Hierarchical Fragmentation and Jet-like Outflows in IRDC G28.34+0.06, a
  Growing Massive Protostar Cluster
  We present Submillimeter Array (SMA) \lambda = 0.88mm observations of an
infrared dark cloud (IRDC) G28.34+0.06. Located in the quiescent southern part
of the G28.34 cloud, the region of interest is a massive ($>10^3$\,\msun)
molecular clump P1 with a luminosity of $\sim 10^3$ \lsun, where our previous
SMA observations at 1.3mm have revealed a string of five dust cores of 22-64
\msun\ along the 1 pc IR-dark filament. The cores are well aligned at a
position angle of 48 degrees and regularly spaced at an average projected
separation of 0.16 pc. The new high-resolution, high-sensitivity 0.88\,mm image
further resolves the five cores into ten compact condensations of 1.4-10.6
\msun, with sizes a few thousands AU. The spatial structure at clump ($\sim 1$
pc) and core ($\sim 0.1$ pc) scales indicates a hierarchical fragmentation.
While the clump fragmentation is consistent with a cylindrical collapse, the
observed fragment masses are much larger than the expected thermal Jeans
masses. All the cores are driving CO(3-2) outflows up to 38 km/s, majority of
which are bipolar, jet-like outflows. The moderate luminosity of the P1 clump
sets a limit on the mass of protostars of 3-7 \msun. Because of the large
reservoir of dense molecular gas in the immediate medium and ongoing accretion
as evident by the jet-like outflows, we speculate that P1 will grow and
eventually form a massive star cluster. This study provides a first glimpse of
massive, clustered star formation that currently undergoes through an
intermediate-mass stage.
