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Isolated dark globules, also known as Bok globules or Barnard objects, are nearby dense interstellar clouds of dust and gas which are primarily heated externally by the interstellar radiation field (ISRF). Many dark globules show an excess in the mid-infrared (MIR; $10 - 100 \mu m$). This excess has generally been attributed to an internal heat source (e.g., a protostellar object), since conventional large grains heated only by the ISRF cannot account for the MIR excess. On the other hand, if very small grains are present in these clouds, they may be responsible for the MIR excess through transient heating by the ISRF. In such a case, the postulation of an internal heat source is not necessary.
To quantitatively determine whether a MIR excess in dark globules uniquely implies the presence of an internal heat source, we have modeled B335 - a source known to have a $60 \mu m$ excess and an internal heat source. We have compared observational data with the results of two models: an internally heated one with only conventional large grains ($a \sim 0.1 \mu m$), and an externally heated one with both large and very small grains ($a > 10 \AA$). We find that both models can reproduce the observed flux spectrum, implying that a MIR excess in dark globules does not uniquely imply the presence of an internal heat source. The parameters for both models are within observational constraints.
We also find that although the flux may be used to differentiate between these two cases, the surface brightness is a more important diagnostic tool. Internally heated sources show extra emission at the center, and strong short wavelength emission at smaller radii. Externally heated sources, on the other hand, show less emission at the center of the cloud, and limb brightening at shorter wavelengths. Our models suggest that these two possibilities could be differentiated at $ 60 \mu m$.
This work has been partially supported by a NASA Grant to CML, and an NDSEG/DoD Fellowship to SDD.
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