HIGH-RESOLUTION STUDIES OF SOLID CO IN THE TAURUS DARK CLOUD - CHARACTERIZING THE ICES IN QUIESCENT CLOUDS

We present spectra with resolving power lambda/Delta lambda similar or equal to 1200 of the 4.67 mu m (2140 cm(-1)) feature of solid carbon monoxide in four held stars and two embedded stars in the line of sight to the Taurus dark cloud. Our data are consistent with the presence of CO in distinct polar and nonpolar ice mixtures. The profiles are fitted with models that use laboratory data for various astrophysical ice analogs. We find that the broad (polar) component of the profile in all sources is generally consistent with CO as a minor constituent in an ice matrix dominated by H2O at 10 K. The narrow (nonpolar) component observed in the field stars is best fitted by a dilute H2O matrix with CO itself as the dominant constituent. Significantly, the embedded stars Elias 18 and (more tentatively) Elias 1 have distinctly different narrow components which are best fitted by CO in a mixture dominated by CO2. Our results may therefore suggest that CO2 is being formed by energetic processing of grain mantles in the dark cloud close to the embedded stars. Column densities are deduced for solid CO in each line of sight. The overall solid state abundance of CO is similar to 30% of that of H2O in the most highly obscured field stars. Comparison of column density estimates for solid and gas phases indicates that up to similar to 40% of the CO is depleted from gas onto grains in quiescent regions of the cloud. We conclude that this is indicative of the functioning of an efficient desorption process throughout the cloud. The correlations of CO and H2O column densities with visual extinction are examined. Considering only field stars in Taurus, we deduce the following correlations: N(CO) = (0.4 +/- 0.1)[A(V) - (6.0 +/- 4.1)] x 10(17) cm(-2) and N(H2O) = (1.5 +/- 0.1)[A(V) - (3.2 +/- 0.5)] x 10(17) cm(-2). Comparing these results with data from the literature for other clouds, we find some uniformity in behavior: our correlation lines for Taurus are reasonably representative of data for sources in the Serpens and R CrA clouds, to within observational scatter. Sources in the rho Oph dark cloud show a greater degree of scatter about these correlation lines and exhibit a trend of lower molecular column densities per unit extinction toward individual embedded sources.