The Interstellar Bubbles of G38.9-0.4 and the Impact of Stellar Feedback
  on Star Formation
  We present a study of the star formation (SF) region G38.9-0.4 using publicly
available multiwavelength Galactic Plane surveys from ground- and space-based
observatories. This region is composed of four bright mid-IR bubbles and
numerous infrared dark clouds. Two bubbles, N 74 and N 75, each host a star
cluster anchored by a single O9.5V star. We identified 162 young stellar
objects (YSOs) and classify 54 as stage I, 7 stage II, 6 stage III, and 32
ambiguous. We do not detect the classical signposts of triggered SF, i.e.,
star-forming pillars or YSOs embedded within bubble rims. We conclude that
feedback-triggered SF has not occurred in G38.9-0.4. The YSOs are
preferentially coincident with infrared dark clouds. This leads to a strong
correlation between areal YSO mass surface density and gas mass surface density
with a power law slope near 1.3, which closely matches the Schmidt-Kennicutt
Law. The correlation is similar inside and outside the bubbles and may mean
that the SF efficiency is neither enhanced nor supressed in regions potentially
influenced by stellar feedback. This suggests that gas density, regardless of
how it is collected, is a more important driver of SF than stellar feedback.
Larger studies should be able to quantify the fraction of all SF that is
feedback-triggered by determining the fraction SF, feedback-compressed gas
surrounding HII regions relative to that already present in molecular clouds.
