Massive young stellar objects in the Galactic center. I. Spectroscopic identification from Spitzer infrared spectrograph observations

We present results from our spectroscopic study, using the Infrared Spectrograph (IRS) on board 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 Infrared Array Camera (IRAC) colors from the high spatial resolution, high sensitivity Spitzer/IRAC images in the Central Molecular Zone, which spans the central ∼300pc region of the Milky Way. We obtained IRS spectra over 5-35 μm using both high- and low-resolution IRS modules. We spectroscopically identify massive YSOs by the presence of a 15.4 μm shoulder on the absorption profile of 15 μm CO₂ ice, suggestive of CO₂ ice mixed with CH₃OH ice on grains. This 15.4 μm shoulder is clearly observed in 16 sources and possibly observed in an additional 19 sources. We show that nine massive YSOs also reveal molecular gas-phase absorption from CO₂, C₂H₂, 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 _⊙, 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 ∼0.07 M_⊙ yr^-1 at the GC.