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We present results from conducting a theoretical chemical analysis of a sample of benchmark companion brown dwarfs whose primary star is of type F, G, or K. We summarize the entire known sample of these types of companion systems, termed “compositional benchmarks,” that are present in the literature or recently published as key systems of study in order to best understand brown dwarf chemistry and condensate formation. Via mass balance and stoichiometric calculations, we predict a median brown dwarf atmospheric oxygen sink of${17.8}_{-2.3}^{+1.7}\%$by utilizing published stellar abundances in the local solar neighborhood. Additionally, we predict a silicate condensation sequence such that atmospheres with bulk Mg/Si ≲0.9 will form enstatite (MgSiO₃) and quartz (SiO₂) clouds, and atmospheres with bulk Mg/Si ≳0.9 will form enstatite and forsterite (Mg₂SiO₄) clouds. The implications of these results on C/O ratio trends in substellar-mass objects and the utility of these predictions in future modeling work are discussed.

 

At the lowest masses, the distinction between brown dwarfs and giant exoplanets is often blurred and literature classifications rarely reflect the deuterium burning boundary. Atmospheric characterization may reveal the extent to which planetary formation pathways contribute to the population of very low mass brown dwarfs, by revealing whether their abundance distributions differ from those of the local field population or, in the case of companions, their primary stars. The T8 dwarf Ross 458c is a possible planetary-mass companion to a pair of M dwarfs, and previous work suggests that it is cloudy. We here present the results of the retrieval analysis of Ross 458c, using archival spectroscopic data in the 1.0–2.4 µm range. We test a cloud-free model as well as a variety of cloudy models and find that the atmosphere of Ross 458c is best described by a cloudy model (strongly preferred). The CH4/H2O is higher than expected at $1.97^{+0.13}_{-0.14}$. This value is challenging to understand in terms of equilibrium chemistry and plausible carbon-to-oxygen (C/O) ratios. Comparisons to thermochemical grid models suggest a C/O of ≈1.35, if CH4 and H2O are quenched at 2000 K, requiring vigorous mixing. We find a [C/H] ratio of +0.18, which matches the metallicity of the primary system, suggesting that oxygen is missing from the atmosphere. Even with extreme mixing, the implied C/O is well beyond the typical stellar regime, suggesting either a non-stellar formation pathway or the sequestration of substantial quantities of oxygen via hitherto unmodelled chemistry or condensation processes.

 

We present results from an atmospheric retrieval analysis of Gl 229B using the Brewster retrieval code. We find the best fit model to be cloud-free, consistent with the T dwarf retrieval work of Line et al.; Zalesky et al. and Gonzales et al. Fundamental parameters (mass, radius, log(L_Bol/L_Sun), log(g)) determined from our model agree within 1σto SED-derived values, except forT_effwhere our retrievedT_effis approximately 100 K cooler than the evolutionary model-based SED value. We find a retrieved mass of${50}_{-9}^{+12}$M_Jup, however, we also find that the observables of Gl 229B can be explained by a cloud-free model with a prior on mass at the dynamical value, 70M_Jup. We are able to constrain abundances for H₂O, CO, CH₄, NH₃, Na and K and find a supersolar C/O ratio as compared to its primary, Gl 229A. We report an overall subsolar metallicity due to atmospheric oxygen depletion, but find a solar [C/H], which matches that of the primary. We find that this work contributes to a growing trend in retrieval-based studies, particularly for brown dwarfs, toward supersolar C/O ratios and discuss the implications of this result on formation mechanisms and internal physical processes, as well as model biases.

