1–4 Micron Spectrophotometry of Dust in the Taurus Dark Cloud: Water Ice Distribution in Heiles Cloud 2

Abstract

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 (AV) and the optical depth of water ice at λ=3.1 μm (τICE) 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 τICE 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 AV threshold for the positive ice detection (AV0), as suggested by previous observations, but with a significant scatter; AV0=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 τICE increases with increasing of AV and the slope of ΔτICE/ΔAV is 0.067, consistent with the previously observed values for the Taurus molecular cloud.