IRDC G030.88+00.13: A Tale of Two Massive Clumps
  Massive stars (M $\gsim 10$ \msun) form from collapse of parsec-scale
molecular clumps. How molecular clumps fragment to give rise to massive stars
in a cluster with a distribution of masses is unclear. We search for cold cores
that may lead to future formation of massive stars in a massive ($> 10^3$
\msun), low luminosity ($4.6 \times 10^2$ \lsun) infrared dark cloud (IRDC)
G030.88+00.13. The \nh3 data from VLA and GBT reveal that the extinction
feature seen in the infrared consists of two distinctive clumps along the same
line of sight: The C1 clump at 97 \kms-1 coincides with the extinction in the
Spitzer 8 and 24 $\mu$m. Therefore, it is responsible for the majority of the
IRDC. The C2 clump at 107 \kms-1 is more compact and has a peak temperature of
45 K. Compact dust cores and \h2O masers revealed in the SMA and VLA
observations are mostly associated with C2, and none is within the IRDC in C1.
The luminosity indicates that neither the C1 nor C2 clump has yet to form
massive protostars. But C1 might be at a precluster forming stage. The
simulated observations rule out 0.1pc cold cores with masses above 8 \msun\
within the IRDC. The core masses in C1 and C2, and those in high-mass
protostellar objects suggest an evolutionary trend that the mass of cold cores
increases over time. Based on our findings, we propose an empirical picture of
massive star formation that protostellar cores and the embedded protostars
undergo simultaneous mass growth during the protostellar evolution.
