Abstract
This study aims to elucidate a possible link between chemical properties of ices in star-forming regions and environmental characteristics of the host galaxy. We performed 3--4 micron spectroscopic observations toward nine embedded high-mass YSOs in the Large Magellanic Cloud (LMC) with the ISAAC at the VLT. Additionally, we analyzed archival ISAAC data of two LMC YSOs. As a result, we detected absorption bands due to solid H2O and CH3OH as well as the 3.47 micron absorption band. The 3.53 micron CH3OH ice absorption band for the LMC YSOs is found to be absent or very weak compared to that seen toward Galactic sources. The result suggests the low abundance of CH3OH ice in the LMC. The 3.47 micron absorption band is detected toward six out of eleven LMC YSOs. We found that the 3.47 micron 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 micron band. For the LMC sources with relatively large H2O ice optical depths, we found that the strength ratio of the 3.47 micron band relative to the water ice band is only marginally lower than those of the Galactic sources. 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. 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 micron 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 micron band carrier in the LMC.
Highlights
Understanding the chemical diversity of materials in star- and planet-forming regions is one of the key topics of present-day astronomy
For the Large Magellanic Cloud (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
We propose that grain surface reactions at a relatively high dust temperature 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
Summary
Understanding the chemical diversity of materials in star- and planet-forming regions is one of the key topics of present-day astronomy. Shimonishi et al (2010), for example, report the results of 2.5–5.0 μm spectroscopic observations of embedded highmass YSOs in the LMC with the Infrared Camera (IRC) on board the AKARI satellite (Onaka et al 2007; Murakami et al 2007) They find that the CO2/H2O ice ratio of LMC YSOs is systematically higher than those measured toward Galactic counterparts. The abundance of CH3OH in the solid phase is of great importance for molecular chemistry since it is believed to be a starting point for the formation of complex organic molecules in circumstellar environments of YSOs (e.g., Nomura & Millar 2004; Herbst & van Dishoeck 2009).
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