225 research outputs found
Measuring Galactic Extinction: A Test
We test the recently published all-sky reddening map of Schlegel, Finkbeiner
& Davis (1998 [SFD]) using the extinction study of a region in the Taurus dark
cloud complex by Arce & Goodman (1999 [AG]). In their study, AG use four
different techniques to measure the amount and structure of the extinction
toward Taurus, and all four techniques agree very well. Thus we believe that
the AG results are a truthful representation of the extinction in the region
and can be used to test the reliability of the SFD reddening map. The results
of our test show that the SFD all-sky reddening map, which is based on data
from COBE/DIRBE and IRAS/ISSA, overestimates the reddening by a factor of 1.3
to 1.5 in regions of smooth extinction with A_V > 0.5 mag. In some regions of
steep extinction gradients the SFD map underestimates the reddening value,
probably due to its low spatial resolution. We expect that the astronomical
community will be using the SFD reddening map extensively. We offer this Letter
as a cautionary note about using the SFD map in regions of high extinction (A_V
> 0.5 mag), as it might not be giving accurate reddening values there.Comment: 14 pages (which include 2 pages of figures
The COMPLETE Survey of Outflows in Perseus
We present a study on the impact of molecular outflows in the Perseus
molecular cloud complex using the COMPLETE survey large-scale 12CO(1-0) and
13CO(1-0) maps. We used three-dimensional isosurface models generated in
RA-DEC-Velocity space to visualize the maps. This rendering of the molecular
line data allowed for a rapid and efficient way to search for molecular
outflows over a large (~ 16 sq. deg.) area. Our outflow-searching technique
detected previously known molecular outflows as well as new candidate outflows.
Most of these new outflow-related high-velocity features lie in regions that
have been poorly studied before. These new outflow candidates more than double
the amount of outflow mass, momentum, and kinetic energy in the Perseus cloud
complex. Our results indicate that outflows have significant impact on the
environment immediately surrounding localized regions of active star formation,
but lack the energy needed to feed the observed turbulence in the entire
Perseus complex. This implies that other energy sources, in addition to
protostellar outflows, are responsible for turbulence on a global cloud scale
in Perseus. We studied the impact of outflows in six regions with active star
formation within Perseus of sizes in the range of 1 to 4 pc. We find that
outflows have enough power to maintain the turbulence in these regions and
enough momentum to disperse and unbind some mass from them. We found no
correlation between outflow strength and star formation efficiency for the six
different regions we studied, contrary to results of recent numerical
simulations. The low fraction of gas that potentially could be ejected due to
outflows suggests that additional mechanisms other than cloud dispersal by
outflows are needed to explain low star formation efficiencies in clusters.Comment: Published in The Astrophysical Journa
An Extinction Study of the Taurus Dark Cloud Complex
We present a study of the detailed distribution of extinction in a region of
the Taurus dark cloud complex. Our study uses new BVR images of the region,
spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60
and 100 micron images. We study the extinction of the region in four different
ways, and we present the first inter-comparison of all these methods, which
are: 1) using the color excess of background stars for which spectral types are
known; 2) using the ISSA 60 and 100 micron images; 3) using star counts; and 4)
using an optical (V and R) version of the average color excess method used by
Lada et al. (1994). We find that all four methods give generally similar
results, with important exceptions. To study the structure in the dust
distribution, we compare the ISSA extinction and the extinction measured for
individual stars. From the comparison, we conclude that in the relatively low
extinction regions studied, with 0.9 < A_V < 3.0 mag (away from filamentary
dark clouds and IRAS cores), there are no fluctuations in the dust column
density greater than 45% (at the 99.7% confidence level), on scales smaller
than 0.2 pc. We also report the discovery of a previously unknown stellar
cluster behind the Taurus dark cloud near R.A 4h19m00s, Dec. 27:30:00 (B1950)Comment: 49 pages (which include 6 pages of tables and 6 pages of figures
Assessing molecular outflows and turbulence in the protostellar cluster Serpens South
Molecular outflows driven by protostellar cluster members likely impact their
surroundings and contribute to turbulence, affecting subsequent star formation.
