2000ApJS..128..603M

We have conducted near infrared spectroscopy of 61 background stars toward Heiles Cloud 2 in the Taurus molecular cloud complex. We used a low-dispersion spectrometer, PASP2, which can simultaneously obtain the spectrum with a wavelength coverage between 1.3 and 4.2 µm. For 56 of 61 objects, the visual extinction (A_V) and the optical depth of water ice at λ=3.1 µm (τ_ICE) have been estimated: for 50 of 56 objects, these were systematically estimated from our data only. In order to investigate the water ice distribution in Heiles Cloud 2, we have constructed a ``water ice map'' in which τ_ICE is plotted at the position of each object. The water ice map is then compared with the C¹⁸O (J=1-0) map obtained by millimeter observations performed by Sunada & Kitamura. We find that the distribution of water ice is closely correlated with that of C¹⁸O. Strong water ice absorption is seen only toward the dense C¹⁸O clumps, while less water ice absorption is detected toward the outer region of the cloud. There is an A_V threshold for the positive ice detection (A_V0), as suggested by previous observations, but with a significant scatter; A_V0=2-5 mag. The scatter might be caused by the different contribution of the inner water-containing portion of the cloud along the line of sight. The value of τ_ICE increases with increasing of A_V and the slope of Δτ_ICE/ΔA_V is 0.067, consistent with the previously observed values for the Taurus molecular cloud.