Optical interstellar line studies of a nearby cold cloud

Interstellar Ca ii lines in the spectra of OB stars in a large region toward the galactic center are shown to arise in the cold hydrogen cloud seen in self-absorption in the 21-cm line. The distances of two of these stars are 150 and 180 pc, which are thus upper limits to the cloud distance. The cold hydrogen is almost certainly associated with the dust at about 150 pc distance which covers this entire area. OH molecules have been detected in this dust at the same velocity as the cold H I. The cold hydrogen is arranged in sheets with multiple velocity components; density fluctuations within these sheets have a typical scale size of about 5 pc, which is the probable thickness of the sheets. The cloud extends for at least 100 pc along the galactic plane and at least 30 pc perpendicular to it. From a comparison of interferometric H I absorption profiles, single-dish emission profiles, and optical Na iline data the set T = 40 K, n(H) = 20 cm -3, n(e) = 0.05 cm -3 is derived for one position in the cloud; these values compare very well with predictions of the twophase model of the interstellar medium. An unexpectedly wide Na I D2 line profile may be explained either as the effect of multiple weak cold-cloud components or as an intermediate temperature ( 800 K) intercloud medium. Subject headings: interstellar matter - 21-cm radiation