1,218 research outputs found
A Case Study of Triggered Star Formation in Cygnus X
Radiative feedback from massive stars can potentially trigger star formation
in the surrounding molecular gas. Inspired by the case of radiatively driven
implosion in M16 or Eagle Nebula, we analyze a similar case of star formation
observed in the Cygnus X region. We present new JCMT observations of
CO(3-2) and CO(3-2) molecular lines of a cometary feature located
at 50 pc north of the Cyg OB2 complex that was previously identified in
CO(3-2) mapping. These data are combined with archival H,
infrared, and radio continuum emission data, from which we measure the mass to
be 110 M. We identify Cyg OB2 as the ionizing source. We measure the
properties of two highly energetic molecular outflows and the photoionized rim.
From this analysis, we argue the external gas pressure and gravitational energy
dominate the internal pressure. The force balance along with previous
simulation results and a close comparison with the case of Eagle Nebula favours
a triggering scenario
Unusually Luminous Giant Molecular Clouds in the Outer Disk of M33
We use high spatial resolution (~7pc) CARMA observations to derive detailed
properties for 8 giant molecular clouds (GMCs) at a galactocentric radius
corresponding to approximately two CO scale lengths, or ~0.5 optical radii
(r25), in the Local Group spiral galaxy M33. At this radius, molecular gas
fraction, dust-to-gas ratio and metallicity are much lower than in the inner
part of M33 or in a typical spiral galaxy. This allows us to probe the impact
of environment on GMC properties by comparing our measurements to previous data
from the inner disk of M33, the Milky Way and other nearby galaxies. The outer
disk clouds roughly fall on the size-linewidth relation defined by
extragalactic GMCs, but are slightly displaced from the luminosity-virial mass
relation in the sense of having high CO luminosity compared to the inferred
virial mass. This implies a different CO-to-H2 conversion factor, which is on
average a factor of two lower than the inner disk and the extragalactic
average. We attribute this to significantly higher measured brightness
temperatures of the outer disk clouds compared to the ancillary sample of GMCs,
which is likely an effect of enhanced radiation levels due to massive star
formation in the vicinity of our target field. Apart from brightness
temperature, the properties we determine for the outer disk GMCs in M33 do not
differ significantly from those of our comparison sample. In particular, the
combined sample of inner and outer disk M33 clouds covers roughly the same
range in size, linewidth, virial mass and CO luminosity than the sample of
Milky Way GMCs. When compared to the inner disk clouds in M33, however, we find
even the brightest outer disk clouds to be smaller than most of their inner
disk counterparts. This may be due to incomplete sampling or a potentially
steeper cloud mass function at larger radii.Comment: Accepted for Publication in ApJ; 7 pages, 4 figure
A statistical study of the mass and density structure of Infrared Dark Clouds
How and when the mass distribution of stars in the Galaxy is set is one of
the main issues of modern astronomy. Here we present a statistical study of
mass and density distributions of infrared dark clouds (IRDCs) and fragments
within them. These regions are pristine molecular gas structures and
progenitors of stars and so provide insights into the initial conditions of
star formation. This study makes use of a IRDC catalogue (Peretto & Fuller
2009), the largest sample of IRDC column density maps to date, containing a
total of ~11,000 IRDCs with column densities exceeding N_{H2} = 1 X10^{22}
cm^{-2} and over 50,000 single peaked IRDC fragments. The large number of
objects constitutes an important strength of this study, allowing detailed
analysis of the completeness of the sample and so statistically robust
conclusions. Using a statistical approach to assigning distances to clouds, the
mass and density distributions of the clouds and the fragments within them are
constructed. The mass distributions show a steepening of the slope when
switching from IRDCs to fragments, in agreement with previous results of
similar structures. IRDCs and fragments are divided into unbound/bound objects
by assuming Larson's relation and calculating their virial parameter. IRDCs are
mostly gravitationally bound, while a significant fraction of the fragments are
not. The density distribution of gravitationally unbound fragments shows a
steep characteristic slope. (see paper for full Abstract).Comment: 15 pages, accepted for publication in Ap
Power Spectrum Analysis of Polarized Emission from the Canadian Galactic Plane Survey
Angular power spectra are calculated and presented for the entirety of the
Canadian Galactic Plane Survey polarization dataset at 1.4 GHz covering an area
of 1060 deg. The data analyzed are a combination of data from the 100-m
Effelsberg Telescope, the 26-m Telescope at the Dominion Radio Astrophysical
Observatory, and the Synthesis Telescope at the Dominion Radio Astrophysical
Observatory, allowing all scales to be sampled down to arcminute resolution.
The resulting power spectra cover multipoles from to and display both a power-law component at low multipoles and a
flattening at high multipoles from point sources. We fit the power spectrum
with a model that accounts for these components and instrumental effects. The
resulting power-law indices are found to have a mode of 2.3, similar to
previous results. However, there are significant regional variations in the
index, defying attempts to characterize the emission with a single value. The
power-law index is found to increase away from the Galactic plane. A transition
from small-scale to large-scale structure is evident at ,
associated with the disk-halo transition in a 15 region around
. Localized variations in the index are found toward HII regions
and supernova remnants, but the interpretation of these variations is
inconclusive. The power in the polarized emission is anticorrelated with bright
thermal emission (traced by H emission) indicating that the thermal
emission depolarizes background synchrotron emission.Comment: Accepted to ApJ; 17 page
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