HCO, c-C₃H and CF⁺: three new molecules in diffuse, translucent and “spiral-arm” clouds

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA, 22903-2475, USA
e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’ Hères, France
³ Observatoire de Paris (CNRS UMR 8112), 61 av. de l’Observatoire, 75014 Paris, France
⁴ LERMA/LRA, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
⁵ Institut de Planétologie et d’Astrophysique de Grenoble (UMR 5274), BP 53, 38041 Grenoble Cedex 9, France

Received: 22 December 2013
Accepted: 11 March 2014

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 km s^-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 3C 111 having E_B − 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 δ_τ ≈ 0.002 at E_B − V = 0.32 mag (A_V = 1 mag) and δ_τ/E_B − V≈ 0.003 mag^-1 overall.

Conclusions. 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 = 1 mag. We find N(HCO)/N(H¹³CO⁺) = 16 toward BL Lac, 3C 111 and the 40 km s^-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 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 3C 111, 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.