Spectroscopic observations of ices around embedded young stellar objects in the Large Magellanic Cloud with AKARI

¹ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan e-mail: shimonishi@astron.s.u-tokyo.ac.jp
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
³ Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya, 464-8602, Japan

Received: 7 December 2009
Accepted: 12 March 2010

Abstract

Aims. The aim of this study is to understand the chemical conditions of ices around embedded young stellar objects (YSOs) in the metal-poor Large Magellanic Cloud (LMC).

Methods. We performed near-infrared (2.5-5 μm) spectroscopic observations toward 12 massive embedded YSOs and their candidates in the LMC using the infrared camera (IRC) onboard AKARI. We estimated the column densities of the H₂O, CO₂, and CO ices based on their 3.05, 4.27, and 4.67 μm absorption features, and we investigated the correlation between ice abundances and physical properties of YSOs.

Results. The ice absorption features of H₂O, CO₂, ¹³CO₂, CO, CH₃OH, and possibly XCN are detected in the spectra. In addition, hydrogen recombination lines and PAH emission bands are detected toward the majority of the targets. The derived typical CO₂/H₂O ice ratio of our samples (~0.36 ± 0.09) is greater than that of Galactic massive YSOs (~0.17 ± 0.03), while the CO/H₂O ice ratio is comparable. It is shown that the CO₂ ice abundance does not correlate with the observed characteristics of YSOs: the strength of hydrogen recombination line and the total luminosity. Likewise, clear correlation is not seen between the CO ice abundance and YSO characteristics, but it is suggested that the CO ice abundance of luminous samples is significantly lower than in other samples.

Conclusions. The systematic difference in the CO2 ice abundance around the LMC's massive YSOs, which was suggested by previous studies, is confirmed with the new near-IR data. We suggest that the strong ultraviolet radiation field and/or the high dust temperature in the LMC are responsible for the observed high abundance of the CO₂ ice. It is suggested that the internal stellar radiation does not play an important role in the evolution of the CO₂ ice around a massive YSO, while more volatile molecules like CO are susceptible to the effect of the stellar radiation.