Water depletion and 15NH3 in the atmosphere of the coldest brown dwarf observed with JWST/MIRI

Abstract

With a temperature of $\sim 285$ K WISE0855 is the coldest brown dwarf observed so far. Using the James Webb Space Telescope (JWST) we obtained observations that allow us to characterize WISE0855s atmosphere focusing on vertical variation in the water steam abundance, measuring trace gas abundances and receiving bulk parameters for this cold object. We observed the ultra cool dwarf WISE0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer (MIRI/MRS) onboard JWST at a spectral resolution of up to 3750. We combined the observation with published data from the Near Infrared Spectrograph (NIRSpec) G395M and PRISM modes yielding a spectrum ranging from 0.8 to 22 um. We apply atmospheric retrievals using petitRADTRANS to measure atmospheric abundances, the pressure-temperature structure, radius and gravity of the brown dwarf. We also employ publicly available clear and cloudy self-consistent grid models to estimate bulk properties of the atmosphere such as the effective temperature, radius, gravity and metallicity. Atmospheric retrievals constrain a variable water abundance profile in the atmosphere, as predicted by equilibrium chemistry. We detect the 15NH3 isotopologue and infer a ratio of mass fraction of 14NH3/15NH3 = 332+63-43 for the clear retrieval. We measure the bolometric luminosity by integrating the presented spectrum and obtain a value of log(L/L${}_{\odot }$) = -7.291+/-0.008. The detected water depletion indicates that water condenses out in the upper atmosphere due to the very low effective temperature of WISE0855. The height in the atmosphere where this occurs is covered by the MIRI/MRS data, and thus demonstrates the potential of MIRI to characterize cold gas giants atmospheres. Comparing the data to retrievals and self-consistent grid models, we do not detect signs for water ice clouds, although their spectral features have been predicted in previous studies.

Submitted to A&A, 29 pages, 21 figures

References in corpus (38)

• petitRADTRANS: a Python radiative transfer package for exoplanet characterization and retrieval
• The James Webb Space Telescope Mission
• The Science Performance of JWST as Characterized in Commissioning
• Atmospheric Retrieval for Super-Earths: Uniquely Constraining the Atmospheric Composition with Transmission Spectroscopy
• A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets
• Fingering convection and cloudless models for cool brown dwarf atmospheres
• JWST MIRI flight performance: The Medium-Resolution Spectrometer
• Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e
• Observing transiting planets with JWST -- Prime targets and their synthetic spectral observations
• Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D
• Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun
• Water Clouds in Y Dwarfs and Exoplanets
• In-orbit Performance of the Near-Infrared Spectrograph NIRSpec on the James Webb Space Telescope
• Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs I: Characterizing Benchmarks, Gl570D and HD3651B
• A 3D Drizzle Algorithm for JWST and Practical Application to the MIRI Medium Resolution Spectrometer
• Abundant molecular gas and inefficient star formation in intracluster regions: Ram pressure stripped tail of the Norma galaxy ESO137-001
• The 13O-rich atmosphere of a young accreting super-Jupiter
• Cloud busting: enstatite and quartz clouds in the atmosphere of 2M2224-0158
• The Sonora Substellar Atmosphere Models. IV. Elf Owl: Atmospheric Mixing and Chemical Disequilibrium with Varying Metallicity and C/O Ratios
• Observations of Disequilibrium CO Chemistry in the Coldest Brown Dwarfs
• Near-infrared photometry of Y dwarfs: low ammonia abundance and the onset of water clouds
• Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans
• Patchy Forsterite Clouds in the Atmospheres of Two Highly Variable Exoplanet Analogs
• A temperate super-Jupiter imaged with JWST in the mid-infrared
• Self-consistent Models of Y Dwarf Atmospheres with Water Clouds and Disequilibrium Chemistry
• Atmospheric retrievals with petitRADTRANS
• Indications of Water Clouds in the Coldest Known Brown Dwarf
• 15NH3 in the atmosphere of a cool brown dwarf
• A blue depression in the optical spectra of M dwarfs
• The Nature of Transition Blazars
• Exploring the Extremes: Characterizing a New Population of Old and Cold Brown Dwarfs
• Photometric Monitoring of the Coldest Known Brown Dwarf with the Spitzer Space Telescope
• Methane Emission From a Cool Brown Dwarf
• Retrieval study of cool, directly imaged exoplanet 51 Eri b
• NEOWISE-R Observation of the Coolest Known Brown Dwarf
• Towards Robust Atmospheric Retrieval on Cloudy L Dwarfs: The Impact of Thermal and Abundance Profile Assumptions
• Microphysics of Water Clouds in the Atmospheres of Y Dwarfs and Temperate Giant Planets
• Jupiter and Saturn as Spectral Analogs for Extrasolar Gas Giants and Brown Dwarfs

Cited by in corpus (9)

• Evidence for SiO cloud nucleation in the rogue planet PSO J318
• The JWST Weather Report: retrieving temperature variations, auroral heating, and static cloud coverage on SIMP-0136
• Extended source fringe flats for the JWST MIRI Medium Resolution Spectrometer
• JWST/MIRI observations of the young TWA 27 system: hydrocarbon disk chemistry, silicate clouds, evidence for a CPD
• Mysteries of Capotauro: investigating the puzzling nature of an extreme F356W-dropout
• Disentangling disc and atmospheric signatures of young brown dwarfs with JWST/NIRSpec
• Time-resolved JWST Retrieval Analysis of the Coldest Brown Dwarf: Temperature and Chemical Variations in WISE 0855-07
• Panchromatic characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 using JWST/MIRI
• Pre-computed aerosol extinction, scattering and asymmetry grids for scalable atmospheric retrievals