  We have performed a multi-wavelength study of the mid-infrared bubble S44 to
investigate the origin of isolated high-mass star(s) and the star-formation
process around the bubble formed by the HII region. In this paper, we report
the 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 ASTE
radio telescopes. We found two velocity components in the direction of the
bubble, at $-84$ km s$^{-1}$ and $-79$ km s$^{-1}$. These two clouds are likely
to 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 by
ultraviolet radiation from embedded high-mass star(s) and from the
morphological correspondence between the 8 $\mu$m emission and the CO
distribution. Assuming a single object, we estimate the spectral type of the
embedded star inside the bubble to be O8.5-9 ($\sim 20 M_{\odot}$) from the
radio-continuum free-free emission. We hypothesize that the two clouds collided
with each other 3 Myr ago, triggering the formation of the isolated high-mass
star in S44, as also occurred with RCW 120 and RCW 79. We argue that this
scenario can explain the origin of the isolated O-star inside the bubble.
