HCO, c-C 3 H and CF +: Three new molecules in diffuse, translucent and "spiral-arm" clouds

Abstract

Aims. We observe molecular absorption from diffuse clouds across the 3 mm receiver band. Methods. We used the EMIR receiver and FTS spectrometer at the IRAM 30 m telescope to construct absorption spectra toward bright extra-galactic background sources at 195 kHz spectral resolution (0.6 kms-1). We used the IRAM Plateau de Bure Interferometer to synthesize absorption spectra of H13CO+ and HCO toward the galactic HII region W49. Results. HCO, c-C3H and CF+ were detected toward the blazars BL Lac and 3C111 having EB-V = 0.32 and 1.65 mag. 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 δτ 0.002 at EB-V = 0.32 mag (A V = 1 mag) and δτ/EB-V0.003 mag -1 overall. Conclusions. As with OH, HCO+ and C 2H, the relative abundance of c-C3H varies little between diffuse and dense molecular gas, with N(c-C3H)/N(o-c-C 3H2) 0.1. We find N(CF+)/N(H 13CO+) 5, N(CF+)/N(C2H) 0.005-0.01 and because N(CF+) increases with EB-V and with the column densities of other molecules we infer that fluorine remains in the gas phase as HF well beyond AV = 1 mag. We find N(HCO)/N(H13CO +) = 16 toward BL Lac, 3C111 and the 40 kms-1 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 AV 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. © 2014 ESO.