High-resolution 3 μm spectroscopy of IRC + 10216

High-resolution $$(\lambda /\Delta \lambda \,\simeq \,100\,000)$$spectra of IRC + 10216 have been obtained with a Fourier transform spectrometer. A search for HCl fundamental vibration–rotation lines near 3 μm and an analysis of the $$\text{CH}_{4}{\nu }_{3}$$ band are described. The upper limit on the column density, $$N(\text{H}\,^{35}\text{Cl})\,=\,2.3\times {10}^{15}\text{cm}^{-2}$$, is a factor of 10 to 20 less than a prediction based on the expectation that Cl is fully associated into HCl in the circumstellar shell. This discrepancy suggests that Cl is not fully associated into HCl but rather may reside in another molecule (e.g. CH₃Cl) or a condensate (e.g. FeC1₂, MgC1₂). The $$\text{CH}_{3}\text{Cl} {\nu }_{1}$$ band is not present but the corresponding limit on the column density does not exclude CH₃Cl as the sink of Cl atoms. The $${CH}_{4}{\nu }_{3}$$ lines provide a rotational temperature $${T}_\text{rot}\,=\,160\pm \,25\,\text{K}$$ and a column density $$N(\text{CH}_{4})\,=\,1.8\times {10}^{16}\text{cm}^{-2}$$ The CH₄ expansion velocity and temperature suggest that methane is formed in cool gas near the terminal velocity of the circumstellar wind.