.
When a massive star reaches the end of its lifetime, its core collapses and releases neutrinos that drive a shock into the outer layers (the stellar envelope). A sufficiently strong shock ejects the envelope, producing a supernova. If the shock fails to eject it, the envelope is predicted to fall back onto the collapsing core, producing a stellar-mass black hole (BH) and causing the star to disappear. We report observations of M31-2014-DS1, a hydrogen-depleted supergiant in the Andromeda Galaxy. In 2014, it brightened in the mid-infrared, then from 2017 to 2022, it faded by factors of [Formula: see text] in optical light (becoming undetectable) and [Formula: see text] in total light. We interpret these observations, and those of a previous event in NGC 6946, as evidence for failed supernovae forming stellar-mass BHs.
Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2 is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra ( R ∼ 2700, 2.87−5.13 μ m) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2 b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major moving group (414 ± 23 Myr) using BANYAN Σ and its full kinematics. We also reestimate the astrometry of COCONUTS-2b using the MIRI data. We support this membership with a comparison of the rotation period, metallicity, and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which, combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2 b is a planetary-mass object (7.5 ± 0.4 M _Jup ), which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7 ± 0.2 M _Jup ). We identify absorption feature differences, which could indicate that 0825+2805 has stronger vertical mixing.
We present near-infrared parallax and proper motion astrometry for 74 L dwarfs and 99 T dwarfs, as single objects or in binary systems, obtained with the ASTROCAM astrometric imager on the USNO, Flagstaff Station 1.55 m telescope over two observing periods. For all 173 objects, the median number of observational epochs was 62 with a median time frame of 5.25 yr, resulting in median uncertainties of sigma(pi abs) = 1.51 mas, sigma(mu abs) = 1.02 mas yr-1, and sigma( Vtan ) = 1.01 km s-1. Our observations provide the first parallax/proper-motion results for 16 objects and the highest-precision parallaxes/proper motions for an additional 116 objects. A serendipitous overlap of 40 objects with Gaia DR3 astrometry allows for direct comparison and confirmation of our results, along with an investigation on the effects of resolved binarity on astrometric results. We also provide a uniform set of J-, H-, KS-band photometry in the UKIRT/MKO system, most of it being from new observations. We use these results to examine objects included in this study of special-interest populations, consisting of binaries, wide companions, young objects, subdwarfs, and brown dwarf spectral standards.
We present a uniform atmospheric retrieval analysis of 22 late-T and Y-type brown dwarfs within 20 pc, observed with the James Webb Space Telescope NIRSpec PRISM and MIRI low-resolution spectrometer. This dataset provides the first continuous similar to 0.95-12 mu m spectroscopic coverage of late-T and Y-type brown dwarfs, which in turn enables precise constraints on their thermal structures and volume mixing ratios (VMRs) of H2O, CH4, CO, CO2, NH3, H2S, K, Na, and PH3. We find positive correlations between the VMR of H2O and CH4, and CO and CO2, consistent with thermochemical equilibrium chemistry. Using the VMRs, we derive atmospheric metallicity, which is positively correlated with H2O and CH4, showing H2O and CH4 trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H)bulk and (C/H)bulk. We also report tentative PH3 detections in roughly half the sample, suggesting potential vertical mixing or nonequilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning similar to 6-77 MJupN and similar to 0.66-1.53 RJupN , respectively. We compare the derived log10(g) (similar to 4-5.5 (cm s-2)) and Teff (similar to 350-1100 K) with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr amongst the sample. Comparing our retrieved thermal profiles with the Elf-Owl forward model thermal profiles, we find a systematic difference between the two, likely arising due to the difference in chemistry treatment.
We present new spectroscopic data for Gaia DR3 2309499817384726016 (WD 0008-350A) and its two wide, comoving, low-mass companions. We confirm the white dwarf is a hydrogen rich DA, with T _eff = 6200 ± 90 K and a mass of 0.63 ± 0.03 M _⊙ , close to that of the average white dwarf. Near-infrared spectra of the two stellar companions to WD 0008-350A reveal that the inner companion is an M dwarf, exhibiting a spectral type of M8. Furthermore, the outer companion is identified as a possible M6 + M9 binary. This paper examines the evidence which suggests the system may be quadruple.
