Grain mantles in the Taurus dark cloud

Abstract

We have obtained low-resolution |$(\lambda /\Delta \lambda \sim 160)$| 3-µm spectra of a selection of field stars lying behind the Taurus dark cloud. Several of these stars have abnormal extinction laws, which are best characterized by |$A_{K}\leq 0.27E(J - K)$| and |$A_{K}\leq 5.4E(J - K)$|⁠. We find a very good correlation between AV and |$\tau_{3.05}$| (the H2O ice feature), confirming a previous result, and between AV and |$\tau_{3.45}$| (the long-wavelength wing), yielding the ratios |$\tau _{3.05}/A_{V} = 0.059\pm 0.003$| and |$\tau _{3.45}/A_{V} = 0.0077\pm 0.0005$|⁠. The threshold extinctions for the H2O ice band and the long-wavelength wing are found to be the same, within the uncertainties, at |${A}_{v_{0}}$| = 2.6. The fact that these thresholds are the same means that the long-wavelength wing cannot be a result of a highly refractory hydrocarbon residue. The lack of substructure in the long- wavelength wing also argues against a hydrocarbon origin for the wing.