VLT/ISAAC infrared spectroscopy of embedded high-mass YSOs in the Large Magellanic Cloud: Methanol and the 3.47 μm band^⋆

¹ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aramakiazaaoba 6-3, Aoba-ku, Sendai, 980-8578 Miyagi Japan
e-mail: shimonishi@astr.tohoku.ac.jp
² Astronomical Institute, Tohoku University, Aramakiazaaoba 6-3, Aoba-ku, Sendai, 980-8578 Miyagi, Japan
³ Institut d’Astrophysique Spatiale, UMR 8617, Université Paris-Sud, Bâtiment 121, 91405 Orsay, France
⁴ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan

Received: 19 May 2015
Accepted: 4 November 2015

Abstract

Aims. This study aims to elucidate a possible link between chemical properties of ices in star-forming regions and environmental characteristics (particularly metallicity) of the host galaxy. The Large Magellanic Cloud (LMC) is an excellent target to study properties of interstellar and circumstellar medium in a different galactic environment thanks to its proximity and low metallicity.

Methods. We performed near-infrared, L-band spectroscopic observations toward embedded high-mass young stellar objects (YSOs) in the LMC with the Infrared Spectrometer And Array Camera (ISAAC) at the Very Large Telescope. The 3.2–3.7 μm spectral region, which is accessible from ground-based telescopes, is important for ice studies, since various C–H stretching vibrations of carbon bearing species fall in this region.

Results. We obtained medium-resolution (R ~ 500) spectra in the 3–4 μm range for nine high-mass YSOs in the LMC. Additionally, we analyzed archival ISAAC data of two LMC YSOs. We detected absorption bands due to solid H₂O and CH₃OH as well as the 3.47 μm absorption band. The properties of these bands are investigated based on comparisons with Galactic embedded sources. The 3.53 μm CH₃OH ice absorption band for the LMC YSOs is found to be absent or very weak compared to that seen toward Galactic sources. The absorption band is weakly detected for two out of eleven objects. We estimate the abundance of the CH₃OH ice, which suggests that solid CH₃OH is less abundant for high-mass YSOs in the LMC than those in our Galaxy. The 3.47 μm absorption band is detected toward six out of eleven LMC YSOs. We found that the 3.47 μm band and the H₂O ice band correlate similarly between the LMC and Galactic samples, but the LMC sources seem to require a slightly higher H₂O ice threshold for the appearance of the 3.47 μm band. For the LMC sources with relatively large H₂O ice optical depths, we found that the strength ratio of the 3.47 μm band relative to the water ice band is only marginally lower than those of the Galactic sources.

Conclusions. We propose that grain surface reactions at a relatively high dust temperature (warm ice chemistry) are responsible for the observed characteristics of ice chemical compositions in the LMC; i.e., the low abundance of solid CH₃OH presented in this work as well as the high abundance of solid CO₂ reported in previous studies. We suggest that this warm ice chemistry is one of the important characteristics of interstellar and circumstellar chemistry in low metallicity environments. The low abundance of CH₃OH in the solid phase implies that formation of complex organic molecules from methanol-derived species is less efficient in the LMC. For the 3.47 μm band, the observed difference in the water ice threshold may suggest that a more shielded environment is necessary for the formation of the 3.47 μm band carrier in the LMC. On the one hand, in well-shielded regions of the LMC, our results suggest that the lower metallicity and different interstellar environment of the LMC have little effect on the abundance ratio of the 3.47 μm band carrier and water ice.