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

TL;DR

This study reports the first detection of methanol ice in the Central Molecular Zone, suggesting that gas-phase methanol is largely produced by desorption from icy grains in this region.

Contribution

It presents the discovery of methanol ice toward a star in the CMZ, providing new insights into methanol formation and distribution in the Galactic center.

Findings

High CH$_3$OH ice abundance (~17%) relative to H$_2$O ice.

Methanol ice likely originates from the MYSO's extended envelope.

Gas-phase CH$_3$OH can be produced by desorption from icy grains.

Abstract

Previous radio observations revealed widespread gas-phase methanol (CH$_3$OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH$_3$OH ice toward a star in the CMZ, based on a Subaru $3.4$-$4.0\ \mu$m spectrum, aided by NASA/IRTF $L'$ imaging and $2$-$4\ \mu$m spectra. The star lies $\sim8000$ au away in projection from a massive young stellar object (MYSO). Its observed high CH$_3$OH ice abundance ($17\%\pm3\%$ relative to H$_2$O ice) suggests that the $3.535\ \mu$m CH$_3$OH ice absorption likely arises in the MYSO's extended envelope. However, it is also possible that CH$_3$OH 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$_3$OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH$_3$OH abundance confirms the prominent $15.4\ \mu$m shoulder absorption observed toward GC MYSOs arises from CO$_2$ ice mixed with CH$_3$OH.