[2.25] vs. [2.25]–[2.34] color magnitude diagram. Red color (positive value) in [2.25]–[2.34] means a weak CO absorption at 2.34 μm which is an indicator for early spectral type stars. A sequence of stars from ([2.25] − [2.34] , [2.25] ) ~ (−0.2,9) to (0,13) is the red giant branch (RGB). Blue ×s are the early-type star candidates found in Nishiyama & Schödel (2013) and they are distributed at the red side of the RGB, indicating earlier spectral type. Most of the bright ([2.25] ≲ 11), very red ([2.25] − [2.34] ≳ 0.1) sources are already known early-type stars and dust-embedded sources. Light green ×s represents the early-type star candidates whose spectrum is obtained in this study, and red crosses are observed red giants as a spectrum reference.