Spectroscopic constraints on CHOH formation: CO mixed with CHOH 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, 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:HO mixtures cannot reproduce the width and position of the observed red component without producing a shoulder at 2152 cm, which is not observed in astronomical spectra. Cuppen et al. showed that CO:CHOH 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 ( = 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 The previously unpublished spectrum of AFGL 7009S shows a wide band of solid CO, the first detection of CO ice in the polar phase. In this source, both the CO and CO ice bands are well fitted with CO:CHOH mixtures, while respecting the profiles and depths of the methanol bands at other wavelengths, whereas mixtures with HO cannot. The presence of a gradient in the CO:CHOH mixing ratio in the grain mantles is also suggested. Towards L1489 IRS, the profile of the CO band is also better fitted with CHOH-containing ices, although the CHOH 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 CHOH by the hydrogenation of CO ices.