Star formation sites toward the Galactic center region: The correlation of CH ₃ OH masers, H ₂ O masers, and near-IR green sources

We present a study of star formation in the central molecular zone (CMZ) of our Galaxy through the association of three star formation indicators: 6.7 GHz CH₃OH masers, 22 GHz H₂O masers, and enhanced 4.5 μm emission ('green') sources. We explore how star formation in the CMZ(|ℓ| < 1.3, |b| < 10′) compares with that of the Galactic disk (6 > ℓ > 345, |b| < 2). Methods. Using an automated algorithm, we search for green sources toward 6.7 GHz CH₃OH masers detected in the Parkes Methanol Multibeam Survey. We combine these results with lists of 22 GHz H ₂O masers, including our Mopra survey of the CMZ. Results. We find that the correlation of CH₃OH masers with green sources is a function of Galactic latitude, with a minimum close to b = 0 and increasing with |b| (toward the central part of the Galaxy, 6 > ℓ > 345, |b| < 2). We find no significant difference between the correlation rate of CH₃OH masers with green sources in the CMZ and the disk. This suggests that although the physical conditions of the gas are different in the CMZ from that of the Galactic disk, once gravitational instability sets in at sufficiently high densities, signatures of star formation appear to be similar in both regions. Moreover, the detection of green sources, even at the distance of the Galactic center, shows that our technique can easily identify the early stages of star formation, especially in low-extinction regions of the Galaxy. Through the association of H₂O and CH₃OH masers, we identify 15 star-forming sites in the CMZ. We find a higher correlation rate of coincident H₂O and CH₃OH masers within the CMZ compared to the Galactic disk, indicating a difference in the maser evolutionary sequence for star-forming cores in these two regions.