March 25, 1999 CfA Colloquium


Title: Seeing Light Through the Dark: Infrared Extinction and the Nature of Dark Nebulae

Speaker: Charlie Lada

Abstract: First observed and cataloged by William and Caroline Hershcel in the late eighteenth century, dark nebulae were originally believed to be holes or voids in the distribution of stars in the Milky Way. A century later E.E. Barnard obtained deep, wide-field photographs of these objects which conclusively demonstrated that the mysterious "holes" were in fact interstellar clouds of obscuring matter. Observations in the first half of the twentieth century indicated that the obscuring material was in the form of small particulate matter or dust grains. By the mid-1970s millimeter-wave spectroscopy showed that these clouds were primarily molecular and the coldest objects in the universe. At the same time, infrared observations revealed their true astrophysical significance as the sites of galactic star (and planet) formation. Over the last quater century knowledge of the physical nature and stucture of these objects has been obtained almost exclusively from millimeter and submillimeter-wave observations of the cold gas and dust within them. However, due to various limitations inherent in such observations, a clear picture of the detailed nature of these clouds has not yet emerged from these studies. Yet, such knowledge is essential for the development of a general theory of star formation. The advent of sensitive imaging detectors at infrared wavelengths has provided an important new tool for investigating the structure of dark clouds. Surveys with large format infrared array cameras on small and modest sized telescopes are capable of detecting thousands of extincted background field stars observed through a large fraction of the surface area of a typical dark cloud. Accurate, high precision, measurements of the extinction caused by dust in the cloud can be made along the pencil beam toward each reddened star. As a result the distribution of dust in the cloud can be determined over an unprecedented range of optical depth and angular scale. Because of its high abundance the distribution of dust may provide the most faithful surrogate for tracing the overall distribution of molecular hydrogen in a dark cloud. In my colloquium I will discuss the results of the first detailed investigations of the distribution of infrared extinction in nearby filamentary dark clouds. I will report the new and somewhat surpising finding that the cloud structure can be described by a simple and very smooth volume density profile with little evidence for random fluctuations on even relatively small angular scales. I will also discuss the application of such extinction measurements to the investigation of molecular abundances and cloud chemistry and report the first evidence for depletion of gaseous CO in cold cloud cores. Finally, I will discuss a new and significantly improved method of measuring the emissivity of dust grains at millimeter and submillimeter wavelengths.

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