Evidence for large grains in the star-forming filament OMC 2/3

We present a new 3.3 mm continuum map of the Orion Molecular Cloud (OMC) 2/3 region. When paired with previously published maps of 1.2 mm continuum and NH3-derived temperature, we derive the emissivity spectral index of dust emission in this region, tracking its changes across the filament and cores. We find that the median value of the emissivity spectral index is 0.9, much shallower than previous estimates in other nearby molecular clouds. We find no significant difference between the emissivity spectral index of dust in the OMC 2/3 filament and the starless or protostellar cores. Furthermore, the temperature and emissivity spectral index, β, are anticorrelated at the 4σ level. The low values of the emissivity spectral index found in OMC 2/3 can be explained by the presence of millimetre-sized dust grains in the dense regions of the filaments to which these maps are most sensitive. Alternatively, a shallow dust emissivity spectral index may indicate non-power-law spectral energy distributions, significant free-free emission, or anomalous microwave emission. We discuss the possible implications of millimetre-sized dust grains compared to the alternatives. © 2014 The Authors.