Dust emissivity in the star-forming filament OMC 2/3

We present new measurements of the dust emissivity index, beta, for the high-mass, star-forming OMC 2/3 filament. We combined 160-500 mu m data from Herschel with long-wavelength observations at 2 mm and fit the spectral energy distributions across a similar or equal to 2 pc long, continuous section of OMC 2/3 at 15 000 AU (0.08 pc) resolution. With these data, we measured beta and reconstructed simultaneously the filtered-out large-scale emission at 2 mm. We implemented 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 (similar to 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 of 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 reliable determinations of beta in nearby cold molecular clouds.