PDRs4All: XVII. Formation and excitation of HD in photodissociation regions: Application to the Orion Bar

Journal article, 2026

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 a→ 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 Tex ~ 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.