Molecular absorption lines toward star-forming regions: a comparative study of HCO⁺, HNC, HCN, and CN

Abstract

Aims. The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in unraveling the coupled chemical and dynamical evolution of the ISM. Methods. The lowest rotational lines of HCO + , HCN, HNC, and CN were observed at the IRAM-30m telescope in absorption against the λ3 mm and λ1.3 mm continuum emission of massive star-forming regions in the Galactic plane. The absorption lines probe the gas over kiloparsecs along these lines of sight. The excitation temperatures of HCO + are inferred from the comparison of the absorptions in the two lowest transitions. The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components. Most appear in all the spectra of a given line of sight. For each component, we derived the central opacity, the velocity dispersion, and computed the molecular column density. We compared our results to the predictions of UV-dominated chemical models of photodissociation regions (PDR models) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy (TDR models). )= 18 ± 9. These ratios are similar to those inferred from observations of high Galactic latitude lines of sight, suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood. The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s −1 and those of the HCO + lines are found to be 30% broader than those of CN-bearing molecules. The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO + , even for high-density material (100 cm −3 < n H < 10 4 cm −3 ). The TDR models, in turn, are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities (30 cm −3 < n H < 200 cm −3 ) and the turbulent energy observed in the diffuse interstellar medium. Conclusions. Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy. The details of the dissipation mechanisms still need to be investigated.

The tight correlation of CCH and c-C3H2in diffuse and translucent clouds

“…We checked that the excitation temperature of the observed CCH transition stays below 3 K, for most of the domain of physical conditions applicable to the line of sight clouds: the absorption features are formed in moderately dense gas: n(H 2 ) = 100-3000 cm −3 (Plume et al 2004;Vastel et al 2000;Godard et al 2010). These analyses have shown that, with such physical conditions, the collisional excitation of polar molecules is rather ineffective, and the excitation temperature remains close to the cosmic background temperature.…”

“…Models with elevated cosmic ray ionization fluxes are drawn in red, using larger symbols. The gray bars indicate the observed abundances of CH (Sheffer et al 2008), CCH (Gerin et al 2010) and c-C 3 H 2 (this work, using the scaling with CCH). massive stars.…”

“…The same sources can be observed at millimeter wavelengths, although the continuum emission is produced by the embedded HII regions rather than by dust. While a first set of observations, covering HCO + , HCN, HNC and CN spectra, has been presented by Godard et al (2010), the data relative to the hydrocarbon chemistry are presented in this paper. The analyzed objects are listed in Table 1.…”

Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6–0.4 (W31C), W49N, and W51

“…3), the overlap of the hyperfine components is not as severe as for the other sources, and we fitted the 532 GHz absorption profile with four Gaussian velocity components. We used the position and width as derived from the fit of the HF and HCO + spectra Godard et al 2010). …”

First detection of [N II] 205μm absorption in interstellar gas

“…Godard et al 2010;Gerin et al 2011;Flagey et al 2013). The first results of the ionised and neutral carbon observations have recently been presented in Gerin et al (2012Gerin et al ( , 2014.…”