions with short orbital arcs; this typically results in broad constraints for individual eccentricity distributions, but together as an ensemble, these systems provide valuable insight into their collective underlying orbital patterns. The shape of the eccentricity distribution function for our full sample of substellar companions is approximately flat from
e
= 0–1. When subdivided by companion mass and mass ratio, the underlying distributions for giant planets and brown dwarfs show significant differences. Low mass ratio companions preferentially have low eccentricities, similar to the orbital properties of warm Jupiters found with radial velocities and transits. We interpret this as evidence for in situ formation on largely undisturbed orbits within massive extended disks. Brown dwarf companions exhibit a broad peak at
e
≈ 0.6–0.9 with evidence for a dependence on orbital period. This closely resembles the orbital properties and period-eccentricity trends of wide (1–200 au) stellar binaries, suggesting that brown dwarfs in this separation range predominantly form in a similar fashion. We also report evidence that the “eccentricity dichotomy” observed at small separations extends to planets on wide orbits: the mean eccentricity for the multi-planet system HR 8799 is lower than for systems with single planets. In the future, larger samples and continued astrometric orbit monitoring will help establish whether these eccentricity distributions correlate with other parameters such as stellar host mass, multiplicity, and age.
Abstract
Dynamical masses of giant planets and brown dwarfs are critical tools for empirically validating substellar evolutionary models and their underlying assumptions. We present a measurement of ...the dynamical mass and an updated orbit of PZ Tel B, a young brown dwarf companion orbiting a late-G member of the
β
Pic moving group. PZ Tel A exhibits an astrometric acceleration between Hipparcos and Gaia EDR3, which enables the direct determination of the companion’s mass. We have also acquired new Keck/NIRC2 adaptive optics imaging of the system, which increases the total baseline of relative astrometry to 15 yr. Our joint orbit fit yields a dynamical mass of
27
−
9
+
25
M
Jup
, semimajor axis of
27
−
4
+
14
au
, eccentricity of
0.52
−
0.10
+
0.08
, and inclination of
91.73
−
0.32
+
0.36
°
. The companion’s mass is consistent within 1.1
σ
of predictions from four grids of hot-start evolutionary models. The joint orbit fit also indicates a more modest eccentricity of PZ Tel B than previous results. PZ Tel joins a small number of young (<200 Myr) systems with benchmark substellar companions that have dynamical masses and precise ages from moving group membership.
Abstract
Accreting protoplanets provide key insights into how planets assemble from their natal protoplanetary disks. Recently, Zhou et al. (2021) used angular differential imaging (ADI) with Hubble ...Space Telescope’s Wide Field Camera 3 (HST/WFC3) to recover the young accreting planet PDS 70 b in F656N (H
α
) at a signal-to-noise ratio (S/N) of 7.9. In this paper, we demonstrate a promising approach to efficiently imaging accreting planets by applying reference star differential imaging (RDI) to the same data set. We compile a reference library from the database of WFC3 point-spread functions (PSFs) provided by the Space Telescope Science Institute and develop a set of morphology-significance criteria for preselection of reference frames to improve RDI subtraction. RDI with this PSF library results in a detection of PDS 70 b at an S/N of 5.3. Astrometry and photometry of PDS 70 b are calibrated using a forward-modeling method and injection-recovery tests, resulting in a separation of 186 ± 13 mas, a position angle of 142° ± 5°, and a H
α
flux of (1.7 ± 0.3) × 10
−15
erg s
−1
cm
−2
. The lower detection significance with RDI can be attributed to the ∼100 times lower peak-to-background ratios of the reference PSFs compared to the ADI PSFs. Building a high-quality reference library with WFC3 will provide unique opportunities to study accretion variability on short timescales not limited by roll angle scheduling constraints and efficiently search for actively accreting protoplanets in H
α
around targets inaccessible to ground-based adaptive optics systems, such as faint transition disk hosts.
