1,584 research outputs found
CO Abundance Variations in the Orion Molecular Cloud
Infrared stellar photometry from 2MASS and spectral line imaging observations
of 12CO and 13CO J = 1-0 line emission from the FCRAO 14m telescope are
analysed to assess the variation of the CO abundance with physical conditions
throughout the Orion A and Orion B molecular clouds. Three distinct Av regimes
are identified in which the ratio between the 13CO column density and visual
extinction changes corresponding to the photon dominated envelope, the strongly
self-shielded interior, and the cold, dense volumes of the clouds. Within the
strongly self-shielded interior of the Orion A cloud, the 13CO abundance varies
by 100% with a peak value located near regions of enhanced star formation
activity. The effect of CO depletion onto the ice mantles of dust grains is
limited to regions with AV > 10 mag and gas temperatures less than 20 K as
predicted by chemical models that consider thermal-evaporation to desorb
molecules from grain surfaces.
Values of the molecular mass of each cloud are independently derived from the
distributions of Av and 13CO column densities with a constant 13CO-to-H2
abundance over various extinction ranges. Within the strongly self-shielded
interior of the cloud (Av > 3 mag), 13CO provides a reliable tracer of H2 mass
with the exception of the cold, dense volumes where depletion is important.
However, owing to its reduced abundance, 13CO does not trace the H2 mass that
resides in the extended cloud envelope, which comprises 40-50% of the molecular
mass of each cloud. The implied CO luminosity to mass ratios, M/L_{CO}, are 3.2
and 2.9 for Orion A and Orion B respectively, which are comparable to the value
(2.9), derived from gamma-ray observations of the Orion region. Our results
emphasize the need to consider local conditions when applying CO observations
to derive H2 column densities.Comment: Accepted for publication in MNRAS. 21 pages, 14 figure
Magnetostrictive Neel ordering of the spin-5/2 ladder compound BaMn2O3: distortion-induced lifting of geometrical frustration
The crystal structure and the magnetism of BaMnO have been studied by
thermodynamic and by diffraction techniques using large single crystals and
powders. BaMnO is a realization of a spin ladder as the
magnetic interaction is dominant along 180 Mn-O-Mn bonds forming the
legs and the rungs of a ladder. The temperature dependence of the magnetic
susceptibility exhibits well-defined maxima for all directions proving the
low-dimensional magnetic character in BaMnO. The susceptibility and
powder neutron diffraction data, however, show that BaMnO exhibits a
transition to antiferromagnetic order at 184 K, in spite of a full frustration
of the nearest-neighbor inter-ladder coupling in the orthorhombic
high-temperature phase. This frustration is lifted by a remarkably strong
monoclinic distortion which accompanies the magnetic transition.Comment: 9 pages, 8 figures, 2 tables; in V1 fig. 2 was included twice and
fig. 4 was missing; this has been corrected in V
Turbulent Driving Scales in Molecular Clouds
Supersonic turbulence in molecular clouds is a dominant agent that strongly
affects the clouds' evolution and star formation activity. Turbulence may be
initiated and maintained by a number of processes, acting at a wide range of
physical scales. By examining the dynamical state of molecular clouds, it is
possible to assess the primary candidates for how the turbulent energy is
injected. The aim of this paper is to constrain the scales at which turbulence
is driven in the molecular interstellar medium, by comparing simulated
molecular spectral line observations of numerical magnetohydrodynamic (MHD)
models and molecular spectral line observations of real molecular clouds. We
use principal component analysis, applied to both models and observational
data, to extract a quantitative measure of the driving scale of turbulence. We
find that only models driven at large scales (comparable to, or exceeding, the
size of the cloud) are consistent with observations. This result applies also
to clouds with little or no internal star formation activity. Astrophysical
processes acting on large scales, including supernova-driven turbulence,
magnetorotational instability, or spiral shock forcing, are viable candidates
for the generation and maintenance of molecular cloud turbulence. Small scale
driving by sources internal to molecular clouds, such as outflows, can be
important on small scales, but cannot replicate the observed large-scale
velocity fluctuations in the molecular interstellar medium.Comment: 8 pages, 7 figures, accepted for publication in A&
WFPC2 Observations of NGC 454: an Interacting Pair of Galaxies
We present WFPC2 images in the F450W, F606W and F814W filters of the
interacting pair of galaxies NGC 454. Our data indicate that the system is in
the early stages of interaction. A population of young star-clusters has formed
around the late component, and substantial amounts of gas have sunk into the
center of the earlier component, where it has not yet produced significant
visible star formation or nuclear activity. We have photometric evidence that
the star-clusters have strong line emission, which indicate the presence of a
substantial component of hot, massive stars which formed less than 5-10 Myrs
ago.Comment: 14 pages, 4 figures, Latex (AAS macros), ApJL in pres
The Density Variance Mach Number Relation in the Taurus Molecular Cloud
Supersonic turbulence in molecular clouds is a key agent in generating
density enhancements that may subsequently go on to form stars. The stronger
the turbulence - the higher the Mach number - the more extreme the density
fluctuations are expected to be. Numerical models predict an increase in
density variance with rms Mach number of the form: sigma^{2}_{rho/rho_{0}} =
b^{2}M^{2}, where b is a numerically-estimated parameter, and this prediction
forms the basis of a large number of analytic models of star formation. We
provide an estimate of the parameter b from 13CO J=1-0 spectral line imaging
