Evidence for Large Grains in the Star-forming Filament OMC-2/3
  We present a new 3.3 mm continuum map of the OMC-2/3 region. When paired with
previously published maps of 1.2mm 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,
beta, are anti-correlated at the 4 sigma level. The low values of the
emissivity spectral index found in OMC-2/3 can be explained by the presence of
millimeter-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-powerlaw spectral energy distributions,
significant free-free emission, or anomalous microwave emission. We discuss the
possible implications of millimeter-sized dust grains compared to the
alternatives.
