Molecular and Atomic Clouds toward the Wolf-Rayet Nebula NGC 2359:
  Possible Evidence for Isolated High-Mass Star Formation Triggered by a
  Cloud-Cloud Collision
  NGC 2359 is an HII region located in the outer Galaxy that contains the
isolated Wolf-Rayet (WR) star HD 56925. We present millimeter/submillimeter
observations of $^{12}$CO($J$ = 1-0, 3-2) line emission toward the entire
nebula. We identified that there are three molecular clouds at VLSR $\sim$37,
$\sim$54, and $\sim$67 km s$^{-1}$, and three HI clouds: two of them are at
VLSR $\sim$54 km s$^{-1}$ and the other is at $\sim$63 km s$^{-1}$. These
clouds except for the CO cloud at 67 km s$^{-1}$ are limb-brightened in the
radio continuum, suggesting part of each cloud has been ionized. We newly found
an expanding gas motion of CO/HI, whose center and expansion velocities are
$\sim$51 and $\sim$4.5 km s$^{-1}$, respectively. This is consistent with large
line widths of the CO and HI clouds at 54 km s$^{-1}$. The kinematic
temperature of CO clouds at 37 and 54 km s$^{-1}$ are derived to be 17 and 61
K, respectively, whereas that of the CO cloud at 67 km s$^{-1}$ is only 6 K,
indicating that the former two clouds have been heated by strong UV radiation.
We concluded that the 37 and 54 km s$^{-1}$ CO clouds and three HI clouds are
associated with NGC 2359, even if these clouds have different velocities.
Although the velocity difference including the expanding motion are typical
signatures of the stellar feedback from the exciting star, our analysis
revealed that the observed large momentum for the 37 km s$^{-1}$ CO cloud
cannot be explained only by the total wind momentum of the WR star and its
progenitor. We therefore propose an alternative scenario that the isolated
high-mass progenitor of HD 56925 was formed by a collision between the CO
clouds at 37 and 54 km s$^{-1}$. If we apply the collision scenario, NGC 2359
corresponds to the final phase of the cloud-cloud collision.
