A Search for Atomic Hydrogen and Molecular Gas Absorption in High-velocity Clouds

We have searched for H I, CO, and HCO` in absorption along 16 lines of sight toward high-velocity clouds (HVCs). Of these, seven lines of sight are toward true HVCs, two are toward the Magellanic Stream, six are toward the outer arm complex, and one is toward a probable Galactic cloud. We detected absorption along two lines of sight toward the outer arm complex, at and 147 K, T S \ 38 respectively. The detections are consistent with normal disk H I absorption. The nondetections suggest that HVCs are weak H I absorbers and that they contain primarily warm H I, with a surface Ðlling fraction of cold (D50 K) gas of less than 7%. In one case, we obtained a 3 p lower limit to the spin temperature of 180È320 K. Upper limits for CO and HCO` emission along seven lines of sight are the most sensitive probes of molecular emission yet toward HVCs. The best upper limits imply N(CO)\ 5.5] 1014 and cm~2, assuming solar abundances and in one case a molecN(H2)\ 5.5] 1018 ular gas fraction less than 17% at the 3 p level. Averaging the CO nondetections to test for broader, low-level absorption implies N(CO)\ 4.8] 1014 cm~2. Subject headings : galaxies : kinematics and dynamics È radio lines : ISM È ISM: structure