The very young Serpens South cluster consists of a particularly high density
and fraction of protostars, yielding a relevant case study for protostellar
outflows and their impact on the cluster environment. We combined CO
observations of this region using the Combined Array for Research in
Millimeter-wave Astronomy (CARMA) and the Institut de Radioastronomie
Millim\'{e}trique (IRAM) 30 m single dish telescope. The combined map allows us
to probe CO outflows within the central, most active region at size scales of
0.01 pc to 0.8 pc. We account for effects of line opacity and excitation
temperature variations by incorporating CO and CO data for the
and transitions (using Atacama Pathfinder Experiment and
Caltech Submillimeter Observatory observations for the higher CO transitions),
and we calculate mass, momentum, and energy of the molecular outflows in this
region. The outflow mass loss rate, force, and luminosity, compared with
diagnostics of turbulence and gravity, suggest that outflows drive a sufficient
amount of energy to sustain turbulence, but not enough energy to substantially
counter the gravitational potential energy and disrupt the clump. Further, we
compare Serpens South with the slightly more evolved cluster NGC 1333, and we
propose an empirical scenario for outflow-cluster interaction at different
evolutionary stages.Comment: 26 pages, 15 figures, accepted for publication in the Astrophysical
Journa
CMR exploration II -- filament identification with machine learning
We adopt magnetohydrodynamics (MHD) simulations that model the formation of
filamentary molecular clouds via the collision-induced magnetic reconnection
(CMR) mechanism under varying physical conditions. We conduct radiative
transfer using RADMC-3D to generate synthetic dust emission of CMR filaments.
We use the previously developed machine learning technique CASI-2D along with
the diffusion model to identify the location of CMR filaments in dust emission.
Both models showed a high level of accuracy in identifying CMR filaments in the
test dataset, with detection rates of over 80% and 70%, respectively, at a
false detection rate of 5%. We then apply the models to real Herschel dust
observations of different molecular clouds, successfully identifying several
high-confidence CMR filament candidates. Notably, the models are able to detect
high-confidence CMR filament candidates in Orion A from dust emission, which
have previously been identified using molecular line emission.Comment: ApJ accepte
ALMA Cycle 1 Observations of the HH46/47 Molecular Outflow: Structure, Entrainment and Core Impact
We present ALMA Cycle 1 observations of the HH46/47 molecular outflow using
combined 12m array and ACA observations. The improved angular resolution and
sensitivity of our multi-line maps reveal structures that help us study the
entrainment process in much more detail and allow us to obtain more precise
estimates of outflow properties than previous observations. We use 13CO(1-0)
and C18O(1-0) emission to correct for the 12CO(1-0) optical depth to accurately
estimate the outflow mass, momentum and kinetic energy. This correction
increases the estimates of the mass, momentum and kinetic energy by factors of
about 9, 5 and 2, respectively, with respect to estimates assuming optically
thin emission. The new 13CO and C18O data also allow us to trace denser and
slower outflow material than that traced by the 12CO maps, and they reveal an
outflow cavity wall at very low velocities (as low as 0.2km/s with respect to
the cores central velocity). Adding with the slower material traced only by
13CO and C18O, there is another factor of 3 increase in the mass estimate and
50% increase in the momentum estimate. The estimated outflow properties
indicate that the outflow is capable of dispersing the parent core within the
typical lifetime of the embedded phase of a low-mass protostar, and that it is
responsible for a core-to-star efficiency of 1/4 to 1/3. We find that the
outflow cavity wall is composed of multiple shells associated with a series of
jet bow-shock events. Within about 3000AU of the protostar the 13CO and C18O
emission trace a circumstellar envelope with both rotation and infall motions,
which we compare with a simple analytic model. The CS(2-1) emission reveals
tentative evidence of a slowly-moving rotating outflow, which we suggest is
entrained not only poloidally but also toroidally by a disk wind that is
launched from relatively large radii from the source.Comment: Accepted for publication in ApJ. 26 pages, 20 figure
Recommended from our members
Evidence for Grain Growth in Molecular Clouds: A Bayesian Examination of the Extinction Law in Perseus
We investigate the shape of the extinction law in two square fields of the Perseus molecular cloud complex. We combine deep red-optical (r, i and z band) observations obtained using Megacam on the MMT with UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey near-infrared (J, H and K band) data to measure the colours of background stars. We develop a new hierarchical Bayesian statistical model, including measurement error, intrinsic colour variation, spectral type and dust reddening, to simultaneously infer parameters for individual stars and characteristics of the population. We implement an efficient Markov chain Monte Carlo algorithm utilizing generalized Gibbs sampling to compute coherent probabilistic inferences. We find a strong correlation between the extinction and the slope of the extinction law (parametrized by . Because the majority of the extinction towards our stars comes from the Perseus molecular cloud, we interpret this correlation as evidence of grain growth at moderate optical depths. The extinction law changes from the ‘diffuse’ value of to the 'dense cloud' value of as the column density rises from to 10 mag. This relationship is similar for the two regions in our study, despite their different physical conditions, suggesting that dust grain growth is a fairly universal process.Astronom
- …