A new model for the v ₁ vibrational band of HCN in cometary comae, with application to three comets

Aims. Hydrogen cyanide (HCN) radiates effectively at infrared wavelengths in cometary atmospheres, and a new quantum-band model is needed to properly interpret high-resolution spectra. HCN spectra of comets 8P/Tuttle, C/2007 W1 (Boattini), and C/2008 Q3 (Garradd) have been recorded by our team using the high-resolution CRyogenic InfraRed Echelle Spectometer (CRIRES) at the Very Large Telescope (VLT), ultimately posing an excellent test for our newly developed model. Methods. We developed a quantum-band model for the ν₁ fundamental of HCN using the latest spectroscopic parameters available and with it retrieved HCN in the above mentioned three comets. For each comet, we sampled several lines of HCN in the spectral region near 3 μm, and retrieved molecular production rates, mixing ratios, and rotational temperatures. Results. When compared to other comets, 8P/Tuttle is relatively depleted in HCN, while C/2007 W1 (Boattini) appears to be enriched and C/2008 Q3 (Garradd) normal. The spatial profile of HCN observed in 8P/Tuttle is symmetric, consistent with isotropic outgassing from the nucleus, while in comet C/2007 W1 we observed an asymmetric excess of HCN in the anti-solar direction. We investigated the HCN-CN parentage by comparing our production rate ratios (HCN/H₂O) with those of CN/OH derived at optical wavelengths. In comet C/2007 W1 the two mixing ratios are comparable, while in 8P/Tuttle our derived HCN abundance is too low to support the HCN molecule as the only parent of the CN radical.