AKARI Near-Infrared Spectroscopy: Detection of H2O and CO2 Ices toward Young Stellar Objects in the Large Magellanic Cloud


First Author:
Takashi Shimonishi
Email: shimonishi AT astron.s.u-tokyo.ac.jp
University of Tokyo
Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku
Tokyo, 113-0003, JAPAN
Coauthors:
Onaka, Takashi,
Kato, Daisuke,
Sakon, Itsuki,
Ita, Yoshifusa,
Kawamura, Akiko,
Kaneda, Hidehiro,

Abstract

We present the results of near-infrared (NIR) spectroscopic observations of Young Stellar Objects (YSOs) in the Large Magellanic Cloud with the Infrared Camera (IRC) on board AKARI satellite. Properties of extragalactic Young Stellar Objects (YSOs) provide us important information on the understanding of the diversity of YSOs in different galactic environments. An infrared spectrum of YSOs shows absorption features of various ices which are thought to be an important reservoir of heavy elements and complex molecules in the circumstellar environments of YSOs. We have performed a low resolution (R ~20) NIR spectroscopic survey of the LMC with the IRC on board AKARI. We detected absorption features of the H2O ice 3.05um and the CO2 ice 4.27 um stretching mode toward seven massive YSOs. These samples are included in the recent YSO candidates catalog based on Spitzer SAGE photometric database and for the first time spectroscopically confirmed to be YSOs by the present study. This is the first detection of the 4.27 um stretching mode of CO2 ice toward extragalactic YSOs. Studying the compositions of ices as functions of physical environments is crucial to understand the chemical evolution in circumstellar environments of YSOs. We evaluated the column densities of the ices toward these seven YSOs and derived the CO2/H2O ratio to be 0.59+-0.31. This is clearly higher than that seen in Galactic massive YSOs (0.17+-0.03). We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC may be responsible for the observed high CO2 ice abundance. We report the detail of the present study in our presentation. The studies of the ices and the extragalactic YSOs have been conducted actively by Spitzer. The present NIR spectroscopic study by AKARI is expected to play an important and complementary role in these areas.
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