HCN J = 4-3, HNC J = 1-0, H¹³CN J = 1-0, and HC₃N J = 10-9 maps of galactic center region. II. Physical properties of dense-gas clumps and probability of star formation

We report a statistical analysis exploring the origin of the overall low star formation efficiency (SFE) of the Galactic central molecular zone (CMZ) and the SFE diversity among the CMZ clouds using a wide-field HCN J = 4-3 map, whose optically thin critical density (∼107 cm-3) is the highest among the tracers ever used in CMZ surveys. Logistic regression is performed to empirically formulate star formation probability of 195 HCN clumps, 13 of which contain star formation signatures. The explanatory parameters in the best-fit model are reduced into the virial parameter αvir without significant contribution from other parameters, whereas the performance of the model without αvir is no better than that using randomly generated data. The threshold αvir is 6, which translates into a volume density (nH2) of 104.6 cm-3 with the nH2-αvir correlation. The scarcity of the low-αvir clumps, whose fraction to all HCN clumps is 0.1, can be considered as one of the immediate causes of the suppressed SFE. No correlation between the clump size or mass and star formation probability is found, implying that HCN J = 4-3 does not immediately trace the mass of star-forming gas above a threshold density. Meanwhile, star-forming and non-star-forming clouds are degenerate in the physical parameters of the CS J = 1-0 clouds, highlighting the efficacy of the HCN J = 4-3 line to probe star-forming regions in the CMZ. The timescale of the high-αvir to low-αvir transition is ≲2 Myr, which is consistent with the tidal compression and X1/X2 orbit transition models but possibly does not fit the cloud-cloud collision picture.