15,585 research outputs found
Helium Recombination Lines as a Probe of Abundance and Temperature Problems
The paper presents a simplified formula to determine an electron temperature,
Te(He I), for planetary nebulae (PNe) using the He I 7281/6678 line flux ratio.
In our previous studies of Te(He I) (Zhang et al. 2005), we used the He I line
emission coefficients given by Benjamin et al. (1999). Here we examine the
results of using more recent atomic data presented by Porter et al. (2005). A
good agreement is shown, suggesting that the effect of uncertainties of atomic
data on the resultant Te(He I) is negligible. We also present an analytical
formula to derive electron temperature using the He I discontinuity at 3421 A.
Our analysis shows that Te(He I) values are significantly lower than electron
temperatures deduced from the Balmer jump of H I recombination spectra, Te(H
I), and that inferred from the collisionally excited [O III] nebular-to-auroral
forbidden line flux ratio, Te([O III]). In addition, Te(H I) covers a wider
range of values than either Te(He I) or Te([O III]). This supports the
two-abundance nebular model with hydrogen-deficient material embedded in
diffuse gas of a ``normal'' chemical composition (i.e. ~solar).Comment: 5 pages, 3 figures. To appear in the RevMexAA proceedings of "The
Ninth Texas-Mexico Conference on Astrophysics
How well do we know the neutron structure function?
We present a detailed analysis of the uncertainty in the neutron F2n
structure function extracted from inclusive deuteron and proton deep-inelastic
scattering data. The analysis includes experimental uncertainties as well as
uncertainties associated with the deuteron wave function, nuclear smearing, and
nucleon off-shell corrections. Consistently accounting for the Q^2 dependence
of the data and calculations, and restricting the nuclear corrections to
microscopic models of the deuteron, we find significantly smaller uncertainty
in the extracted F2n/F2p ratio than in previous analyses. In addition to
yielding an improved extraction of the neutron structure function, this
analysis also provides an important baseline that will allow future,
model-independent extractions of neutron structure to be used to examine
nuclear medium effects in the the deuteron.Comment: 5 pages, 6 figure
Inelastische Streuung kalter Neutronen an Vanadiumhydrid. EUR 522. = Inelastic scattering of cold neutrons on vanadium hydride. EUR 522.
Directed Rh(I)-Catalyzed Asymmetric Hydroboration of Prochiral 1-Arylcycloprop-2-ene-1-carboxylic Acid Derivatives
This is the peer reviewed version of the following article: A. Edwards, M. Rubina, M. Rubin, Chem. Eur. J. 2018, 24, 1394., which has been published in final form at http://doi.org/10.1002/chem.201704443. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.A full account on rhodium-catalyzed asymmetric, directed hydroboration of functionalized prochiral cyclopropenes affording enantiomerically enriched cyclopropylboronates is reported. The scope and limitations of two alternate directing groups, ester and carboxamide, are evaluated. It was found that hydroboration of esters appeared to be more sensitive to substitution in the aromatic ring of the substrates. Specifically, ortho-halogens were detrimental for diastereo- and enantioselectivity, possibly because of additional coordination with rhodium. In contrast, more Lewis-basic amide directing groups allowed for stronger chelation to the transition metal, leading to consistently high diastereo- and enantioselectivity in hydroboration across a broader range of substrates
Macroscopic detection of the strong stochasticity threshold in Fermi-Pasta-Ulam chains of oscillators
The largest Lyapunov exponent of a system composed by a heavy impurity
embedded in a chain of anharmonic nearest-neighbor Fermi-Pasta-Ulam oscillators
is numerically computed for various values of the impurity mass . A
crossover between weak and strong chaos is obtained at the same value
of the energy density (energy per degree of freedom)
for all the considered values of the impurity mass . The threshold \epsi
lon_{_T} coincides with the value of the energy density at which a
change of scaling of the relaxation time of the momentum autocorrelation
function of the impurity ocurrs and that was obtained in a previous work ~[M.
Romero-Bastida and E. Braun, Phys. Rev. E {\bf65}, 036228 (2002)]. The complete
Lyapunov spectrum does not depend significantly on the impurity mass . These
results suggest that the impurity does not contribute significantly to the
dynamical instability (chaos) of the chain and can be considered as a probe for
the dynamics of the system to which the impurity is coupled. Finally, it is
shown that the Kolmogorov-Sinai entropy of the chain has a crossover from weak
to strong chaos at the same value of the energy density that the crossover
value of largest Lyapunov exponent. Implications of this result
are discussed.Comment: 6 pages, 5 figures, revtex4 styl
CT and ultrasonographic findings in jugular vein ectasia.
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135231/1/jum198439417.pd
False-negative upper extremity ultrasound in the initial evaluation of patients with suspected subclavian vein thrombosis due to thoracic outlet syndrome (Paget-Schroetter syndrome)
OBJECTIVE: To assess the utilization and consequences of upper extremity Duplex ultrasound in the initial diagnostic evaluation of patients with suspected subclavian vein (SCV) thrombosis and venous thoracic outlet syndrome (VTOS).
METHODS: A retrospective single-center review was conducted for patients that underwent primary surgical treatment for VTOS between 2008 and 2017, in whom an upper extremity ultrasound had been performed as the initial diagnostic test (n = 214). Clinical and treatment characteristics were compared between patients with positive and false-negative ultrasound studies.
RESULTS: There were 122 men (57%) and 92 women (43%) that had presented with spontaneous idiopathic arm swelling, including 28 (13%) with proven pulmonary embolism, at a mean age of 30.7 ± 0.8 years (range 14-69). Upper extremity ultrasound had been performed 23.8 ± 12.2 days after the onset of symptoms, with confirmation of axillary-SCV thrombosis in 169 patients (79%) and negative results in 45 (21%). Of the false-negative ultrasound study reports, only 8 (18%) acknowledged limitations in visualizing the central SCV. Definitive diagnostic imaging (DDI) had been obtained by upper extremity venography in 175 (82%), computed tomography angiography in 24 (11%), and magnetic resonance angiography in 15 (7%), with 142 (66%) undergoing catheter-directed axillary-SCV thrombolysis. The mean interval between initial ultrasound and DDI was 48.9 ± 14.2 days with no significant difference between groups, but patients with a positive ultrasound were more likely to have DDI within 48 hours than those with a false-negative ultrasound (44% vs 24%; P = .02). At the time of surgical treatment, the SCV was widely patent following paraclavicular decompression and external venolysis alone in 74 patients (35%). Patch angioplasty was performed for focal SCV stenosis in 76 (36%) and bypass graft reconstruction for long-segment axillary-SCV occlusion in 63 (29%). Patients with false-negative initial ultrasound studies were significantly more likely to require SCV bypass reconstruction than those with a positive ultrasound (44% vs 25%; P = .02).
CONCLUSIONS: Duplex ultrasound has significant limitations in the initial evaluation of patients with suspected SCV thrombosis, with false-negative results in 21% of patients with proven VTOS. This is rarely acknowledged in ultrasound reports, but false-negative ultrasound studies have the potential to delay definitive imaging, thrombolysis, and further treatment for VTOS. Initial false-negative ultrasound results are associated with progressive thrombus extension and a more frequent need for SCV bypass reconstruction at the time of surgical treatment
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