@misc{oai:ir.soken.ac.jp:00000371,
author = {村川, 幸史 and ムラカワ, コウジ and MURAKAWA, Koji},
month = {2016-02-17, 2016-02-17},
note = {In this paper, I will discuss the distribution of solid water in the molecular cloud, based on the spectroscopic observations toward the Taurus dark cloud.
In the interstellar space, dust surface works as catalysis of molecular formation and thus plays important role in the chemical evolution of the cloud. The solid water, or water ice, is the most abundant and ubiquitous ice component. Therefore water ice is a useful indicator of the chemical and physical condition in the interstellar space, since it is sensitive to the temperature, the density, and the ultraviolet radiation field. Recent observations in the millimeter wavelengths have resolved the detailed structures within the molecular clouds. Thus I attempted to identify the relationship between the water ice and small scale structures in the cloud.
Near infrared spectroscopy is used to study water ice that has a characteristic absorption band near 3.1 μm caused by O-H stretching mode. Usually the absorption band can be seen toward the background stars shining from behind the molecular cloud. Whittet et al. started such an measurement of stars in the direction of the Taurus molecular cloud complex, a nearby and prototype of the galactic molecular cloud of 10 pc scale.
I concentrated on a smaller region of 3 pc scale, Heiles Cloud 2 in the Taurus region. The star formation activity in Heiles Cloud 2 is moderate : only low mass stars are being formed and there are no OB stars which would produce destructive UV radiation. One can assume that the water ice in this region is controlled by the density of the molecular cloud and the external ultraviolet photon. 62 sources were detected by using PASP2 spectrometer covering 1.3 to 4.2 μm simultaneously, mounted on the 2.3 m Wyoming telescope and 1.5 m Mt. Lemmon telescope.
In order to estimate two parameters that characterize the molecular cloud, visual extinction Av and water ice opacity τICE, spectral type and continuum level of each object are determined. Spectral type of 53 out of 62 objects are derived from spectral energy distribution by standard method. Data for 2 stars are from the literature. The determination of continuum level relies on template stars with the same spectral type. Thus Av and τICE are systematically estimated for 55 objects.
The correlation between Av and τICE shows that absorption of water ice is detected above Av of 2～5 magnitude and τICE increases proportionally with Av. Scatters in the data are real, not caused by the observational errors nor the data reduction procedure. This result is consistent with trend in the larger scale. Whittet et al. derived similar Av vs. τICE correlation although with a sample of 17 field stars, and their result showed threshold of Av=3.3 magnitude above which the water ice absorption is detected.
Comparison of the spatial distribution of 62 infrared sources with respect to the gas distribution is made. The denser part of the molecular gas is represented in the C18O distribution, while the outer, less dense regions are delineated in the 13CO data. Recent millimeter observations by Sunada et al. revealed 4 dense clumps in Heiles Cloud 2. They show filamentary, elongated, or flattened shapes in 1 pc scale. Water ice detection is strongly correlated with the radio data: 6 sources of τICE larger than 0.3 are associated with the dense C18O clumps. Outside of the C18O or 13CO detection area, the water ice is not seen.
Simple geometrical structure of the dense clump is modeled to explain the Av threshold. Density of dust in the ellipsoidal clump is assumed to decrease with square of the distance from the clump center. The result shows that Av threshold varies from 2 to 5 magnitudes and τICE increases with increasing Av when the ratio of major to minor axis of the clump is changed. The variation in the threshold is due to the different contribution of the water-containing, inner portion of the clump along the line of sight. When the elongation is larger, the overall opacity increases while the contribution of the interior is smaller, and thus the threshold of water ice detection becomes larger.
This study implies that water ice detection level could differentiate the geometry of individual clumps in the molecular cloud without resolving the true spatial structure., application/pdf, 総研大甲第292号},
title = {Infrared studies on water ice distribution in the Taurus dark cloud},
year = {}
}