  We present Heinrich Hertz Telescope CO observations of the shell structure
near the active star-forming complex W51A to investigate the process of star
formation triggered by the expansion of an HII region. The CO observations
confirm that dense molecular material has been collected along the shell
detected in Spitzer IRAC images. The CO distribution shows that the shell is
blown out toward a lower density region to the northwest. Total hydrogen column
density around the shell is high enough to form new stars. We find two CO
condensations with the same central velocity of 59 km/s to the east and north
along the edge of the IRAC shell. We identify two YSOs in early evolutionary
stages (Stage 0/I) within the densest molecular condensation. From the CO
kinematics, we find that the HII region is currently expanding with a velocity
of 3.4 km/s, implying that the shell's expansion age is ~1 Myr. This timescale
is in good agreement with numerical simulations of the expansion of the HII
region (Hosokawa et al. 2006). We conclude that the star formation on the
border of the shell is triggered by the expansion of the HII region.
