A sensitive search for molecular gas in high-velocity clouds

We report on observations of the J = 1-0 line of (CO)-C-12 at 115 GH, toward several high-velocity H I clouds (HVCs). High angular resolution observations at 21 cm show the presence of dense cores with N(H I) = (2-4) x 10(20) cm(-2) and n(H I) = 30-800(kpc)(-1) cm(-3). We observed the CO line toward six of the densest HVC cores. No (CO)-C-12 emission is detected down to a best 5 sigma limit of 0.077 K km s(-1). Using the median I(CO) to N(H-2) conversion factor appropriate for high-latitude translucent clouds, this limit corresponds to a column density limit for N(H-2) of 9 x 10(18) cm(-2). We calculate the expected value of N(H-2) assuming a standard formation mechanism for the molecules and accounting for the run of the interstellar radiation held with the assumed distance to the HVC. The nondetections then indicate that the observed HVCs are more distant than similar to 2 kpc. This is consistent with the results of interstellar absorption line studies, which yield lower limits of 2-5 kpc for most of the observed HVCs. The nondetections also suggest that HVC cores are not likely to be current sites of star formation in the halo.