The Life and Death of Dense Molecular Clumps in the Large Magellanic
  Cloud
  We report the results of a high spatial (parsec) resolution HCO+ (J = 1-0)
and HCN (J = 1-0) emission survey toward the giant molecular clouds of the star
formation regions N105, N113, N159, and N44 in the Large Magellanic Cloud. The
HCO+ and HCN observations at 89.2 and 88.6 GHz, respectively, were conducted in
the compact configuration of the Australia Telescope Compact Array. The
emission is imaged into individual clumps with masses between 10^2 and 10^4
solar masses and radii of <1 pc to ~2 pc. Many of the clumps are coincident
with indicators of current massive star formation, indicating that many of the
clumps are associated with deeply-embedded forming stars and star clusters. We
find that massive YSO-bearing clumps tend to be larger (>1 pc), more massive (M
> 10^3 solar masses), and have higher surface densities (~1 g cm^-2), while
clumps without signs of star formation are smaller (<1 pc), less massive (M <
10^3 solar masses), and have lower surface densities (~0.1 g cm^-2). The dearth
of massive (M >10^3 solar masses) clumps not bearing massive YSOs suggests the
onset of star formation occurs rapidly once the clump has attained physical
properties favorable to massive star formation. Using a large sample of LMC
massive YSO mid-IR spectra, we estimate that ~2/3 of the massive YSOs for which
there are Spitzer mid-IR spectra are no longer located in molecular clumps; we
estimate that these young stars/clusters have destroyed their natal clumps on a
time scale of at least 3 x 10^{5}$ yrs.
