Testing Young Brown Dwarf Atmospheric Properties with L-Band Spectroscopy

One of the greatest challenges in the study of directly-imaged imaged exoplanets has been the determination of their atmospheric properties from observed spectroscopy and photometry. In particular, the extremely red near-IR colors and lack of methane for exoplanets have been difficult to model. There are several explanations for these properties of directly-imaged exoplanets, such as thermo-chemical instability, disequilibrium chemistry, and dust clouds. The difficulty in observing exoplanet atmospheres, however, have limited the wavelength coverage and detail with which these theories have been tested. With the intent to test these theories, we use young brown dwarf spectra as proxies for these exoplanet spectra. The objects we have chosen are of spectral types, masses, and ages that overlap with the red directly-imaged exoplanets. Specifically we are looking at the L-band where the 3.3 micron methane feature provides a sensitive probe for disequilibrium chemistry and the spectral slope probes for dust-grain size. We detect methane at spectral type L4 and later, which matches the onset of methane in field brown dwarfs. This work showcases the diagnostic power of L-band spectroscopy and the potential for observations with future facilities (such as JWST) to aid in our understanding of planetary atmospheres

Probing the Heights and Depths of Y Dwarf Atmospheres : A Retrieval Analysis of the JWST Spectral Energy Distribution of WISE J035934.06−-540154.6

© 2024. The Author(s). Published by the American Astronomical Society. This article is licensed under a Creative Commons Attribution 4.0 International License, To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06$-$540154.6 using the low-resolution 0.96--12 $\mu$m JWST spectrum presented in \citet{Beiler_2023}. We obtain volume number mixing ratios of the major gas-phase absorbers (H$_2$O, CH$_4$, CO, CO$_2$, PH$_3$, and H$_2$S) that are 3--5$\times$ more precise than previous work that used HST spectra. We also find an order-of-magnitude improvement in the precision of the retrieved thermal profile, a direct result of the broad wavelength coverage of the JWST data. We used the retrieved thermal profile and surface gravity to generate a grid of chemical forward models with varying metallicity, (C/O)$_\textrm{atm}$, and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient $K_\textrm{zz}$. Comparison of the retrieved abundances with this grid of models suggests that the deep atmosphere of WISE 0359$-$54 shows signs of vigorous vertical mixing with $K_\textrm{zz}=10^9$ [cm$^{2}$ s$^{-1}$]. To test the sensitivity of these results to our 5-knot spline thermal profile model, we performed a second retrieval using the \citet{Madhusudhan_2009} thermal profile model. While the results of the two retrievals generally agree well, we do find differences between the retrieved values of mass and volume number mixing ratio of H$_2$S with fractional differences of the median values of $-$0.64 and $-$0.10, respectively. In addition, the 5-knot thermal profile is consistently warmer at pressure between 1 and 70 bar. Nevertheless, our results underscore the power that the broad-wavelength infrared spectra obtainable with the James Webb Space Telescope have to characterize the atmospheres of cool brown dwarfs.Peer reviewe

An L Band Spectrum of the Coldest Brown Dwarf

The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. (2016) presented a spectrum of WISE 0855 from 4.5–5.1 µm (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in L band, from 3.4–4.14 µm. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. JWST will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes

An L Band Spectrum of the Coldest Brown Dwarf

The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. 2016 presented a spectrum of WISE 0855 from 4.5-5.1 micron (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in L band, from 3.4-4.14 micron. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. JWST will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.Comment: 19 pages, 21 figures. Accepted for publication in Ap

The First JWST Spectral Energy Distribution of a Y Dwarf

We present the first JWST spectral energy distribution of a Y dwarf. This spectral energy distribution of the Y0 dwarf WISE J035934.06−540154.6 consists of low-resolution ( λ /Δ λ ∼100) spectroscopy from 1–12 μ m and three photometric points at 15, 18, and 21 μ m. The spectrum exhibits numerous fundamental, overtone, and combination rotational–vibrational bands of H _2 O, CH _4 , CO, CO _2 , and NH _3 , including the previously unidentified ν _3 band of NH _3 at 3 μ m. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21 μ m, we measure a bolometric luminosity of 1.523 ± 0.090 × 10 ^20 W. We determine a semiempirical effective temperature estimate of ${467}_{-18}^{+16}$ K using the bolometric luminosity and evolutionary models to estimate a radius. Finally, we compare the spectrum and photometry to a grid of atmospheric models and find reasonably good agreement with a model having T _eff = 450 K, log g = 3.25 [cm s ^−2 ], and [M/H] = −0.3. However, the low surface gravity implies an extremely low mass of 1 M _Jup and a very young age of 20 Myr, the latter of which is inconsistent with simulations of volume-limited samples of cool brown dwarfs