A Cold Nearby Cloud Inside the Local Bubble

The high-latitude Galactic H I cloud toward the extragalactic radio source 3C 225 is characterized by very narrow 21 cm emission and absorption indicative of a very low H I spin temperature of about 20 K. Through high-resolution optical spectroscopy, we report the detection of strong, very narrow Na I absorption corresponding to this cloud toward a number of nearby stars. Assuming that the turbulent H I and Na I motions are similar, we derive a cloud temperature of 20 −8 K (in complete agreement with the 21 cm results) and a line-of-sight turbulent velocity of 0.37 ± 0.08 km s from a comparison of the H I and Na I absorption linewidths. We also place a firm upper limit of 45 pc on the distance of the cloud, which situates it well inside the Local Bubble in this direction and makes it the nearest-known cold diffuse cloud discovered to date. Subject headings: ISM: atoms – ISM: clouds – ISM: structure – Galaxy: solar neighborhood Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation. Current Address: Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292