  The expansion of interstellar bubbles is suggested to be an important
mechanism of triggering material accumulation and star formation. In this work,
we study the gaseous environment of a large sample of bubbles, aiming to
explore the possible evidence of triggered gas accumulation and star formation
in a statistical sense. By cross-matching 6124 Spitzer bubbles from the Milky
Way Project (MWP) and more than 2500 Galactic HII regions, we obtain the
velocity information for 818 MWP bubbles. To study the gaseous environment of
the interstellar bubbles and get rid of the projection effect as much as
possible, we constrain the velocity difference between the bubbles and the
13CO(1-0) emission extracted from the Galactic Ring Survey (GRS). Three
methods: the mean azimuthally averaged radial profile of 13CO emission, the
surface number density of molecular clumps, and the angular cross-correlation
function of MWP bubbles and the GRS molecular clumps are adopted. Significant
over density of molecular gas is found to be close to the bubble rims. 60
percent of the studied bubbles were found to have associated molecular clumps.
By comparing the clump-associated and the clump-unassociated MWP bubbles, we
reveal that the bubbles in associations tend to be larger and thicker in
physical sizes. We speculate that some of the bubble-associated clumps result
from the expansion of bubbles. The fraction of the molecular clumps associated
with the MWP bubbles is estimated to be about 20 percent after considering the
projection effect. For the bubble-clump complexes, we found that the bubbles in
the complexes with associated massive young stellar object(s) (MYSO(s)) have
larger physical sizes, hence the complexes tend to be older. We propose that an
evolutionary sequence might exist between the relative younger
MYSO-unassociated bubble-clump complexes and the MYSO-associated complexes.

Astrophysics of Galaxies