ABSTRACT
The Transiting Exoplanet Survey Satellite (TESS) has already begun to discover what will ultimately be thousands of exoplanets around nearby cool bright stars. These potential host stars ...must be well understood to accurately characterize exoplanets at the individual and population levels. We present a catalogue of the chemo-kinematic properties of 2218 434 stars in the TESS Candidate Target List using survey data from Gaia DR2, APOGEE, GALAH, RAVE, LAMOST, and photometrically derived stellar properties from SkyMapper. We compute kinematic thin disc, thick disc, and halo membership probabilities for these stars and find that though the majority of TESS targets are in the thin disc, 4 per cent of them reside in the thick disc and <1 per cent of them are in the halo. The TESS Objects of Interest in our sample also display similar contributions from the thin disc, thick disc, and halo with a majority of them being in the thin disc. We also explore metallicity and α/Fe distributions for each Galactic component and show that each cross-matched survey exhibits metallicity and α/Fe distribution functions that peak from higher to lower metallicity and lower to higher α/Fe from the thin disc to the halo. This catalogue will be useful to explore planet occurrence rates, among other things, with respect to kinematics, component membership, metallicity, or α/Fe.
We present results from a high-contrast adaptive optics imaging search for giant planets and brown dwarfs (> ~1 M sub(Jup)) around 122 newly identified nearby (<, ~40 pc) young M dwarfs. Half of our ...targets are younger than 135 Myr and 90% are younger than the Hyades (620 Myr). After removing 44 close stellar binaries (implying a stellar companion fraction of >35.4% + or - 4.3% within 100 AU), 27 of which are new or spatially resolved for the first time, our remaining sample of 78 single M dwarfs makes this the largest imaging search for planets around young low-mass stars (0.1-0.6 M sub(middot in circle)) to date. Our H- and K-band coronagraphic observations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of 12-14 mag and 9-13 mag at 1", respectively, which correspond to limiting planet masses of 0.5-10 M sub(Jup) at 5-33 AU for 85% of our sample. We discovered four young brown dwarf companions: 1RXS J235133.3+312720 B (32 + or - 6 M sub(Jup); L0 super(+2) sub(-1); 120 + or - 20 AU), GJ 3629 B (64 super(+30) sub(-23) M sub(Jup); M7.5 + or - 0.5; 6.5 + or - 0.5 AU), 1RXS J034231.8+121622 B (35 + or - 8 M sub(Jup); L0 + or - 1; 19.8 + or - 0.9 AU), and 2MASS J15594729+4403595 B (43 + or - 9 M sub(Jup); M8.0 + or - 0.5; 190 + or - 20 AU). Over 150 candidate planets were identified; we obtained follow-up imaging for 56% of these but all are consistent with background stars. Our null detection of planets enables strong statistical constraints on the occurrence rate of long-period giant planets around single M dwarfs. We infer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13 M sub(Jup) planets between 10-100 AU for hot-start and cold-start (Fortney) evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbor massive gas giants in the 5-13 M sub(Jup) range like those orbiting HR 8799 and beta Pictoris between 10-100 AU for a hot-start (cold-start) formation scenario. The frequency of brown dwarf (13-75 M sub(Jup)) companions to single M dwarfs between 10-100 AU is 2.8 super(+2.4) sub(-1.5)%. Altogether we find that giant planets, especially massive ones, are rare in the outskirts of M dwarf planetary systems. Although the first directly imaged planets were found around massive stars, there is currently no statistical evidence for a trend of giant planet frequency with stellar host mass at large separations as predicted by the disk instability model of giant planet formation.
