Abstract
Recently discovered scattered light at 3-5 $\mu$m from low-mass cores (so-called "coreshine") reveals the presence of grains around 1 $\mu$m, which is larger than the grains found in the low-density interstellar medium. But only about half of the 100+ cores investigated so far show the effect. This prompts further studies on the origin of this detection rate. From the 3D continuum radiative transfer equation, we derive the expected scattered light intensity from a core placed in an arbitrary direction seen from Earth. We use the approximation of single scattering, consider extinction up to 2nd-order Taylor approximation, and neglect spatial gradients in the dust size distribution. The impact of the directional characteristics of the scattering on the detection of scattered light from cores is calculated for a given grain size distribution, and local effects like additional radiation field components are discussed. The surface brightness profiles of a core with a 1D density profile are calculated for various Galactic locations, and the results are compared to the approximate detection limits. We find that for optically thin radiation and a constant size distribution, a simple limit for detecting scattered light from a low-mass core can be derived that holds for grains with sizes smaller than 0.5 $\mu$m. The extinction by the core prohibits detection in bright parts of the Galactic plane, especially near the Galactic center. For scattered light received from low-mass cores with grain sizes beyond 0.5 $\mu$m, the directional characteristics of the scattering favors the detection of scattered light above and below the Galactic center, and to some extent near the Galactic anti-center. We identify the local incident radiation field as the major unknown causing deviations from this simple scheme.
Highlights
Dense regions in molecular clouds have been identified as the site where star formation starts
We find that for optically thin radiation and a constant size distribution, a simple limit for detecting scattered light from a low-mass core can be derived that holds for grains with sizes smaller than 0.5 μm
We find that the detection of scattered light at 3.6 μm from lowmass molecular cloud cores is both influenced by local effects like the radiation field in the vicinity of the core and global effects like the interplay of core position with respect to the Galactic center (GC) and the directional characteristics of the scattering
Summary
Dense regions in molecular clouds have been identified as the site where star formation starts. For the diffuse ISM, Mathis et al (1977) were the first to suggest a power-law distribution with a size limit of 0.25 μm for non-graphite grains (hereinafter called “MRN(0.25)”). They emphasized that only weak constraints could be derived for the larger grains in the distribution. The analysis of scattered light based on IRAC photometry is compromised by PAH emission that in principle can contaminate all four IRAC bands This is true for any star forming region like ρ Oph or Vela that is irradiated by close medium or high-mass stars.
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