High-resolution mappings of the l =1.3 complex in molecular lines: Discovery of a proto-superbubble

We report on the results of molecular line observations toward the l = 1,3 complex, an anomalous cloud complex in the central molecular zone of the Galaxy, The CO J=3-2 survey recently performed with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope has found that the complex has an enhanced CO J=3-2/J=1-0 intensity ratio. We have made high-resolution maps of the CO J=1-0, HCN J=1-0, HCO⁺ J=1-0, SiO J=1-0, and J=2-1 lines with the Nobeyama 45 m telescope. The complex is found to be rich in shells and arcs of dense molecular gas. We identified 9 expanding shells in HCN maps and compact SiO features associated with the shells. The intensity ratios of HCN/CO, HCO ⁺/CO, and CO J=3-2/J=1-0 are coherently enhanced by a factor of a few in gas with an LSR velocity higher than 110 km s^-1. The high-velocity gas has a high density (n_H ∼ 10^4.5 cm^-3) and high SiO/¹³CO intensity ratio, indicating that the gas was shocked. The typical HCN/HCO⁺ intensity ratio is found to be 2.3, being higher by a factor of a few than those in the Galactic disk clouds. The typical kinetic energy and expansion time of the shells are estimated to be 10^50.9-52.5 erg and 10^4.6-5.3 yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions at a rate of 10^-3-4 yr^-1. These estimates suggest that the expanding shells as a whole may be in the early stage of superbubble formation. This proto-superbubble may have originated in a massive cluster formation that took place 10^6.8-7.6 yr ago.