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