Extinction curves, distances, and clumpiness of diffuse interstellar dust clouds

We present CCD photometry in UBVRI of several thousand Galactic held stars in four large (>1 deg(2)) regions centered on diffuse interstellar dust clo uds, commonly referred to as "cirrus'' clouds (with optical depth A(V) less than unity). Our goal in studying these stars is to investigate the proper ties of the cirrus clouds. A comparison of the observed stellar surface den sity between on-cloud and off-cloud regions as a function of apparent magni tude in each of the five bands effectively yields a measure of the extincti on through each cloud. For two of the cirrus clouds, this method is used to derive UBVRI star-count-based extinction curves, and U-band counts are use d to place constraints on the cloud distance. The color distribution of sta rs and their location in (U-B, B-V) and (B-V, V-I) color-color space are an alyzed in order to determine the amount of selective extinction (reddening) caused by the cirrus. The color excesses, A(lambda)-A(V), derived from ste llar color histogram offsets for the four clouds, are better fitted by a re ddening law that rises steeply toward short wavelengths [R-V = A(V)/E(B - V ) less than or similar to 2] than by the standard law (R-V = 3.1). This may be indicative of a higher than average abundance of small dust grains rela tive to larger grains in diffuse cirrus clouds. The shape of the count-base d effective extinction curve and a comparison of different estimates of the dust optical depth (extinction optical depth derived from background star counts/colors; emission optical depth derived from far infrared measurement s) are used to measure the degree of clumpiness in clouds. The set of techn iques explored in this gaper can be eadily adapted to the Sloan Digital Sky Survey data set in order to carry out a systematic, large-scale study of c irrus clouds.