JWST Edge-on Disk Ice (JEDIce): Vibrationally Hot, Rotationally Cold H 2 in the Outer Disk of Oph 163131 Nonthermally Excited by Ultraviolet and Cosmic Rays
Abstract
Constraining ionization and excitation processes in protoplanetary disks is essential for understanding the chemical structure and evolution of disk material, shaping planet formation pathways. We present JWST/NIRSpec integral-field-unit observations of the edge-on disk Oph 163131, which reveal an unusual rovibrational H 2 spectrum dominated by the 1–0 O(2) line (2.627 μ m), with suppressed higher- J emission despite excitation to v = 2 and 3. This vibrationally hot, rotationally cold H 2 emission is spatially extended, broadly following the molecular disk traced by CO( J = 2–1), with emission increasing above and below a thin midplane dark lane and extending radially beyond ∼200 au, where near-IR scattered-light emission is no longer dominant. We interpret the observed H 2 emission as arising from nonthermal excitation in cold, dense outer-disk gas, where collisions depopulate higher- J rotational levels within each vibrational manifold prior to emission, producing the characteristic “ v -hot, J -cold” spectrum. We consider both ultraviolet irradiation and cosmic-ray excitation as contributors to the H 2 emission and find that their combined action, together with collisional deexcitation of high- J level populations, broadly reproduces the observed line ratios and morphology. Within this framework, we infer a rather high effective cosmic-ray ionization rate of ∼(1–10) × 10 −15 s −1 in the presence of a moderate UV field ( χ UV = 100−1000, in Draine units). These results for disks, together with the recent findings by Bialy et al. for the lower-density starless core B68, highlight the potential of ro-vibrational H 2 emission as a novel probe of cosmic-ray ionization.
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BibTeX
@article{assani2026jwst,
title = {{JWST Edge-on Disk Ice (JEDIce): Vibrationally Hot, Rotationally Cold H 2 in the Outer Disk of Oph 163131 Nonthermally Excited by Ultraviolet and Cosmic Rays}},
author = {Korash Assani and Zhi-Yun Li and Jennifer Bergner and David Neufeld and Daniel Harsono and Maria Drozdovskaya and Marco Padovani and Emmanuel Dartois and Jennifer A. Noble and Nicole Arulanantham and Alice Booth and Yao-Lun Yang and Mayank Narang and Will Thompson and Elizabeth Yunerman and Karin Öberg and Julia Santos and Charles Mentzer and Jon Ramsey and Lukas Welzel and Klaus Pontoppidan and Melissa Mcclure},
journal = {The Astronomical Journal},
year = {2026},
month = aug,
doi = {10.3847/1538-3881/ae947a},
eprint = {2607.09407},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05850293v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - JWST Edge-on Disk Ice (JEDIce): Vibrationally Hot, Rotationally Cold H 2 in the Outer Disk of Oph 163131 Nonthermally Excited by Ultraviolet and Cosmic Rays
AU - Korash Assani
AU - Zhi-Yun Li
AU - Jennifer Bergner
AU - David Neufeld
AU - Daniel Harsono
AU - Maria Drozdovskaya
AU - Marco Padovani
AU - Emmanuel Dartois
AU - Jennifer A. Noble
AU - Nicole Arulanantham
AU - Alice Booth
AU - Yao-Lun Yang
AU - Mayank Narang
AU - Will Thompson
AU - Elizabeth Yunerman
AU - Karin Öberg
AU - Julia Santos
AU - Charles Mentzer
AU - Jon Ramsey
AU - Lukas Welzel
AU - Klaus Pontoppidan
AU - Melissa Mcclure
PY - 2026
DA - 2026/08/24
JO - The Astronomical Journal
DO - 10.3847/1538-3881/ae947a
UR - https://hal.science/hal-05850293v1
AB - Constraining ionization and excitation processes in protoplanetary disks is essential for understanding the chemical structure and evolution of disk material, shaping planet formation pathways. We present JWST/NIRSpec integral-field-unit observations of the edge-on disk Oph 163131, which reveal an unusual rovibrational H 2 spectrum dominated by the 1–0 O(2) line (2.627 μ m), with suppressed higher- J emission despite excitation to v = 2 and 3. This vibrationally hot, rotationally cold H 2 emission is spatially extended, broadly following the molecular disk traced by CO( J = 2–1), with emission increasing above and below a thin midplane dark lane and extending radially beyond ∼200 au, where near-IR scattered-light emission is no longer dominant. We interpret the observed H 2 emission as arising from nonthermal excitation in cold, dense outer-disk gas, where collisions depopulate higher- J rotational levels within each vibrational manifold prior to emission, producing the characteristic “ v -hot, J -cold” spectrum. We consider both ultraviolet irradiation and cosmic-ray excitation as contributors to the H 2 emission and find that their combined action, together with collisional deexcitation of high- J level populations, broadly reproduces the observed line ratios and morphology. Within this framework, we infer a rather high effective cosmic-ray ionization rate of ∼(1–10) × 10 −15 s −1 in the presence of a moderate UV field ( χ UV = 100−1000, in Draine units). These results for disks, together with the recent findings by Bialy et al. for the lower-density starless core B68, highlight the potential of ro-vibrational H 2 emission as a novel probe of cosmic-ray ionization.
SN - 0004-6256
SN - 1538-3881
ER -