1999ApJ...523..163A

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 T_S=38 and 147 K, 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 filling fraction of cold (∼50 K) gas of less than 7%. In one case, we obtained a 3 σ lower limit to the spin temperature of 180-320 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)<5.5x10¹⁴ and N(H₂)<5.5x10¹⁸ cm^–2, assuming solar abundances and in one case a molecular gas fraction less than 17% at the 3 σ level. Averaging the CO nondetections to test for broader, low-level absorption implies N(CO)<4.8x10¹⁴ cm^–2.