PDRs4All

XVII. Formation and excitation of HD in photodissociation regions: Application to the Orion Bar

¹ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, B â timent 121, 91405 Orsay Cedex, France
² Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
³ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
⁴ Université Paris-Cité, Paris, France
⁵ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
⁶ Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
⁷ Department of Physics, College of Science, United Arab Emirates University (UAEU), Al-Ain 15551, USA
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 11 November 2025

Abstract

Context. The James Webb Space Telescope (JWST), with its high spatial resolution and sensitivity, enabled the first detection of several v = 1–0 rovibrational emission lines of hydrogen deuteride HD in the Orion Bar, a prototypical photodissociation region (PDR). This provides an incentive to examine the physics of HD in dense and strongly irradiated PDRs.

Aims. Using the latest data available on HD excitation by collisional, radiative, and chemical processes, our goal is to unveil HD formation and excitation processes in PDRs by comparing our state-of-the-art PDR model with observations made in the Orion Bar and discuss if and how HD can be used as a complementary tracer of physical parameters (thermal pressure and intensity of the UV field) in the emitting region.

Methods. We computed detailed PDR models using an upgraded version of the Meudon PDR code (including radiative, collisional, and formation pumping excitation of HD rovibrational levels). Model results were then compared to spectro-imaging data acquired with the NIRSpec instrument on board JWST using population–excitation diagrams and synthetic emission spectra.

Results. The models predict that HD is mainly produced in the gas phase via the reaction D + H₂ → H + HD at the front edge of the PDR, contrary to H₂ (which forms on grain surfaces), and that the D/HD transition is located slightly closer to the edge than the H/H₂ transition. Rovibrational levels are excited by UV pumping. In the observations, HD rovibrational emission is detected close to the H/H₂ dissociation fronts of the Orion Bar, and it peaks where vibrationally excited H₂ peaks, rather than at the maximum emission of pure rotational H₂ levels. We detected lines emitted from five different levels of HD (v = 1) from which we can derive an excitation temperature around T_ex ~ 480–710 K. Our comparison to PDR models showed that a range of thermal pressure P = (3–9) × 10⁷ K cm⁻³ with no strong constraints on the intensity of the UV field G₀ are compatible with HD observations. This range of pressure is consistent with previous estimates from H₂ observations with JWST.

Conclusions. This study provides a new detailed analysis of HD formation and excitation in PDRs. State-of-the-art PDR models with parameters best reproducing other tracers’ emission are compatible with HD observations, highlighting the coherence of the different studies. This is also the first time that observations of HD emission lines in the near-infrared have been used to put constraints on the thermal pressure in the PDR, even though the lines are very faint.