High-Resolution Mappings of the $l=1{\rlap{.}{^\circ}}3$ Complex in Molecular Lines: Discovery of a Proto-Superbubble

Abstract

Abstract We report on the results of molecular line observations toward the $l=1{\rlap{\hskip.75pt{^\circ}}\hskip.75pt.\hskip1pt}3$ 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 $J$=$1$–$0, {\rm HCO^{+}} J$=$1$–$0$, 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, ${\rm HCO^{+}}$/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 $110\ {\rm kms^{-1}}$. The high-velocity gas has a high density (${n_{\rm H}}\sim10^{4.5}\ {\rm cm^{-3}}$) and high SiO/$^{13}{\rm CO}$ intensity ratio, indicating that the gas was shocked. The typical HCN/${\rm HCO^{+}}$ 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 $10^{50.9\mbox{-}52.5}$ erg and $10^{4.6\mbox{-}5.3}$ yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions at a rate of $10^{-3\mbox{-}-4}$ 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 $10^{6.8\mbox{-}7.6}\ {\rm yr}$ ago.