JWST observations of photodissociation regions III. Dust modelling at the illuminated edge of the Horsehead PDR

TL;DR

JWST observations of the Horsehead PDR edge reveal that carbonaceous nano-grains cannot be explained by diffuse ISM dust and require evolved grain populations. The authors integrate 3D radiative transfer with the THEMIS dust model to constrain hydrogenation state, size distribution, and abundance of nano-grains, using a sequential, grid-based approach anchored by NIRSpec/MRS spectroscopy and NIRCam/MIRI imaging. They find a high-density edge with $n_0 \sim 4\times10^5\, mathrm{H\,cm^{-3}}$, a nano-grain minimum size around $0.35$–$0.45\,\mathrm{nm}$, and a less steep size distribution with $\alpha \approx -3.5$, plus a small-grain abundance $M_{\mathrm{a-C}}/M_{\mathrm{H}} \gtrsim 0.003$ and a PDR path length $l_{\rm PDR} \lesssim 0.015$ pc. The results imply that nano-grain destruction is less efficient in the Horsehead’s moderate-UV field than in more intense PDRs and support a picture where nano-grain population recovery is slower in such environments, with implications for dust evolution in other low- to moderate-UV regions.

Abstract

Carbonaceous nano-grains are a significant component of interstellar dust and dominate the mid-infrared emission of photodissociation regions (PDRs). We study the evolution of nano-grains across the illuminated edge of the Horsehead PDR, especially their abundance and size properties. This work is part of the Physics and Chemistry of PDR Fronts program studying dust and gas in PDRs with JWST. We use NIRCam+MIRI photometric bands and NIRSpec+MRS spectroscopy to map the illuminated edge. We model dust emission using the THEMIS dust model with the SOC radiative transfer code. Detailed modeling of high angular resolution JWST data allows us to obtain constraints on nano-grain properties. We find that diffuse ISM dust cannot account for the observed data, requiring evolved grains. A sharp density increase is observed at the illuminated edge, consistent with ALMA observations revealing a sharp transition between molecular and ionized gas. Although the PDR length could not be directly determined, we estimate an upper limit of approximately 0.015 pc. This implies a lower limit on small grain abundance (greater than 0.003), showing small grains are not depleted at the Horsehead edge, unlike in the Orion Bar. Our findings indicate a high-density environment and less steep size distribution for nano-grains at the illuminated edge versus the diffuse ISM. This implies nano-grain destruction mechanisms might be less efficient in the Horsehead's moderate-UV field than in more intense PDRs. These results support a model where nano-grain population recovery is slower in moderate-UV environments, leading to a unique dust size distribution at the edge of the Horsehead Nebula.

Figures (11)

• Figure 1: Top: (Left) Hubble’s view of the Horsehead Nebula at near-infrared wavelengths of 1.1 $\mu$m (blue/cyan) and 1.6 $\mu$m (red/orange); NASA, ESA, and the Hubble Heritage Team (STScI/AURA). (Middle) a zoom-in image of part of the Horsehead nebula as seen by the NIRCam instrument. This image is composed of light at wavelengths of 1.4 and 2.5 $\mu$m (blue), 3.0 and 3.23 $\mu$m (cyan), 3.35 $\mu$m (green), 4.3 $\mu$m(yellow), and 4.7 and 4.05 $\mu$m (red). (Right) The same zoom-in region visualised by the JWST instrument MIRI. In this image, blue represents light at wavelengths of 5.6, 7.7, and 10 $\mu$m ; green is 11, 12, and 15 $\mu$m; and red is 18, 21, and 25 $\mu$m. The NIRCam and MIRI images are from the recent ESA/Webb release, NASA, CSA, K. Misselt (University of Arizona) and A. Abergel (IAS/University Paris-Saclay, CNRS). The blue solid lines correspond to the cut used in our study. Bottom: Profiles of the relative brightness (normalized at a distance of 2 from the edge) for NIRCam and MIRI filters, adapted from Abergel2024.
• Figure 2: Schematic illustration of the Horsehead PDR illuminated by a radiation field (from the right). $l_\text{PDR}$ parameter is the length of the PDR along the line of sight. Refer to Table \ref{['soc_setup']} for the corresponding values.
• Figure 3: Top: Assumed density profile across the illuminated edge of the Horsehead PDR (see Sect. \ref{['subsec:structure']}).
• Figure 4: Size distributions of the dust mixtures from THEMIS using the dust emission tool DustEM compiegne2011 (parameters are listed in Table. \ref{['tab:size_distribution']}). The red, black dotted line, and black dashed line correspond to a-C, a-C:H/a-C and a-Sil/a-C respectively. The Blue dashed line corresponds to the distribution of a-C grains for the best-fit parameters at the edge of the Horsehead.
• Figure 5: Comparison of observed surface brightness and DustEM models normalized to the 3.3 $\mu$m observed band (without radiative transfer) in the illuminated edge of the Horsehead PDR. The black data points represent observations from NIRSpec, while the coloured lines show model predictions with varying band gap energies. The dust parameters used are those of the DISM model, $a_{\mathrm{min,\,a-C}}$ = 0.4 nm, $\alpha$ = -5.