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We have performed a multi-wavelength study of the mid-infrared bubble S44 toinvestigate the origin of isolated high-mass star(s) and the star-formationprocess around the bubble formed by the HII region. In this paper, we reportthe results of new CO observations ($^{12}$CO, $^{13}$CO $J=$1-0, and $^{12}$CO$J=$3-2) toward the isolated bubble S44 using the NANTEN2, Mopra, and ASTEradio telescopes. We found two velocity components in the direction of thebubble, at $-84$ km s$^{-1}$ and $-79$ km s$^{-1}$. These two clouds are likelyto be physically associated with the bubble,both because of the enhanced$^{12}$CO $J=$3-2/1-0 intensity ratio from a ring-like structure affected byultraviolet radiation from embedded high-mass star(s) and from themorphological correspondence between the 8 $\mu$m emission and the COdistribution. Assuming a single object, we estimate the spectral type of theembedded star inside the bubble to be O8.5-9 ($\sim 20 M_{\odot}$) from theradio-continuum free-free emission. We hypothesize that the two clouds collidedwith each other 3 Myr ago, triggering the formation of the isolated high-massstar in S44, as also occurred with RCW 120 and RCW 79. We argue that thisscenario can explain the origin of the isolated O-star inside the bubble.

CO observations toward the isolated mid-infrared bubble S44: External triggering of O-star formation by a cloud-cloud collision