Initial Fragmentation in the Infrared Dark Cloud G28.53-0.25
  To study the fragmentation and gravitational collapse of dense cores in
infrared dark clouds (IRDCs), we have obtained submillimeter continuum and
spectral line data as well as multiple inversion transitions of NH3 and H2O
maser data of four massive clumps in an IRDC G28.53-0.25. Combining single dish
and interferometer NH3 data, we derive the rotation temperature of G28.53. We
identity 12 dense cores at 0.1 pc scale based on submillimeter continuum, and
obtain their physical properties using NH3 and continuum data. By comparing the
Jeans masses of cores with the core masses, we find that turbulent pressure is
important in supporting the gas when 1 pc scale clumps fragment into 0.1 pc
scale cores. All cores have a virial parameter smaller than 1 assuming a
inverse squared radial density profile, suggesting they are gravitationally
bound, and the three most promising star forming cores have a virial parameter
smaller than 1 even taking magnetic field into account. We also associate the
cores with star formation activities revealed by outflows, masers, or infrared
sources. Unlike what previous studies suggested, MM1 turns out to harbor a few
star forming cores and is likely a progenitor of high-mass star cluster. MM5 is
intermediate while MM7/8 are quiescent in terms of star formation, but they
also harbor gravitationally bound dense cores and have the potential of forming
stars as in MM1.
