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 H ii 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 ^ { -1 } 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 H ii region is currently expanding with a velocity of 3.4 km s ^ { -1 } , implying that the shell ’ s expansion age is \sim 1 Myr .
This timescale is in good agreement with numerical simulations of the expansion of the H ii region ( Hosokawa & Inutsuka ) .
We conclude that the star formation on the border of the shell is triggered by the expansion of the H ii region .