1,938 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
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&
A method for reconstructing the variance of a 3D physical field from 2D observations: Application to turbulence in the ISM
We introduce and test an expression for calculating the variance of a
physical field in three dimensions using only information contained in the
two-dimensional projection of the field. The method is general but assumes
statistical isotropy. To test the method we apply it to numerical simulations
of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and
demonstrate that it can recover the 3D normalised density variance with ~10%
accuracy if the assumption of isotropy is valid. We show that the assumption of
isotropy breaks down at low sonic Mach number if the turbulence is
sub-Alfvenic. Theoretical predictions suggest that the 3D density variance
should increase proportionally to the square of the Mach number of the
turbulence. Application of our method will allow this prediction to be tested
observationally and therefore constrain a large body of analytic models of star
formation that rely on it.Comment: 8 pages, 9 figures, accepted for publication in MNRA
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&
Ciliated hepatic foregut cyst: A report of 6 cases and a review of the English literature
BACKGROUND: Ciliated hepatic foregut cyst (CHFC) is a rare cystic lesion most commonly identified in segment 4 of the liver that arises from the embryonic foregut. The classic histologic pattern is comprised of 4 distinct layers (inner ciliated epithelial lining, smooth muscle, loose connective tissue, fibrous capsule). Although rare, cases of metaplastic and malignant epithelial lining have been described in CHFC. METHODS: We report 6 additional cases of CHFC, one of which had gastric metaplasia of the cyst lining, and review all reported cases of CHFC in the English literature. We describe the clinicopathologic analysis of 6 cases, with selective immunohistochemical analysis on 1 case with gastric metaplasia. RESULTS: Cases occurred in 4 women and 2 men (average age 55Â years, range 42 to 67Â years). Cysts ranged in size from 0.7 to 17Â cm (average 7.2Â cm) and were grossly tan-pink to white with blood-filled contents. The majority were located in segment 4 of the liver, however 2 were located in the porta hepatis. Tumor serologies (CA19-9 and/or CEA) were performed in 3 cases; 1 case demonstrated elevated CA19-9, and 2 cases had laboratory values within normal limits. All cases showed the classic histologic findings, however one case additionally had extensive gastric metaplasia. CONCLUSIONS: In conclusion, CHFC is a rare diagnostic entity that should be considered in the differential diagnosis for cystic hepatic lesions, particularly those located in segment 4 of the liver. Metaplasia and squamous carcinoma can occur, therefore complete surgical excision is the recommended treatment. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13000-015-0321-1) contains supplementary material, which is available to authorized users
The Supernova Remnant CTB104A : Magnetic Field Structure and Interaction with the Environment
We present new, high resolution 1420 and 408 MHz continuum images and HI and
12CO (J=1-0) spectral line maps of the diffuse supernova remnant CTB104A
(G93.7-0.3). Analysis of the complex continuum emission reveals no significant
spectral index variations across the remnant. Three prominences around CTB104A
are found to be related to the SNR, while one extension to the east is
identified as an HII region associated with a background molecular shell. Small
scale polarization and rotation measure (RM) structures are turbulent in
nature, but we find a well-ordered RM gradient across the remnant, extending
from southeast to northwest. This gradient does not agree with the direction of
the global Galactic magnetic field, but does agree with a large-scale RM
anomaly inferred from rotation measure data by Cleg et al. (1992). We show that
the observed morphology of CTB104A is consistent with expansion in a uniform
magnetic field, and this is supported by the observed RM distribution. By
modeling the RM gradient with a simple compression model we have determined the
magnetic field strength within the remnant as Bo ~ 2.3 micro G. We have
identified signatures of the interaction of CTB104A with the surrounding
neutral material, and determined its distance, from the kinematics of the HI
structure encompassing the radio emission, as 1.5 kpc. We also observed clear
breaks in the HI shell that correspond well to the positions of two of the
prominences, indicating regions where hot gas is escaping from the interior of
the SNR.Comment: 7 pages, Latex with aastex and emulateapj5, 12 figures, ApJ accepte
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