PLATEAU-DE-BURE OBSERVATIONS OF MM-WAVE MOLECULAR ABSORPTION TOWARD BL LACERTAE

We have used the Plateau de Bure Interferometer to observe lambda3mm 1 -0 absorption line profiles of (CO)-C-13, HCO+, C2H, CN, HCN, HNC, and N2H+ formed in a nearby molecular cloud occulting BL Lac (2200+420): only N2H+ was not seen. The observed optical depths, typically 0.3-1.5 , provide highly reliable total column densities because the excitatio n is quite weak. The relative abundances of (CO)-C-13, HCO+, HCN, CN, and C2H follow those cited for TMC-1 but N2H+ and HNC are deficient to ward BL Lac by factors of 3-6. Only (CO)-C-13 and HCO+ were seen in em ission with the 12m NRAO telescope. The latter provides an estimate of the pressure n(H-2) T(K) = 4 -5 x 10(4) cm-3 K.