Two-level hierarchical fragmentation in the Orion Molecular Cloud 1
  northern filament
  [Abridged] We have recently reported on the collapse and fragmentation
properties of the northernmost part of this structure, located ~2.4pc north of
Orion KL -- the Orion Molecular Cloud 3 (OMC 3, Takahashi et al. 2013). As part
of our project to study the integral-shaped filament, we analyze the
fragmentation properties of the northern OMC 1 filament. This filament is a
dense structure previously identified by JCMT/SCUBA submillimeter continuum and
VLA ammonia observations and shown to have fragmented into clumps. We observed
OMC1 n with the Submillimeter Array (SMA) at 1.3mm and report on our analysis
of the continuum data. We discovered 24 new compact sources, ranging in mass
from 0.1 to 2.3, in size from 400 to 1300au, and in density from 2.6 x 10^7 to
2.8 x 10^6 cm^{-3}. The masses of these sources are similar to those of the SMA
protostars in OMC3, but their typical sizes and densities are lower by a factor
of ten. Only 8% of the new sources have infrared counterparts, yet there are
five associated CO molecular outflows. These sources are thus likely in the
Class 0 evolutionary phase yet it cannot be excluded that some of the sources
might still be pre-stellar cores. The spatial analysis of the protostars shows
that these are divided into small groups that coincide with previously
identified JCMT/SCUBA 850 micron and VLA ammonia clumps, and that these are
separated by a quasi-equidistant length of ~30arcmin (0.06pc). This separation
is dominated by the Jeans length, and therefore indicates that the main
physical process in the filament's evolution was thermal fragmentation. Within
the protostellar groups, the typical separation is ~6arcsec (~2500\,au), which
is a factor 2-3 smaller than the Jeans length of the parental clumps within
which the protostars are embedded. These results point to a hierarchical
(2-level) thermal fragmentation process of the OMC1n filament.
