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:H_2O 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:CH_3OH 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 = 25 000) spectra of the massive YSO AFGL 7009S and of the low-mass YSO L1489 IRS. The previously unpublished spectrum of AFGL 7009S shows a wide band of solid ^(13)CO, the first detection of ^(13)CO ice in the polar phase. In this source, both the ^(12)CO and ^(13)CO ice bands are well fitted with CO:CH_3OH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with H_2O cannot. The presence of a gradient in the CO:CH_3OH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the ^(12)CO band is also better fitted with CH_3OH-containing ices, although the CH_3OH 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 CH_3OH by the hydrogenation of CO ices.