Combination of CN(1-0), HCN(1-0), and HNC(1-0): A possible indicator for a high-mass star formation sequence in the Milky Way

Context. CN, HCN, and HNC have been used to discuss the star formation sequence in galaxies, but recent studies of large samples involving these lines did not yet provide convincing results. Aims. We intend to determine whether the global line ratios CN/HCN, CN/HNC, and HCN/HNC can be used to trace the high-mass star formation sequence in the Milky Way. Methods. We performed map observations of CN(1-0), HCN(1-0), and HNC(1-0) toward 38 high-mass star-forming regions, which includes high-mass starless cores (HMSC), high-mass protostars (HMPO), UCHII (ultra-compact HII) and normal HII regions. We identified the molecular clumps associated with them, and removed the clumps that were affected by environment. We averaged all the detected emission from each clump to obtain global line ratios and investigated their variations with the evolutionary stages of high-mass star-forming clumps. Results. The global line ratios of I-CN/I-HNC and I-HCN/I-HNC for HMSC clumps (HMSCCs), HMPO clumps (HMPOCs), UCHII region clumps (UCHIICs), and HII region clumps display an increasing trend. We tentatively divide the star-forming regions into two types. Type 1 sources include HMSCCs and HMPOCs that are not associated with external 20 cm continuum emission. Type 2 sources include all UCHIICs and HIICs. Our analysis shows that the I-CN/I-HNC and I-HCN/I-HNC line ratios can trace the evolution from type 1 to type 2 well. The same method may be used to study the evolution of external galaxies. Conclusions. I-CN/I-HNC and I-HCN/I-HNC appear to be good tracers for the evolution of high-mass star-forming regions in the Milky Way.