observations and extinction mapping of the Taurus molecular cloud, using a
recently developed technique that needs information contained solely in the
projected column density field to calculate sigma^{2}_{rho/rho_{0}}. We find b
~ 0.48, which is consistent with typical numerical estimates, and is
characteristic of turbulent driving that includes a mixture of solenoidal and
compressive modes. More conservatively, we constrain b to lie in the range
0.3-0.8, depending on the influence of sub-resolution structure and the role of
diffuse atomic material in the column density budget. We also report a break in
the Taurus column density power spectrum at a scale of ~1pc, and find that the
break is associated with anisotropy in the power spectrum. The break is
observed in both 13CO and dust extinction power spectra, which, remarkably, are
effectively identical despite detailed spatial differences between the 13CO and
dust extinction maps. [ abridged ]Comment: 8 pages, 9 figures. Accepted for publication in A&
Distribution and mass of diffuse and dense CO gas in the Milky Way
This is the final version of the article. Available from American Astronomical Society and IOP Publishing via the DOI in this record.Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star-forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in 13CO and 12CO, and a low column density component detected in 12CO, but not in 13CO. The âdiffuseâ and âdenseâ components are identified using a combination of smoothing, masking, and erosion/dilation procedures, making use of three large-scale 12CO and 13CO surveys of the inner and outer Milky Way. The diffuse component, which globally represents 25% (1.5 Ă 108Mâ) of the total molecular gas mass (6.5 Ă {10}8 Mâ), is more extended perpendicular to the Galactic plane. The fraction of diffuse gas increases from âŒ10%â20% at a galactocentric radius of 3â4 kpc to 50% at 15 kpc, and increases with decreasing surface density. In the inner Galaxy, a yet denser component traced by CS emission represents 14% of the total molecular gas mass traced by 12CO emission. Only 14% of the molecular gas mass traced by 12CO emission is identified as part of molecular clouds in 13CO surveys by cloud identification algorithms. This study indicates that CO emission not only traces star-forming clouds, but also a significant diffuse molecular ISM component.R.S. and R.S.K. acknowledge support from the Deutsche
Forschungsgemeinschaft (DFG) for funding through the SPP
1573 âThe Physics of the Interstellar Mediumâ as well as via
SFB 881 âThe Milky Way Systemâ (sub-projects B12, and
B8). R.S.K. also receives funding from the European Research
Council under the European Communitys Seventh Framework
Program (FP7/2007-2013) via the ERC Advanced Grant
âSTARLIGHTâ (project number 339177)
Embedded Stellar Clusters in the W3/W4/W5 Molecular Cloud Complex
We analyze the embedded stellar content in the vicinity of the W3/W4/W5 HII
regions using the FCRAO Outer Galaxy 12CO(J=1-0) Survey, the IRAS Point Source
Catalog, published radio continuum surveys, and new near-infrared and molecular
line observations. Thirty-four IRAS Point Sources are identified that have
far-infrared colors characteristic of embedded star forming regions, and we
have obtained K' mosaics and 13CO(J=1-0) maps for 32 of them. Ten of the IRAS
sources are associated with an OB star and 19 with a stellar cluster, although
three OB stars are not identified with a cluster. Half of the embedded stellar
population identified in the K' images is found in just the 5 richest clusters,
and 61% is contained in IRAS sources associated with an embedded OB star. Thus
rich clusters around OB stars contribute substantially to the stellar
population currently forming in the W3/W4/W5 region. Approximately 39% of the
cluster population is embedded in small clouds with an average mass of ~130 Mo
that are located as far as 100 pc from the W3/W4/W5 cloud complex. We speculate
that these small clouds are fragments of a cloud complex dispersed by previous
episodes of massive star formation. Finally, we find that 4 of the 5 known
embedded massive star forming sites in the W3 molecular cloud are found along
the interface with the W4 HII region despite the fact that most of the
molecular mass is contained in the interior regions of the cloud. These
observations are consistent with the classical notion that the W4 HII region
has triggered massive star formation along the eastern edge of the W3 molecular
cloud.Comment: to appear in ApJS, see http://astro.caltech.edu/~jmc/papers/w
Larson's third law and the universality of molecular cloud structure
Larson (1981) first noted a scaling relation between masses and sizes in
molecular clouds that implies that these objects have approximately constant
column densities. This original claim, based upon millimeter observations of
carbon monoxide lines, has been challenged by many theorists, arguing that the
apparent constant column density observed is merely the result of the limited
dynamic range of observations, and that in reality clouds have column density
variations over two orders of magnitudes. In this letter we investigate a set
of nearby molecular clouds with near-infrared excess methods, which guarantee
very large dynamic ranges and robust column density measurements, to test the
validity of Larson's third law. We verify that different clouds have almost
identical average column densities above a given extinction threshold; this
holds regardless of the extinction threshold, but the actual average surface
mass density is a function of the specific threshold used. We show that a
second version of Larson's third law, involving the mass-radius relation for
single clouds and cores, does not hold in our sample, indicating that
individual clouds are not objects that can be described by constant column
density. Our results instead indicate that molecular clouds are characterized
by a universal structure. Finally we point out that this universal structure
can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter
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