Peculiar Dust Emission within the Orion Molecular Cloud

TL;DR

This study investigates the spectral energy distribution of dust in the Orion Molecular Cloud, revealing flattened opacity indices at millimeter wavelengths likely due to large dust grains or emission contamination, emphasizing the complexity of dust properties.

Contribution

It provides new high-resolution observations of dust emission in OMC 2/3, demonstrating flattened opacity indices and highlighting the importance of multi-scale analysis for understanding dust grain properties.

Findings

Confirmed flattened dust opacity indices between 2.9 mm and 3.6 mm.

Detected significant emission from large dust grains in embedded disks.

Highlighted the need for careful interpretation when combining multi-scale data.

Abstract

It is widely assumed that dust opacities in molecular clouds follow a power-law profile with an index, $\beta$. Recent studies of the Orion Molecular Cloud (OMC) 2/3 complex, however, show a flattening in the spectral energy distribution (SED) at $ \lambda > 2$ mm implying non-constant indices on scales $\gtrsim$ 0.08 pc. The origin of this flattening is not yet known but it may be due to the intrinsic properties of the dust grains or contamination from other sources of emission. We investigate the SED slopes in OMC 2/3 further using observations of six protostellar cores with NOEMA from 2.9 mm to 3.6 mm and ALMA-ACA in Band 4 (1.9 -- 2.1 mm) and Band 5 (1.6 -- 1.8 mm) on core and envelope scales of $\sim 0.02 - 0.08$ pc. We confirm flattened opacity indices between 2.9 mm and 3.6 mm for the six cores with $\beta \approx -0.16 - 1.45$, which are notably lower than the $\beta$ values of $> 1.3$ measured for these sources on $0.08$ pc scales from single-dish data. Four sources have consistent SED slopes between the ALMA data and the NOEMA data. We propose that these sources may have a significant fraction of emission coming from large dust grains in embedded disks, which biases the emission more at longer wavelengths. Two sources, however, had inconsistent slopes between the ALMA and NOEMA data, indicating different origins of emission. These results highlight how care is needed when combining multi-scale observations or extrapolating single-band observations to other wavelengths.