Dust emission and extinction in the Orion OMC-3 cloud

TL;DR

This study uses three-dimensional radiative transfer modelling to dissect dust evolution in the OMC-3 filament, combining Herschel far-infrared dust emission with near-infrared extinction constraints to test multiple dust models. While many dust prescriptions can reproduce the 160–500 μm emission, the near-infrared extinction data sharply constrain grain growth, disfavouring large grains in the outer filament and allowing potential growth in the central ridge. The Mix family of large porous grains (Mix 1, Mix 1:50) and the Mix 1:Ice variant provide the best overall fits, yielding FIR-to-NIR opacity ratios around τ(250 μm)/τ(J) ≈ 1.6×10^-3 and implying moderate grain growth in dense regions with possible ice mantles. The results show that dust properties substantially affect inferred cloud masses and the local radiation field, though the high column density of OMC-3 mitigates sensitivity to LOS geometry and spectral shape, supporting a scenario of evolved grains with limited growth in the outer gas and possible enhancement in the densest core.

Abstract

Dust is an important tracer of the structure of interstellar clouds, as well as a central factor in the thermal balance and chemistry of the clouds. Our knowledge of the dust properties is nevertheless incomplete, especially regarding the dense star-forming clouds. The aim is to study dust evolution in the Orion Molecular Cloud 3 (OMC-3) and how uncertainty regarding dust properties affects estimates of the radiation field and the cloud mass. We constructed three-dimensional radiative transfer (RT) models to fit the far-infrared (FIR) observations of dust emission in the OMC-3 field and used near-infrared (NIR) extinction measurements as additional constraints. We examined fits to the dense star-forming filaments and to the surrounding cloud, including some tests with spatial dust property variations.The 160-250 $μ$m observations of dust emission could be fitted moderately well with any of the dust models tested, but few models are consistent with the measured NIR extinction. The best match to observations is found with dust models such as the THEMIS model of large porous grains, with or without ice mantles, and with mean grain sizes up to ~ 0.3$μ$m. The flattening of the NIR extinction curve excludes larger grain sizes, except possibly in the central ridge. Compared to models of lower column density clouds, the results were relatively insensitive to the line-of-sight (LOS) cloud size and the spectral shape of the heating radiation field. In addition, the effect of embedded stars remained very localised in OMC-3. The results suggest that the dust in the OMC-3 region is evolved with a grain of average size $a$=0.1-0.3 $μ$m, potentially with ice mantles.

Figures (16)

• Figure 1: Examples of OMC-3 observations. The maps show background-subtracted surface brightness values (non-linear colour scale) smoothed to the resolution used in the model fits. The plotted area is identical to previous figures. Frame c shows the outline of the PACS coverage and the reference areas for the SPIRE background subtraction (larger white circle) and for the adjustment of the PACS zero level (smaller white circle).
• Figure 2: Masks for the OMC-3 field. The levels correspond to areas with low emission ($Q=-1$), extended cloud ($Q=0$), filament ($Q=1$), ridge ($Q=2$), bright parts of the ridge ($Q=3$), FIR point sources and high-surface brightness areas affected by local heating ($Q=4$), and MIR point sources ($Q=5$). The small dots correspond to the NIR star-like (cyan) and potentially extended (blue) sources that are the basis of the NIR extinction map Meingast2016.
• Figure 3: Comparison of $\tau \rm(250\,\mu m)$ optical depth map derived from Herschel observations at 41$\arcsec$ resolution (frame a) and NICER extinction map of Meingast2016 at $1\arcmin$ resolution (frame b). The white and grey contours show, respectively, the outlines of the filament ($Q=1$) and the ridge ($Q=2$) regions. The black contours separate other high column density areas that are excluded from the analysis ($Q>2$).
• Figure 4: Normalised density profiles (blue lines) for cloud models A-D, for LOS towards filament (position indicated in Fig. \ref{['fig:combo']}). Examples of profiles at FWHM distance from the filament spine are plotted with dashed black lines. The density profiles for the other orthogonal directions, along constant right ascension (magenta lines) and constant declination (cyan), are for the model A with THEMIS dust. The quoted $FWHM$ values correspond to the blue curve.
• Figure 5: OMC-3 surface brightness at 350 $\mu$m (colour scale). The grey contours correspond to NIR extinction $A_{\rm J}$=2.5 mag and 5 mag. The black crosses show the locations of potential embedded radiation sources that were included in some models. The magenta lines trace the filament spines that were used to generate the initial model density distributions. The white plus sign shows the position for the LOS density profiles in Fig. \ref{['fig:plot_LOS_profiles']}.