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

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 $3.4$-$4.0\hat{I}¼$m spectrum, aided by NASA/IRTF $L^{\prime }$ imaging and $2$-$4\hat{I}¼$m spectra. The star lies $\sim 8000$ au away in projection from a massive young stellar object (MYSO). Its observed high CH${}_3$OH ice abundance ($17\%\pm 3\%$ relative to H${}_2$O ice) suggests that the $3.535\hat{I}¼$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 $15.4\hat{I}¼$m shoulder absorption observed toward GC MYSOs arises from CO${}_2$ ice mixed with CH${}_3$OH.

Accepted for publication in the Astrophysical Journal Letters