Brown Dwarfs vs Giant Planets - Distinct populations from direct imaging?

Population-Level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels
https://arxiv.org/abs/1911.10569

There are updates to the orbits of several directly imaged brown dwarfs and planets and they suggest that two distinct populations are emerging in the eccentricity - mass-ratio parameter space.

Unveiling wide-orbit companions to K-type stars in Sco-Cen with Gaia EDR3

Quote :

We aim to identify new low-mass companions to young stars using the astrometric measurements provided by the Gaia space mission and complementary VLT/SPHERE data. We identify companion candidates from a sample of K-type, pre-main sequence stars in the Scorpius Centaurus association using the early version of the third data release of the Gaia space mission. Based on the provided positions, proper motions, and magnitudes, we identify all objects within a predefined radius whose differential proper motions are consistent with a gravitationally bound system. We derive companion masses through comparison with evolutionary tracks. For seven identified companion candidates we use additional data collected with VLT/SPHERE and VLT/NACO to assess the accuracy of the properties of the companions based on Gaia photometry alone. We identify 110 comoving companions that have a companionship likelihood of more than 95%. We identify ten especially intriguing companions that have masses in the brown dwarf regime down to 20M_Jup. Our high-contrast imaging data confirm both astrometry and photometric masses derived from Gaia alone. We discover a new brown dwarf companion, TYC 8252-533-1 B, with a projected separation of approximately 570au from its Sun-like primary. SED modeling provides a companion mass of 52⁺¹⁷_-11M_Jup. We show that the Gaia database can identify low-mass companions at wide separations from their host stars. For K-type Sco-Cen members Gaia can detect sub-stellar objects at projected separations larger than 300au and is sensitivity limited beyond 1,000au with a lower mass limit down to 20M_Jup. A similar analysis of other star-forming regions could significantly enlarge the sample size of such objects and test formation and evolution theories of planetary systems.

Besides TYC 8252-533-1, imaged BD candidates around following young stars too:

- 1RXS J103137.1-690205
- 1RXS J131752.0-505845
- 2MASS J16175569-3828132
- ASAS J160835-1912.0
- HD 144432 A
- HT Lupi
- RX J1511.6-3249B
- RX J1608.9-3905
- ScoPMS 42b
- TYC 6784-39-1
- TYC 9245-535-1  
- VSS II-28
- WRAY 15-1380

Updated orbital parameters for HD 984 b in this new paper wherein an overlooked nearby circumbinary brown dwarf is mentioned, GJ 802 (dating back to 2008!).

At least seven BD-BD and likely BD-planet (if not planet-planet) binaries in Orion Nebula

Binary Formation in the Orion Nebula Cluster: Exploring the Sub-stellar Limit

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We present results constraining the multiplicity of the very low mass stars and sub-stellar objects in the Orion Nebula Cluster (ONC). Our sample covers primary masses 0.012-0.1M⊙ using archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys using multiple filters. Studying the binary populations of clusters provides valuable constraints of how the birth environment affects binary formation and evolution. Prior surveys have shown that the binary populations of high-mass, high-density star clusters like the ONC may substantially differ from those in low-mass associations. Very low mass stellar and sub-stellar binaries at wide separations, >20AU, are statistically rare in the Galactic field and have been identified in stellar associations like Taurus-Auriga and Ophiuchus. They also may be susceptible to dynamical interactions, and their formation may be suppressed by feedback from on-going star formation. We implement a double point-spread function (PSF) fitting algorithm using empirical, position dependent PSF models to search for binary companions at projected separations > 10 AU (25 mas). With this technique, we identify 7 very low mass binaries, 5 of which are new detections, resulting in a binary frequency of 12⁺⁶_−3.2% over mass ratios of 0.5 - 1.0 and projected separations of 20 - 200 AU. We find an excess of very low mass binaries in the ONC compared to the Galactic field, with a probability of 10⁻⁶ that the populations are statistically consistent. The sub-stellar population of the ONC may require further dynamical processing of the lowest binding energy binaries to resemble the field population

Underlined pair is the least massive of the batch.

Precise masses for nine exoplanet candidates confirm their planetary/substellar nature. Brown dwarf HD 221420 b is confirmed too and turns out farther away but less eccentric than previously thought as well as suitable for imaging search.

Precise Masses and Orbits for Nine Radial Velocity Exoplanets

Quote :

Radial velocity (RV) surveys have discovered hundreds of exoplanetary systems but suffer from a fundamental degeneracy between planet mass Mp and orbital inclination i. In this paper we break this degeneracy by combining RVs with complementary absolute astrometry taken from the Gaia EDR3 version of the cross-calibrated Hipparcos-Gaia Catalog of Accelerations (HGCA). We use the Markov Chain Monte Carlo orbit code orvara to simultaneously fit literature RVs and absolute astrometry from the HGCA. We constrain the orbits, masses, and inclinations of nine single and massive RV companions orbiting nearby G and K stars. We confirm the planetary nature of six companions: HD 29021 b (4.47^+0.67_−0.65 M_Jup), HD 81040 b (7.24^+1.0_−0.37 M_Jup), HD 87883 b (6.31^+0.31_−0.32 M_Jup), HD 98649 b (9.7^+2.3_−1.9 M_Jup), HD 106252 b (10.00^+0.78_-0.73 M_Jup), and HD 171238 b (8.8^+3.6_−1.3 M_Jup). We place one companion, HD 196067 b (12.5^+2.5_−1.8 M_Jup) on the planet-brown dwarf boundary, and two companions in the low mass brown dwarf regime: HD 106515 Ab (18.9^+1.5_−1.4 M_Jup), and HD 221420 b (20.6^+2.0_−1.6 M_Jup). The brown dwarf HD 221420 b, with a semi-major axis of 9.99^+0.74_−0.70 AU, a period of 27.7^+3.0_−2.5 years, and an eccentricity of 0.162^+0.035_−0.030 represents a promising target for high-contrast imaging. The RV orbits of HD 87883 b, HD 98649 b, HD 171238 b, and HD 196067 b are not fully constrained yet because of insufficient RV data. We find two possible inclinations for each of these orbits due to difficulty in separating prograde from retrograde orbits, but we expect this will change decisively with future Gaia data releases.