We present results from our spectroscopic study , using the Infrared Spectrograph ( IRS ) onboard the Spitzer Space Telescope , designed to identify massive young stellar objects ( YSOs ) in the Galactic Center ( GC ) .
Our sample of 107 YSO candidates was selected based on IRAC colors from the high spatial resolution , high sensitivity Spitzer /IRAC images in the Central Molecular Zone ( CMZ ) , which spans the central \sim 300 pc region of the Milky Way Galaxy .
We obtained IRS spectra over 5 \mu m to 35 \mu m using both high- and low-resolution IRS modules .
We spectroscopically identify massive YSOs by the presence of a 15.4 \mu m shoulder on the absorption profile of 15 \mu m CO _ { 2 } ice , suggestive of CO _ { 2 } ice mixed with CH _ { 3 } OH ice on grains .
This 15.4 \mu m shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources .
We show that 9 massive YSOs also reveal molecular gas-phase absorption from CO _ { 2 } , C _ { 2 } H _ { 2 } , and/or HCN , which traces warm and dense gas in YSOs .
Our results provide the first spectroscopic census of the massive YSO population in the GC .
We fit YSO models to the observed spectral energy distributions and find YSO masses of 8 - 23 M _ { \odot } , which generally agree with the masses derived from observed radio continuum emission .
We find that about 50 % of photometrically identified YSOs are confirmed with our spectroscopic study .
This implies a preliminary star formation rate of \sim 0.07 M _ { \odot } yr ^ { -1 } at the GC .