Abundant Methanol Ice toward a Massive Young Stellar Object in the Central Molecular Zone

Abstract

Previous radio observations revealed widespread gas-phase methanol (CH_3OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH_3OH ice toward a star in the CMZ, based on a Subaru 3.4–4.0 μm spectrum, aided by NASA/IRTF L' imaging and 2–4 μm spectra. The star lies ~8000 au away in projection from a massive young stellar object (MYSO). Its observed high CH_3OH ice abundance (17% ± 3% relative to H_2O ice) suggests that the 3.535 μm CH_3OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH_3OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH_3OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH_3OH abundance confirms the prominent 15.4 μm shoulder absorption observed toward GC MYSOs arises from CO_2 ice mixed with CH_3OH.

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