Infrared spectroscopy of dust in the Taurus dark clouds: ice and silicates

We present low-resolution spectra of the 3 μm water-ice and 10 μm silicate dust features for stars in the direction of the extensive dark cloud complex in Taurus. A total of 22 stars were observed at 3 μm, and 16 at 10 μm. Our sample includes both dust-embedded objects and background field stars seen through the cloud. New and previously published results are combined to investigate the correlation of the strengths of both features with visual extinction AV, and we demonstrate the existence of a very close linear correlation between the peak optical depth in the 3 μm feature and AV for field stars. Ice is detected in all cases where AV exceeds a threshold value of 3.3±0.1 mag, a result which provides a firm observational basis for models of volatile mantle growth on grains in the dark cloud environment. In contrast, the silicate feature is rather poorly correlated with AV, and its relative weakness per unit AV in the spectra of field stars with strong ice bands may reflect the dilution of silicates as a fraction of total grain mass as mantles accrete in the dark cloud.