Near-infrared spectroscopic observations of massive young stellar object
  candidates in the Central Molecular Zone
  We present a spectroscopic follow-up of photometrically-selected young
stellar object (YSO) candidates in the Central Molecular Zone of the Galactic
center. Our goal is to quantify the contamination of this YSO sample by
reddened giant stars with circumstellar envelopes and to determine the star
formation rate in the CMZ. We obtained KMOS low-resolution near-infrared
spectra (R ~4000) between 2.0 and 2.5 um of sources, many of them previously
identified, by mid-infrared photometric criteria, as massive YSOs in the
Galactic center. Our final sample consists of 91 stars with good
signal-to-noise ratio. We separate YSOs from cool late-type stars based on
spectral features of CO and Br_gamma at 2.3 um and 2.16 um respectively. We
make use of SED model fits to the observed photometric data points from 1.25 to
24 um in order to estimate approximate masses for the YSOs. Using the
spectroscopically identified YSOs in our sample, we confirm that existing
colour-colour diagrams and colour-magnitude diagrams are unable to efficiently
separate YSOs and cool late-type stars. In addition, we define a new
colour-colour criterion that separates YSOs from cool late-type stars in the
H-Ks vs H-[8.0] diagram. We use this new criterion to identify YSO candidates
in the |l| < 1.5, |b|<0.5 degree region and use model SED fits to estimate
their approximate masses. By assuming an appropriate initial mass function
(IMF) and extrapolating the stellar IMF down to lower masses, we determine a
star formation rate (SFR) of ~0.046 +/- 0.026 Msun/yr assuming an average age
of 0.75 +/- 0.25 Myr for the YSOs. This value is lower than estimates found
using the YSO counting method in the literature. Our SFR estimate in the CMZ
agrees with the previous estimates from different methods and reaffirms that
star formation in the CMZ is proceeding at a lower rate than predicted by
various star forming models.
