Physical conditions in PDRs revealed by IGRINS ro-vibrational H2 observations
Abstract
We compare H2 high ro-vibrational observations of 5 PDRs (S140, IC63, Horsehead Nebula, NGC 2023 and Orion Bar) obtained with the IGRINS spectrograph to PDR models produced with the Meudon PDR code, in order to estimate the physical conditions and to constrain the physical and chemical processes that govern PDRs. We use newly flux-calibrated and extinction-corrected IGRINS H2 observations covering the 1.45-2.45 um range at a resolution of 45000, and adjust Meudon PDR models using both a simple chi2 minimization approach and a robust method using Bayesian inversion followed by posterior exploration via an advanced MCMC technique. The PDR models use specific incident FUV spectra for each PDRs based on theoretical stellar spectra from the Pollux database. The Meudon PDR code is able to reproduce more than 85% of the observed H2 ro-vibrational line intensities within a factor of two. We find that (1) a realistic modeling of the incident FUV field (both in term of geometry and of spectral shape) and (2) the inclusion of recent data on collisional de-excitation rates for high vibrational levels of H2 are key to this success. H2 ro-vibrational emission lines are found to provide good constraints on both the thermal pressure and the incident FUV field strength, G0, although with a non-negligible remaining degeneracy in most PDRs. Additional constraints, such as the spatial scales of the PDR derived from ALMA or JWST observations, are found to be able to lift this degeneracy. For low excitation PDRs, the observations provide evidence that nascent H2 molecules formed on grains have relatively low rotational energy and high vibrational energy, as predicted by theoretical and experimental studies of the surface formation of H2.
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BibTeX
@misc{piluso2026physical,
title = {{Physical conditions in PDRs revealed by IGRINS ro-vibrational H2 observations}},
author = {Aurélien Piluso and Vincent Maillard and Raphael Meshaka and Emeric Bron and Franck Le Petit and Kyle Kaplan and Pierre Palud and Emilie Habart and Evelyne Roueff and Jacques Le Bourlot and Aristide Doussot and David Languignon and Nicolas Moreau},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.24630},
eprint = {2609.24630},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05781656v1},
note = {CC0},
}
RIS
TY - UNPB
TI - Physical conditions in PDRs revealed by IGRINS ro-vibrational H2 observations
AU - Aurélien Piluso
AU - Vincent Maillard
AU - Raphael Meshaka
AU - Emeric Bron
AU - Franck Le Petit
AU - Kyle Kaplan
AU - Pierre Palud
AU - Emilie Habart
AU - Evelyne Roueff
AU - Jacques Le Bourlot
AU - Aristide Doussot
AU - David Languignon
AU - Nicolas Moreau
PY - 2026
DA - 2026/09/22
PB - HAL
DO - 10.48550/arxiv.2609.24630
UR - https://hal.science/hal-05781656v1
AB - We compare H2 high ro-vibrational observations of 5 PDRs (S140, IC63, Horsehead Nebula, NGC 2023 and Orion Bar) obtained with the IGRINS spectrograph to PDR models produced with the Meudon PDR code, in order to estimate the physical conditions and to constrain the physical and chemical processes that govern PDRs. We use newly flux-calibrated and extinction-corrected IGRINS H2 observations covering the 1.45-2.45 um range at a resolution of 45000, and adjust Meudon PDR models using both a simple chi2 minimization approach and a robust method using Bayesian inversion followed by posterior exploration via an advanced MCMC technique. The PDR models use specific incident FUV spectra for each PDRs based on theoretical stellar spectra from the Pollux database. The Meudon PDR code is able to reproduce more than 85% of the observed H2 ro-vibrational line intensities within a factor of two. We find that (1) a realistic modeling of the incident FUV field (both in term of geometry and of spectral shape) and (2) the inclusion of recent data on collisional de-excitation rates for high vibrational levels of H2 are key to this success. H2 ro-vibrational emission lines are found to provide good constraints on both the thermal pressure and the incident FUV field strength, G0, although with a non-negligible remaining degeneracy in most PDRs. Additional constraints, such as the spatial scales of the PDR derived from ALMA or JWST observations, are found to be able to lift this degeneracy. For low excitation PDRs, the observations provide evidence that nascent H2 molecules formed on grains have relatively low rotational energy and high vibrational energy, as predicted by theoretical and experimental studies of the surface formation of H2.
ER -