Panchromatic characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 using JWST/MIRI

TL;DR

This work targets the atmospheric characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 by performing a joint retrieval over JWST/MIRI mid-infrared spectra and near-infrared data from HST/WFC3 and Gemini/GNIRS. Using Neural Posterior Estimation with a cloud-free petitRADTRANS forward model, the authors constrain not only the bulk physical parameters (Teff, log g, mass, radius, luminosity, and age) but also the molecular abundances, including CO, CO2, and 15NH3, revealing disequilibrium chemistry driven by vertical mixing. The combined analysis yields a consistent Teff ≈ 402 K, log g ≈ 4.43, mass ≈ 13 M_Jup, radius ≈ 1.14 R_Jup, and log L/L_sun ≈ -6.52, with C/O ≈ 1.35 and [M/H] ≈ 0.34 (subject to oxygen sequestration corrections that bring these closer to solar values). The results illustrate the power of broad wavelength coverage to tighten constraints on the $P$–$T$ profile and chemical abundances, reduce biases relative to single-wavelength studies, and reveal atmospheric processes that are not captured by equilibrium models, thereby advancing our understanding of cold substellar atmospheres and their evolution.

Abstract

Cold brown dwarf atmospheres are good training grounds for analyzing temperate giant planets. WISEP J173835.52+273258.9 (WISE 1738) is an isolated Y0 brown dwarf with a temperature between 350-400 K, at the T-Y transition. While its near-infrared spectrum has been studied, bulk properties and chemistry remain uncertain. We analyze new JWST MIRI medium-resolution spectra (5-18 micron), combined with near-infrared spectra (0.98-2.2 micron) from HST/WFC3 and Gemini/GNIRS, to better constrain WISE 1738's atmosphere and physical parameters. We use Neural Posterior Estimation (NPE) with a cloud-free petitRADTRANS model and evaluate results using posterior checks, coverage, and L-C2ST diagnostics. Our retrieval confirms previous constraints on H2O, CH4, and NH3, and for the first time constrains CO, CO2, and 15NH3. We find evidence of disequilibrium chemistry through CO and CO2 abundances not expected under equilibrium. Estimated properties are temperature 402 (+12,-9) K, log g 4.43 (+0.26,-0.34) cm/s2, mass 13 (+11,-7) M_Jup, radius 1.14 (+0.03,-0.03) R_Jup, and bolometric luminosity -6.52 (+0.05,-0.04) log L/L_sun. Evolutionary models suggest an age between 1 and 4 Gyr, consistent with a 6-hour rotation. We place an upper bound on 15NH3, implying a 3-sigma lower limit on the 14N/15N ratio of 275. We also derive a C/O ratio of 1.35 (+0.39,-0.31) and metallicity of 0.34 (+0.12,-0.11), without accounting for oxygen sequestration.

Figures (12)

• Figure 1: Top. WFC3 ($J$ and $H$ bands, red), GNIRS ($Y$, $J$, $H$ and $K$ bands, black), and MIRI (black) observations $x_\text{obs}$, overlaid with the simulated noiseless spectrum $f(\theta)$ associated with the most probable parameters from the posterior. Bottom. Residuals of the sample normalized by the inflated standard deviation of the noise distribution for each spectral channel.
• Figure 2: Left. Cloud-free retrieval using neural posterior estimation on the WISE 1738 spectra. The corner plot shows the full 1D and 2D marginal posterior distributions obtained for the WISE 1738 observations $x_\text{obs}$, WFC3+GNIRS+MIRI spectra.
• Figure 3: Comparing individual spectral retrievals of WISE 1738 across different wavelength regions with the combined retrievals. The corner plot shows 1D and 2D marginal posterior distributions obtained for the WFC3, GNIRS and MIRI spectrum along with the combined WFC3+GNIRS+MIRI spectra. The top right figure illustrates the posterior distribution of the $P-T$ profile of the combined retrieval, while also highlighting the 99.7% credible intervals of the three independent retrievals.
• Figure 4: Chemical equilibrium abundances for a solar-composition atmosphere, calculated using the retrieved most probable $P$-$T$ profile. These are compared with the retrieved molecular abundances in mass fractions (in dashed lines, including 1$\sigma$ uncertainties as colored bars) for key opacity-contributing species: H$_2$O (red), CO$_2$ (green), CO (purple), CH$_4$ (brown), and NH$_3$ (orange). The grey regions are the estimated 1$\sigma$ of the emission contribution functions for the MIRI probed photosphere (at the top) and the near-infrared probed photosphere (at the bottom).
• Figure 5: Top. WFC3+GNIRS+MIRI consistency plot. The posterior predictive distribution $p(f(\theta)+ \epsilon |x_\text{obs})$ of noisy simulations spectra for the $99.7\%$, $95\%$ and $68.7\%$ quartiles (hues of blue), overlaid on top of the the WFC3+GNIRS+MIRI observation $x_\text{obs}$ (black line). Bottom. Residuals of the posterior predictive samples, normalized by the inflated standard deviation of the noise distribution for each spectral channel and a horizontal line at 0 for reference (in black).