Abstract
We present new measurements of the dust emissivity index, beta, for the high-mass, star-forming OMC 2/3 filament. We combine 160-500 um data from Herschel with long-wavelength observations at 2 mm and fit the spectral energy distributions across a ~ 2 pc long, continuous section of OMC 2/3 at 15000 AU (0.08 pc) resolution. With these data, we measure beta and reconstruct simultaneously the filtered-out large-scale emission at 2 mm. We implement both variable and fixed values of beta, finding that beta = 1.7 - 1.8 provides the best fit across most of OMC 2/3. These beta values are consistent with a similar analysis carried out with filtered Herschel data. Thus, we show that beta values derived from spatial filtered emission maps agree well with those values from unfiltered data at the same resolution. Our results contradict the very low beta values (~ 0.9) previously measured in OMC 2/3 between 1.2 mm and 3.3 mm data, which we attribute to elevated fluxes in the 3.3 mm observations. Therefore, we find no evidence or rapid, extensive dust grain growth in OMC 2/3. Future studies with Herschel data and complementary ground-based long-wavelength data can apply our technique to obtain robust determinations of beta in nearby cold molecular clouds.
Highlights
Dust grains are excellent tracers of mass and structure in molecular clouds
The dust emissivity index, β, represents the efficiency at which dust grains radiate at long wavelengths, where the value β = 2 is expected for bare dust grains in the interstellar medium (e.g., Draine & Lee 1984)
The column densities were determined from spectral energy distributions (SEDs) fits to Herschel data alone, assuming a fixed dust opacity law based on the models of Ossenkopf & Henning (1994)
Summary
Thermal emission from dust can characterize structures over various scales, from the diffuse cloud to the dense, star-forming cores (e.g., Di Francesco et al 2008; Enoch et al 2009; André et al 2010; Stutz & Kainulainen 2015). The dust emissivity index, β, represents the efficiency at which dust grains radiate at long wavelengths, where the value β = 2 is expected for bare dust grains in the interstellar medium (e.g., Draine & Lee 1984). This efficiency, will evolve with density and temperature. Dust grains in cold, dense cores are likely to coagulate, (e.g., Ossenkopf & Henning 1994; Ormel et al 2011), leading to values of β < 2 toward molecular clouds and dense cores (e.g., Shirley et al 2011; Planck Collaboration XXIV 2011; Sadavoy et al 2013) and values of β < 1 toward protoplanetary disks (e.g., Beckwith & Sargent 1991; Wright et al 2015)
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