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

Marion Zannese, Jacques Le Bourlot, Evelyne Roueff, Emeric Bron, Franck Le Petit, Dries Van De Putte, Maryvonne Gerin, Naslim Neelamkodan, Javier R. Goicoechea, John Black, Ryan Chown, Ameek Sidhu, Emilie Habart, Els Peeters, Olivier Berné

TL;DR

The paper develops a state-of-the-art Meudon PDR model that includes rovibrational HD chemistry and excitation, and applies it to the Orion Bar to interpret JWST/NIRSpec HD detections. It finds that HD forms primarily at the PDR front via the D + H2 → HD + H channel, with UV pumping driving rovibrational excitation and a vibrationally excited H2 reservoir shaping the formation. The observed HD v=1 lines yield an excitation temperature around Tex ≈ 480–710 K, and the HD data favor a thermal pressure range of $(3-9)\times 10^7$ K cm$^{-3}$, largely independent of the UV field strength. This work demonstrates that near-IR HD emission can serve as a complementary tracer of PDR physical conditions, consistent with H2-based pressure estimates, and highlights the need for deeper observations to map HD across the dissociation fronts more precisely.

Abstract

The James Webb Space Telescope enabled the first detection of several rovibrational emission lines of HD in the Orion Bar, a prototypical photodissociation region. This provides an incentive to examine the physics of HD in dense and strong PDRs. 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 in the emitting region. We compute detailed PDR models, using an upgraded version of the Meudon PDR code, which are compared to NIRSpec data using excitation diagrams and synthetic emission spectra. 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 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 peaks where vibrationally excited H2 peaks, rather than at the maximum emission of pure rotational H2 levels. We derive an excitation temperature around Tex ~ 480 - 710 K. Due to high continuum in the Orion Bar, fringes lead to high noise levels beyond 15 $μ$m, no pure rotational lines of HD are detected. The comparison to PDR models shows that a range of thermal pressure P = (3-9)x10$^7$ K cm$^{-3}$ with no strong constraints on the intensity of the UV field are compatible with HD observations. This range of pressure is compatible with previous estimates from H2 observations with JWST. This is the first time that observations of HD emission lines in the near-infrared are used to put constraints on the thermal pressure in the PDR.

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

TL;DR

The paper develops a state-of-the-art Meudon PDR model that includes rovibrational HD chemistry and excitation, and applies it to the Orion Bar to interpret JWST/NIRSpec HD detections. It finds that HD forms primarily at the PDR front via the D + H2 → HD + H channel, with UV pumping driving rovibrational excitation and a vibrationally excited H2 reservoir shaping the formation. The observed HD v=1 lines yield an excitation temperature around Tex ≈ 480–710 K, and the HD data favor a thermal pressure range of K cm, largely independent of the UV field strength. This work demonstrates that near-IR HD emission can serve as a complementary tracer of PDR physical conditions, consistent with H2-based pressure estimates, and highlights the need for deeper observations to map HD across the dissociation fronts more precisely.

Abstract

The James Webb Space Telescope enabled the first detection of several rovibrational emission lines of HD in the Orion Bar, a prototypical photodissociation region. This provides an incentive to examine the physics of HD in dense and strong PDRs. 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 in the emitting region. We compute detailed PDR models, using an upgraded version of the Meudon PDR code, which are compared to NIRSpec data using excitation diagrams and synthetic emission spectra. 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 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 peaks where vibrationally excited H2 peaks, rather than at the maximum emission of pure rotational H2 levels. We derive an excitation temperature around Tex ~ 480 - 710 K. Due to high continuum in the Orion Bar, fringes lead to high noise levels beyond 15 m, no pure rotational lines of HD are detected. The comparison to PDR models shows that a range of thermal pressure P = (3-9)x10 K cm with no strong constraints on the intensity of the UV field are compatible with HD observations. This range of pressure is compatible with previous estimates from H2 observations with JWST. This is the first time that observations of HD emission lines in the near-infrared are used to put constraints on the thermal pressure in the PDR.

Paper Structure

This paper contains 34 sections, 15 equations, 22 figures, 12 tables.

Table of Contents

  1. Introduction
  2. $\mathrm{HD}$ chemistry and excitation
  3. Formation and destruction processes
  4. Main reactions
  5. State-to-state chemical reactions
  6. Excitation/de-excitation processes
  7. Radiative rovibrational transitions within $X$
  8. Ultraviolet electronic radiative transitions
  9. Collisions
  10. A fiducial model
  11. Parameters
  12. Structure
  13. Heating and cooling processes
  14. $\mathrm{HD}$ abundance
  15. $\mathrm{HD}$ excitation
  16. ...and 19 more sections

Figures (22)

  • Figure 1: JWST NIRCam composite image of the Orion Bar, located in the Orion molecular cloud 2024AA...685A..73H. Red is the $3.35 \,\mu\mathrm{m}$ emission (F335M filter), blue is the emission of Pa$\alpha$ (F187N filter subtracted by F182M filter) and green is the emission of the $\mathrm{H}_2$ 0–0 S(9) line at $4.70\,\mu\mathrm{m}$ (F470N filter subtracted by F480M filter). The white box represents the Field of view of NIRSpec. The blue boxes correspond to the aperture where the spectra are averaged in the three dissociation fronts (DF1, DF2 and DF3).
  • Figure 2: (a) Temperature (red line, left axis) and density profiles (blue line, right axis) as a function of distance from the ionization front, $d$, for the fiducial model. See Sect. \ref{['sect:structure']} for the definitions of zones $I$ to $V$. (b) Relative abundances of $\mathrm{H}$ and $\mathrm{H}_2$, (resp. $\mathrm{D}$ and $\mathrm{HD}$), normalized by $n(\mathrm{H}) + n(\mathrm{H}_2)$ (resp. $n(\mathrm{D}) + n(\mathrm{HD})$). Values at the transitions are given in Table \ref{['tab:d-T-n-Values.']}.
  • Figure 3: Main contributions to (a) heating ($\Gamma$) and (b) cooling ($\Lambda$) of the gas as a function of distance $d$ from the ionization front. Each heating (resp. cooling) term is normalized by the total heating/cooling.
  • Figure 4: Radiative energy density $u_{\lambda}$ at d= 0.05 pc in the $96-96.3\, \mathrm{nm}$ far ultraviolet wavelength range. Blue: $\mathrm{H}_2$ lines, red: $\mathrm{HD}$ lines. The positions of three strong $\mathrm{HD}$ absorption lines are shown using black vertical lines, coming from $v = 0$, $J = 1$ and $J = 2$.
  • Figure 5: Relative contribution of $\mathrm{HD}$ formation and destruction reactions normalized to the total formation rate. (a): individual reactions \ref{['Eq:HD_F1']} to \ref{['Eq:HD_D4']}. (b): sum of direct and reverse reactions. The $y$ axis is dimensionless.
  • ...and 17 more figures