30 research outputs found
Measuring and Replicating the 1-20 um Energy Distributions of the Coldest Brown Dwarfs: Rotating, Turbulent and Non-Adiabatic Atmospheres
Cold, low-mass, field brown dwarfs are important for constraining the
terminus of the stellar mass function, and also for optimizing atmospheric
studies of exoplanets. In 2020 new model grids for such objects were made
available: Sonora-Bobcat and ATMO 2020. Also, new candidate cold brown dwarfs
were announced, and new spectroscopic observations at lambda ~4.8 um were
published. In this paper we present new infrared photometry for some of the
coldest brown dwarfs, and put the new data and models together to explore the
properties of these objects. We reconfirm the importance of mixing in these
atmospheres, which leads to CO and NH_3 abundances that differ by orders of
magnitude from chemical equilibrium values. We also demonstrate that the new
models retain the known factor >~3 discrepancy with observations at 2 <~ lambda
um <~ 4, for brown dwarfs cooler than 600 K. We show that the entire 1 <~
lambda um <~ 20 energy distribution of six brown dwarfs with 260 <= T_eff K <=
475 can be well reproduced, for the first time, by model atmospheres which
include dis-equilibrium chemistry as well as a photospheric temperature
gradient which deviates from the standard radiative/convective equilibrium
value. This change to the pressure-temperature profile is not unexpected for
rotating and turbulent atmospheres which are subject to diabatic processes. A
limited grid of modified-adiabat model colors is generated, and used to
estimate temperatures and metallicities for the currently known Y dwarfs. A
compilation of the photometric data used here is given in the Appendix.Comment: 40 pages which includes 16 Figures and 10 Tables. The Journal
publication will include data behind the Figures for Figures 5, 8 and 9, and
a machine readable version of Table 1
Expanding the Y Dwarf Census with Spitzer Follow-up of the Coldest CatWISE Solar Neighborhood Discoveries
We present Spitzer 3.6 and 4.5 μm follow-up of 170 candidate extremely cool brown dwarfs newly discovered via the combination of Wide-field Infrared Survey Explorer (WISE) and NEOWISE imaging at 3–5 μm. CatWISE, a joint analysis of archival WISE and NEOWISE data, has improved upon the motion measurements of AllWISE by leveraging a >10× time baseline enhancement, from 0.5 yr (AllWISE) to 6.5 yr (CatWISE). As a result, CatWISE motion selection has yielded a large sample of previously unrecognized brown dwarf candidates, many of which have archival detections exclusively in the WISE 4.6 μm (W2) channel, suggesting that they could be both exceptionally cold and nearby. Where these objects go undetected in WISE W1 (3.4 μm), Spitzer can provide critically informative detections at 3.6 μm. Of our motion-confirmed discoveries, 17 have a best-fit Spitzer [3.6]–[4.5] color most consistent with spectral type Y. It is likely that CWISEP J144606.62–231717.8 (μ ≈ 1.”3 yr⁻¹) is the reddest, and therefore potentially coldest, member of our sample with a very uncertain [3.6]–[4.5] color of 3.71 ± 0.44 mag. We also highlight our highest proper-motion discovery, WISEA J153429.75–104303.3, with μ ≈ 2.”7 yr⁻¹. Given that the prior list of confirmed and presumed Y dwarfs consists of just 27 objects, the Spitzer follow-up presented in this work has substantially expanded the sample of identified Y dwarfs. Our new discoveries thus represent significant progress toward understanding the bottom of the substellar mass function, investigating the diversity of the Y dwarf population, and selecting optimal brown dwarf targets for James Webb Space Telescope spectroscopy
WISEA J083011.95+283716.0: A Missing Link Planetary-Mass Object
We present the discovery of WISEA J083011.95+283716.0, the first Y dwarf
candidate identified through the Backyard Worlds: Planet 9 citizen science
project. We identified this object as a red, fast-moving source with a faint
detection in multi-epoch \textit{AllWISE} and unWISE images. We have
characterized this object with Spitzer Space Telescope and \textit{Hubble Space
Telescope} follow-up imaging. With mid-infrared detections in
\textit{Spitzer}'s \emph{ch1} and \emph{ch2} bands and flux upper limits in
Hubble Space Telescope and filters, we find that this object is
both very faint and has extremely red colors ( mag,
mag), consistent with a T K source, as
estimated from the known Y dwarf population. A preliminary parallax provides a
distance of pc, leading to a slightly warmer temperature
of K. The extreme faintness and red Hubble Space Telescope and
Spitzer Space Telescope colors of this object suggest it may be a link between
the broader Y dwarf population and the coldest known brown dwarf WISE
J08550714, and highlight our limited knowledge of the true spread of Y dwarf
colors. We also present four additional Backyard Worlds: Planet 9 late-T brown
dwarf discoveries within 30 pc.Comment: 13 pages, 6 figures, 5 table