 

We present an atmospheric retrieval analysis of a pair of highly variable, ∼200 Myr old, early T type planetary-mass exoplanet analogs SIMP J01365662+0933473 and 2MASS J21392676+0220226 using the Brewster retrieval framework. Our analysis, which makes use of archival 1–15μm spectra, finds almost identical atmospheres for both objects. For both targets, we find that the data is best described by a patchy, high-altitude forsterite (Mg₂SiO₄) cloud above a deeper, optically thick iron (Fe) cloud. Our model constrains the cloud properties well, including the cloud locations and cloud particle sizes. We find that the patchy forsterite slab cloud inferred from our retrieval may be responsible for the spectral behavior of the observed variability. Our retrieved cloud structure is consistent with the atmospheric structure previously inferred from spectroscopic variability measurements, but clarifies this picture significantly. We find consistent C/O ratios for both objects, which supports their formation within the same molecular cloud in the Carina-Near moving group. Finally, we note some differences in the constrained abundances of H₂O and CO, which may be caused by data quality and/or astrophysical processes such as auroral activity and their differing rotation rates. The results presented in this work provide a promising preview of the detail with which we will characterize extrasolar atmospheres with JWST, which will yield higher-quality spectra across a wider wavelength range.

 

Abstract We have used data from the UKIRT Hemisphere Survey to search for substellar members of the Hyades cluster. Our search recovered several known substellar Hyades members, and two known brown dwarfs that we suggest may be members based on a new kinematic analysis. We uncovered thirteen new substellar Hyades candidates, and obtained near-infrared follow-up spectroscopy of each with IRTF/SpeX. Six candidates with spectral types between M7 and L0 are ruled out as potential members based on their photometric distances (≳100 pc). The remaining seven candidates, with spectral types between L5 and T4, are all potential Hyades members, with five showing strong membership probabilities based on BANYAN Σ and a convergent point analysis. Distances and radial velocities are still needed to confirm Hyades membership. If confirmed, these would be some of the lowest mass free-floating members of the Hyades yet known, with masses as low as ∼30 M Jup . An analysis of all known substellar Hyades candidates shows evidence that the full extent of the Hyades has yet to be probed for low-mass members, and more would likely be recovered with deeper photometric and astrometric investigations. 

We present the discovery of 34 comoving systems containing an ultracool dwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC’s angular resolution of ∼ 1″ allows for the detection of small separation binaries with significant proper motions. We used the catalog’s accurate proper motion measurements to identify the companions by cross-matching a previously compiled list of brown dwarf candidates with NSC DR2. The comoving pairs consist of either a very low-mass star and an ultracool companion, or a white dwarf and an ultracool companion. The estimated spectral types of the primaries are in the K and M dwarf regimes, those of the secondaries in the M, L, and T dwarf regimes. We calculated angular separations between ∼2″ and ∼ 56″, parallactic distances between ∼43 and ∼261 pc, and projected physical separations between ∼169 and ∼8487 au. The lowest measured total proper motion is 97 mas yr⁻¹, with the highest 314 mas yr⁻¹. Tangential velocities range from ∼23 to ∼187 km s⁻¹. We also determined comoving probabilities, estimated mass ratios, and calculated binding energies for each system. We found no indication of possible binarity for any component of the 34 systems in the published literature. The discovered systems can contribute to the further study of the formation and evolution of low-mass systems as well as to the characterization of cool substellar objects.

 

Abstract High-energy neutrinos are a promising tool for identifying astrophysical sources of high and ultra-high energy cosmic rays (UHECR). Prospects of detecting neutrinos at high energies (≳TeV) from blazars have been boosted after the recent association of IceCube-170922A and TXS 0506+056. We investigate the high-energy neutrino, IceCube-190331A, a high-energy starting event (HESE) with a high likelihood of being astrophysical in origin. We initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and followed up with ATCA radio observations, compiling a multiwavelength SED for the most likely source of origin. NuSTAR observations of the neutrino location and a nearby X-ray source were also performed. We find two promising counterpart in the 90% confidence localisation region and identify the brightest as the most likely counterpart. However, no Fermi/LAT γ-ray source and no prompt Swift/BAT source is consistent with the neutrino event. At this point it is unclear whether any of the counterparts produced IceCube-190331A. We note that the Helix Nebula is also consistent with the position of the neutrino event, and we calculate that associated particle acceleration processes cannot produce the required energies to generate a high-energy HESE neutrino.