2014A&A...564A..64L

We observe molecular absorption from diffuse clouds across the 3mm receiver band. We used the EMIR receiver and FTS spectrometer at the IRAM 30m telescope to construct absorption spectra toward bright extra-galactic background sources at 195kHz spectral resolution (≃0.6km/s). We used the IRAM Plateau de Bure Interferometer to synthesize absorption spectra of H¹³CO⁺ and HCO toward the galactic HII region W49. HCO, c-C₃H and CF⁺ were detected toward the blazars BL Lac and 3C111 having E_B–V=0.32 and 1.65mag. HCO was observed in absorption from ``spiral-arm'' clouds in the galactic plane occulting W49. The complement of detectable molecular species in the 85-110 GHz absorption spectrum of diffuse/translucent gas is now fully determined at rms noise level δ_τ≃0.002 at E_B–V=0.32mag (A_V=1mag) and δ_τ/E_B–V≃0.003mag^–1 overall. As with OH, HCO⁺ and C₂H, the relative abundance of c-C₃H varies little between diffuse and dense molecular gas, with N(c-C₃H)/N(o-c-C₃H₂)≃0.1. We find N(CF⁺)/N(H¹³CO⁺)≃5, N(CF⁺)/N(C₂H)≃0.005-0.01 and because N(CF⁺) increases with E_B–V and with the column densities of other molecules we infer that fluorine remains in the gas phase as HF well beyond A_V=1mag. We find N(HCO)/N(H¹³CO⁺)=16 toward BL Lac, 3C111 and the 40km/s spiral arm cloud toward W49, implying X(HCO)≃10^–9, about 10 times higher than in dark clouds. The behaviour of HCO is consistent with previous suggestions that it forms from C⁺ and H₂, even when A_V is well above 1 mag. The survey can be used to place useful upper limits on some species, for instance N(H₂CO)/N(H₂CS)>32 toward 3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase formation route to H₂CO. In general, however, the hunt for new species will probably be more fruitful at cm- and sub-mm wavelengths for the near future.