Infrared spectroscopy of dust in the Taurus dark clouds: solid carbon monoxide

Spectra centred on the spectral feature of solid CO at 4.67 µm wavelength are presented for eight stars in or behind the quiescent dark cloud complex in Taurus. The solid CO profile is dominated by a sharp component (FWHM ∼ 8 cm⁻¹) centred at 4.673 µm (2140 cm⁻¹). As in previous observations of the feature, asymmetry in the profile is consistent with the presence of a weaker, somewhat broader, overlapping component centred at ∼ 4.682 µm (2136 cm⁻¹). New and previously published data for Taurus stars are combined to study the correlation of the peak optical depth in the CO feature with visual extinction and with the depth of the water-ice feature at 3.0 µm. A threshold extinction of $$A_\nu = 5.3\pm 0.6$$ mag is required for solid CO to be of detectable strength, compared with 3.3±0.1 for water-ice. CO embedded in an H₂O matrix may explain the broad component of the feature, but cannot account for the dominant sharp feature. The best correspondence to the latter is obtained with CO embedded in non-polar molecules such as CO₂ or CH₄. Comparison of our data for solid CO with gas-phase column densities deduced from millimetre-wave observations of CO emission in the same lines-of-sight suggests that ∼ 30 per cent of the available CO is depleted on to grains.