Fragmentation at the Earliest Phase of Massive Star Formation
  We present 1.3mm continuum and spectral line images of two massive molecular
clumps P1 and P2 in the G28.34+0.06 region with the Submillimeter Array. While
the two clumps contain masses of 1000 and 880 \msun, respectively, P1 has a
luminosity $< 10^2$ \lsun, and a lower gas temperature and smaller line width
than P2. Thus, P1 appears to be at a much earlier stage of massive star
formation than P2. The high resolution SMA observations reveal two distinctive
cores in P2 with masses of 97 and 49 \msun, respectively. The 4 GHz spectral
bandpass captures line emission from CO isotopologues, SO, CH$_3$OH, and
CH$_3$CN, similar to hot molecular cores harboring massive young stars. The P1
clump, on the other hand, is resolved into five cores along the filament with
masses from 22 to 64 \msun and an average projected separation of 0.19 pc.
Except $^{12}$CO, no molecular line emission is detected toward the P1 cores at
a 1$\sigma$ rms of 0.1 K. Since strong $^{12}$CO and C$^{18}$O emissions are
seen with the single dish telescope at a resolution of 11$''$, the
non-detection of these lines with the SMA indicates a depletion factor upto
$10^3$. While the spatial resolution of the SMA is better than the expected
Jeans length, the masses in P1 cores are much larger than the thermal Jeans
mass, indicating the importance of turbulence and/or magnetic fields in cloud
fragmentation. The hierarchical structures in the P1 region provide a glimpse
of the initial phase of massive star and cluster formation.