Abstract
Understanding differences between substellar spectral data and models has proven to be a major challenge, especially for self-consistent model grids that are necessary for a thorough ...investigation of brown dwarf atmospheres. Using the random forest supervised machine-learning method, we study the information content of 14 previously published model grids of brown dwarfs (from 1997 to 2021). The random forest method allows us to analyze the predictive power of these model grids, as well as interpret data within the framework of Approximate Bayesian Computation. Our curated data set includes three benchmark brown dwarfs (Gl 570D,
ϵ
Indi Ba, and Bb) as well as a sample of 19 L and T dwarfs; this sample was previously analyzed by Lueber et al. using traditional Bayesian methods (nested sampling). We find that the effective temperature of a brown dwarf can be robustly predicted independent of the model grid chosen for the interpretation. However, inference of the surface gravity is model-dependent. Specifically, the
BT-Settl
,
Sonora Bobcat
, and
Sonora Cholla
model grids tend to predict
log
g
∼
3
–4 (cgs units) even after data blueward of 1.2
μ
m have been disregarded to mitigate for our incomplete knowledge of the shapes of alkali lines. Two major, longstanding challenges associated with understanding the influence of clouds in brown dwarf atmospheres remain: our inability to model them from first principles and also to robustly validate these models.
Abstract
A large suite of 228 atmospheric retrievals is performed on a curated sample of 19 brown dwarfs spanning the L0–T8 spectral types using the open-source
Helios-r2
retrieval code, which ...implements the method of short characteristics for radiative transfer and a finite-element description of the temperature–pressure profile. Surprisingly, we find that cloud-free and cloudy (both gray and nongray) models are equally consistent with the archival SpeX data from the perspective of Bayesian model comparison. Only upper limits for cloud properties are inferred if log-uniform priors are assumed, but the cloud optical depth becomes constrained if a uniform prior is used. Water is detected in all 19 objects, and methane is detected in all of the T dwarfs, but no obvious trend exists across effective temperature. As carbon monoxide is only detected in a handful of objects, the inferred carbon-to-oxygen ratios are unreliable. The retrieved radius generally decreases with effective temperature, but the values inferred for some T dwarfs are implausibly low and may indicate missing physics or chemistry in the models. For the early L dwarfs, the retrieved surface gravity depends on whether the gray-cloud or non-gray-cloud model is preferred. Future data are necessary for constraining cloud properties and the vertical variation of chemical abundances, the latter of which is needed for distinguishing between the chemical instability and traditional cloud interpretation of the L-T transition.
Abstract
HR 8799 is a young A5/F0 star hosting four directly imaged giant planets at wide separations (∼16–78 au), which are undergoing orbital motion and have been continuously monitored with ...adaptive optics imaging since their discovery over a decade ago. We present a dynamical mass of HR 8799 using 130 epochs of relative astrometry of its planets, which include both published measurements and new medium-band 3.1
μ
m observations that we acquired with NIRC2 at Keck Observatory. For the purpose of measuring the host-star mass, each orbiting planet is treated as a massless particle and is fit with a Keplerian orbit using Markov chain Monte Carlo. We then use a Bayesian framework to combine each independent total mass measurement into a cumulative dynamical mass using all four planets. The dynamical mass of HR 8799 is
1.47
−
0.17
+
0.12
M
⊙
assuming a uniform stellar mass prior, or
1.46
−
0.15
+
0.11
M
⊙
with a weakly informative prior based on spectroscopy. There is a strong covariance between the planets’ eccentricities and the total system mass; when the constraint is limited to low-eccentricity solutions of
e
< 0.1, which are motivated by dynamical stability, our mass measurement improves to
1.43
−
0.07
+
0.06
M
⊙
. Our dynamical mass and other fundamental measured parameters of HR 8799 together with Modules for Experiments in Stellar Astrophysics Isochrones and Stellar Tracks grids yields a bulk metallicity most consistent with Fe/H ∼ −0.25–0.00 dex and an age of 10–23 Myr for the system. This implies hot-start masses of 2.7–4.9
M
Jup
for HR 8799 b and 4.1–7.0
M
Jup
for HR 8799 c, d, and e, assuming they formed at the same time as the host star.
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.
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