ULTRAVIOLET, OPTICAL, AND INFRARED OBSERVATIONS OF THE HIGH-LATITUDE MOLECULAR CLOUD TOWARD HD-210121

Low-resolution UV spectra of the B3 V star HD 210121, located behind the high-latitude molecular cloud DBB 80, yield an extinction curve exhibiting a far-UV rise that is among the steepest known. Comparisons of optical absorption-line spectra with H I 21 cm and molecular emission-line spectra suggest that the line of sight is particularly simple, thus affording an excellent opportunity for investigating the absorption and emission characteristics of a single, isolated interstellar cloud characterized by extreme UV extinction. The low ratio I(100-mu-m)/N(H(tot)) suggests that the radiation field incident on the cloud is somewhat lower than the average interstellar field; declines in the ratios of the other IRAS bands with respect to I(100-mu-m), for increasing I(100-mu-m), seem consistent with further attenuation of that incident field within the cloud. The apparent relative enhancement of I(12-mu-m), compared with models of dust emission, and the extremely steep far-UV extinction together are consistent with the presence of an enhanced population of very small grains; the normal calcium depletion suggests that there has been little wholesale grain destruction. The steep far-UV extinction may help to explain the relatively high observed abundances of CO and CN, though observations of CO absorption bands in the UV suggest that the N(CO) and n(H-2) inferred from millimeter-wave emission lines from various rotational levels of CO may be somewhat larger than the values appropriate to the specific line of sight to the star. The disagreement in density for this cloud inferred from C2 absorption versus that inferred from CO emission may thus be due in part to clumping in the gas sampled by the radio beams.