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
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
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 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 (AV = 1 mag) and δτ/EB − 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 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, 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 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 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.
Key words: ISM: molecules / ISM: abundances / ISM: clouds
© ESO, 2014