The Dyn-Atmo Survey: JWST/NIRSpec spectroscopy of dynamical benchmark GJ 758 B
Abstract
We present new JWST/NIRSpec IFU high contrast spectroscopic observations of the brown dwarf companion GJ 758 B. Extensive radial velocity monitoring of the primary has enabled high-precision measurements of the companion mass through characterization of their joint orbital dynamics. The combination of prior space- and ground-based photometry and spectroscopic observations in concert with a spectral model-independent mass makes this target an ideal benchmark for calibrating evolutionary models with known mass-age-luminosity degeneracies. A template-matching analysis on the NIRSpec/IFU data (F290LP,G395H; λ∈ [2.87-5.27] μm) detects the companion at the expected orbital position with incredible significance (265.2σ). Comprehensive spectral analysis including published datasets over a variety of different spectral model families reveal that an inference using a uniform radius prior is plagued by inaccurate posteriors. The atmospheric model degeneracy between metallicity, gravity, and clouds pull the inferred mass/gravity/radius to an unphysical portion of parameter space for all model families tested. A constrained inference with a gaussian prior on the mass can mitigate this issue but models without clouds still struggle to accurately reproduce the K-band photometry and result in mass inferences which are inconsistent with the dynamics. We explore modifying the spectral models with custom post-processed clouds and molecular abundance perturbations of ammonia and hydrogen sulfide. We find the best-fitting model includes clouds with no patchiness (hole fraction fhole = 0), a small fractional depletion in ammonia and moderate enhancement in hydrogen sulfide.
Related
- The Dyn-Atmo Survey: High-contrast Imaging Spectroscopy of the Substellar Companion HD 13724 B with the JWST NIRSpec IFU
- MOCSS: Multi-Object Coronagraphy for JWST/NIRSpec Slitless Spectroscopy
- Source--Line Associations in JWST/NIRSpec Dense-Shutter Spectroscopy
- Metal-poor Brown Dwarf Kinematics from JWST NIRSpec Spectroscopy
- Metal-poor Brown Dwarf Kinematics from JWST NIRSpec Spectroscopy
BibTeX
@misc{madurowicz2026dynatmo,
title = {{The Dyn-Atmo Survey: JWST/NIRSpec spectroscopy of dynamical benchmark GJ 758 B}},
author = {Alexander Madurowicz and Emily Rickman and Daniella Bardalez Gagliuffi and James Mang and Kim Ward-Duong and William O. Balmer and Brendan P. Bowler and Emily Calamari and Henry Dennen and Jacqueline K Faherty and Kyle Franson and Mark R. Giovinazzi and Lisseth Gonzales Quevedo and Kielan K. W. Hoch and Piper Lentz and Elena Manjavacas and Klara Matuszewska and Ashley Messier and Caroline V. Morley and Evert Nasedkin and Marshall Perrin and Laurent Pueyo and Jean-Baptiste Ruffio and Christopher A. Theissen and Jerry W. Xuan},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.11828},
eprint = {2609.11828},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.11828},
}
RIS
TY - UNPB
TI - The Dyn-Atmo Survey: JWST/NIRSpec spectroscopy of dynamical benchmark GJ 758 B
AU - Alexander Madurowicz
AU - Emily Rickman
AU - Daniella Bardalez Gagliuffi
AU - James Mang
AU - Kim Ward-Duong
AU - William O. Balmer
AU - Brendan P. Bowler
AU - Emily Calamari
AU - Henry Dennen
AU - Jacqueline K Faherty
AU - Kyle Franson
AU - Mark R. Giovinazzi
AU - Lisseth Gonzales Quevedo
AU - Kielan K. W. Hoch
AU - Piper Lentz
AU - Elena Manjavacas
AU - Klara Matuszewska
AU - Ashley Messier
AU - Caroline V. Morley
AU - Evert Nasedkin
AU - Marshall Perrin
AU - Laurent Pueyo
AU - Jean-Baptiste Ruffio
AU - Christopher A. Theissen
AU - Jerry W. Xuan
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.11828
UR - https://arxiv.org/abs/2609.11828
AB - We present new JWST/NIRSpec IFU high contrast spectroscopic observations of the brown dwarf companion GJ 758 B. Extensive radial velocity monitoring of the primary has enabled high-precision measurements of the companion mass through characterization of their joint orbital dynamics. The combination of prior space- and ground-based photometry and spectroscopic observations in concert with a spectral model-independent mass makes this target an ideal benchmark for calibrating evolutionary models with known mass-age-luminosity degeneracies. A template-matching analysis on the NIRSpec/IFU data (F290LP,G395H; λ∈ [2.87-5.27] μm) detects the companion at the expected orbital position with incredible significance (265.2σ). Comprehensive spectral analysis including published datasets over a variety of different spectral model families reveal that an inference using a uniform radius prior is plagued by inaccurate posteriors. The atmospheric model degeneracy between metallicity, gravity, and clouds pull the inferred mass/gravity/radius to an unphysical portion of parameter space for all model families tested. A constrained inference with a gaussian prior on the mass can mitigate this issue but models without clouds still struggle to accurately reproduce the K-band photometry and result in mass inferences which are inconsistent with the dynamics. We explore modifying the spectral models with custom post-processed clouds and molecular abundance perturbations of ammonia and hydrogen sulfide. We find the best-fitting model includes clouds with no patchiness (hole fraction f_hole = 0), a small fractional depletion in ammonia and moderate enhancement in hydrogen sulfide.
ER -