Evidence for Large Grains in the Star-forming Filament OMC-2/3

TL;DR

This study presents a detailed analysis of dust grain sizes in the OMC-2/3 star-forming filament, revealing unusually low emissivity spectral indices suggestive of millimeter-sized grains, with implications for dust evolution models.

Contribution

It provides the first comprehensive measurement of the dust emissivity spectral index in OMC-2/3, indicating the presence of large grains in dense star-forming regions.

Findings

Median spectral index is 0.9, shallower than in other clouds.

No significant difference in dust properties between cores and filament.

Anti-correlation between temperature and spectral index at 4 sigma.

Abstract

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.