SIMBAD references

A detailed intercomparison is made between published observations of H₂ O and CO ices towards the Taurus and Ophiuchus dark clouds. The column densities of the ices are intercompared, and each compared to the visual extinction through the clouds, A_v. It is neither clear that the two clouds have different or well defined threshold extinctions for the survival of ice mantles, nor that, for each of the clouds, the thresholds for the survival of CO and H₂ O mantles are different. The inclusion of new objects in Taurus (Teixeira et al., 1998A&A...330..711T) introduces a large scatter in the relations between those quantities relative to results obtained by previous authors. Lines-of-sight towards deeply embedded Young Stellar Objects in Taurus appear to show an enhancement in the amount of water-ice relative to lines-of-sight towards field stars behind that cloud. While for A_v<14mag there is a tight correlation between the water-ice column density, N_s(H₂O), and A_v, the inclusion of those new objects reveals a discontinuity in the relation between N_s(H₂O) and A_v. The interpretation of this discontinuity is discussed.