Considerations for the Interpretation of Infrared Emission from Molecular Clouds

Abstract

The last years have seen a remarkable growth in observational data relating to the dense interstellar clouds composed of molecular hydrogen in which star formation occurs. Though molecular hydrogen itself has no observable radio frequency transitions, the gaseous component in these regions has been mapped using the millimeter wavelength rotational lines of trace molecules like CO. And where the associated dust grains have been heated to temperatures above 30°K, adjacent to either an HII region or an embedded “star”, their emission has been detected in the far infrared. In the absence of such a heat source this emission would be unobservable at wavelengths shorter than 200µ. The far infrared measurements are, therefore, most useful in delineating regions of active star formation. The CO observations (reviewed in §5) indicate that the gas is also heated in the vicinity of the luminous sources. The dust grains and their infrared emission are, therefore, also critical to the energetics and energy transfer in molecular clouds. In this review I would like to describe an analysis we have recently made of the infrared radiative transfer in these molecular cloud sources (Scoville and Kwan 1976). We hope that this study not only aids in interpretion of the infrared data but also may shed light on the relationship between the dust and gas in these fascinating regions.