Spectroscopic constraints on CH₃OH formation: CO mixed with CH₃OH ices towards young stellar objects

The prominent infrared absorption band of solid CO - commonly observed towards young stellar objects (YSOs) - consists of three empirically determined components. The broad 'red component' (2136 cm^-1, 4.681 μm) is generally attributed to solid CO mixed in a hydrogen-bonded environment. Usually, CO embedded in the abundantly present water is considered. However, CO:H₂O mixtures cannot reproduce the width and position of the observed red component without producing a shoulder at 2152 cm^-1, which is not observed in astronomical spectra. Cuppen et al. showed that CO:CH₃OH mixtures do not suffer from this problem. Here, this proposition is expanded by comparing literature laboratory spectra of different CO-containing ice mixtures to high-resolution (R = λ/Δλ = 25000) spectra of the massive YSO AFGL 7009S and of the low-mass YSOL1489 IRS. The previously unpublished spectrum of AFGL 7009S shows a wide band of solid ¹³CO, the first detection of ¹³CO ice in the polar phase. In this source, both the ¹²CO and ¹³CO ice bands are well fitted with CO:CH₃OH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with H₂O cannot. The presence of a gradient in the CO:CH₃OH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the ¹²CO band is also better fitted with CH₃OH-containing ices, although the CH₃OH abundance needed is a factor of 2.4 above previous measurements. Overall, however, the results are reasonably consistent with models and experiments about formation of CH₃OH by the hydrogenation of CO ices.