Earliest Stages of Protocluster Formation: Substructure and Kinematics
  of Starless Cores in Orion
  We study the structure and kinematics of nine 0.1 pc-scale cores in Orion
with the IRAM 30-m telescope and at higher resolution eight of the cores with
CARMA, using CS(2-1) as the main tracer. The single-dish moment zero maps of
the starless cores show single structures with central column densities ranging
from 7 to 42 times 10^23 cm^-2 and LTE masses from 20 solar masses to 154 solar
masses. However, at the higher CARMA resolution (5 arcsec), all of the cores
except one fragment into 3 - 5 components. The number of fragments is small
compared to that found in some turbulent fragmentation models, although
inclusion of magnetic fields may reduce the predicted fragment number and
improve the model agreement. This result demonstrates that fragmentation from
parsec-scale molecular clouds to sub-parsec cores continues to take place
inside the starless cores. The starless cores and their fragments are embedded
in larger filamentary structures, which likely played a role in the core
formation and fragmentation. Most cores show clear velocity gradients, with
magnitudes ranging from 1.7 to 14.3 km/s/pc. We modeled one of them in detail,
and found that its spectra are best explained by a converging flow along a
filament toward the core center; the gradients in other cores may be modeled
similarly. We infer a mass inflow rate of ~ 2 x 10^{-3} Msolar/yr, which is in
principle high enough to overcome radiation pressure and allow for massive star
formation. However, the core contains multiple fragments, and it is unclear
whether the rapid inflow would feed the growth of primarily a single massive
star or a cluster of lower mass objects. We conclude that fast, supersonic
converging flow along filaments play an important role in massive star and
cluster formation.
