Year: 2026
Type: preprint
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; $λ\in$ [2.87-5.27] $μ$m) detects the companion at the expected orbital position with incredible significance (265.2$σ$). 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$_\mathrm{hole}$ = 0), a small fractional depletion in ammonia and moderate enhancement in hydrogen sulfide.
Source: arXiv (Cornell University)
Authors: Madurowicz, Alexander, Rickman, Emily, Gagliuffi, Daniella Bardalez, Mang, James, Ward-Duong, Kim, Balmer, William O., Bowler, Brendan P., Calamari, Emily, Dennen, Henry, Faherty, Jacqueline K, Franson, Kyle, Giovinazzi, Mark R., Quevedo, Lisseth Gonzales, Hoch, Kielan K. W., Lentz, Piper, Manjavacas, Elena, Matuszewska, Klara, Messier, Ashley, Morley, Caroline V., Nasedkin, Evert, Perrin, Marshall, Pueyo, Laurent, Ruffio, Jean-Baptiste, Theissen, Christopher A., Xuan, Jerry W.
Institutions: American Museum of Natural History, Amherst College, The University of Texas at Austin, Smith College, Northwestern University, University of California, Santa Barbara, University of Massachusetts Amherst, Space Telescope Science Institute, Johns Hopkins University, Trinity College Dublin, University of California San Diego, University of California, Los Angeles
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7212296971