Column densities of CO (left) and CO2 (right) ices plotted against that of water ice. The measurements of CO ice toward the Pipe Nebula are all upper limits (yellow triangles) except for 17312818−2631268 (source # 2) shown in the yellow circle. The gray circle is the column density of CO ice on source #2 as well, but measured assuming that the short-wavelength shoulder 4.655–4.665 μm is part of the continuum (see Sect. 2.2.3). No observation of CO2 ice is available in the Pipe Nebula. Other measurements shown are from Taurus (Whittet et al. 2007), L 183 (Whittet et al. 2013), CK 2 (Knez et al. 2005), IC 5146 (Q21-1; Chiar et al. 2011), assorted quiescent clouds (Noble et al. 2013), Lupus (Boogert et al. 2013), and L 429 (Boogert et al. 2011). All the measurements are against the continua of background field stars through quiescent clouds. Only those lines of sight where both CO and CO2 ices are positively detected are shown. Four simulations of ice formation are compared with the observations (Hocuk et al. (in prep.); Vasyunin & Herbst 2013; Garrod & Pauly 2011; Pauly & Garrod 2016). The number of layers presented in the simulations are converted to the column density by assuming that one monolayer corresponds to 9.6 × 1015 cm-2 (Appendix B). Model column densities calculated with conversion factors that are two times larger and smaller are shown by the shaded areas.
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