The Formation of Massive Stars. High Resolution Millimeter and Radio Studies of High-Mass Protostellar Candidates
S. Molinari, L. Testi, L.F. Rodriguez, Q. Zhang
We used the Owens Valley Millimeter Array and the Very Large Array to
obtain interferometric maps at millimeter and centimeter wavelengths,
both in the continuum and in various lines (HCO+, H13CO+, SiO and
H13CN), toward a sample of 11 high-mass protostellar candidates. These
sources are known from a previous study to be associated with dense gas
and dust, and not associated with HII regions.
All 11 sources were
detected in HCO+, 9 in mm-continuum and 5 (out of 8 observed) in
physical parameters confirm the high mass nature of these molecular
clumps and suggest they are gravitationally bound. Molecular outflows
were detected toward 6 sources, with flow masses and momenta much higher
than in low-mass Young Stellar Objects (YSOs). In many of the sources
the molecular emission is organised in sub-structures, resolved both
spatially and in velocity. We find that the sources may be characterised
by their degree of fragmentation, turbulence and outflow activity, with
the sample dividing into two groups: group 1 cores have multiple
peaks but with a clearly
dominant component, larger linewidths, and are systematically associated
with outflows, while group 2 cores have several, comparable
sub-entities, smaller linewidths, and
no association with outflows.
We speculate that more massive cores may form from smaller cores
via coalescence or competitive accretion. Even conservative estimates of
outflow mass loss rates, however, indicate that accretion is the
dominant process in the later formation of massive protostars
from such cores.
We find a flattening of the outflow mass spectra with
increasing flow velocities, at variance with previous
studies that suggest a steepening with
increasing flow velocities.
In the light of this result
we suggest a re-evaluation of the wide-angle wind momentum-driven flow
models to describe the acceleration of outflows in the earliest stages
of massive star formation.