6,961 research outputs found
Properties of Hot Stars in the Wolf-Rayet galaxy NGC5253 from ISO Spectroscopy
ISO-SWS spectroscopy of the WR galaxy NGC5253 is presented, and analysed to
provide estimates of its hot young star population. Our approach differs from
previous investigations in that we are able to distinguish between the regions
in which different infrared fine-structure lines form, using complementary
ground-based observations. The high excitation nebular [SIV] emission is formed
in a very compact region, which we attribute to the central super-star-nucleus,
and lower excitation [NeII] nebular emission originates in the galactic core.
We use photo-ionization modelling coupled with the latest theoretical O-star
flux distributions to derive effective stellar temperatures and ionization
parameters of Teff>38kK, logQ=8.25 for the compact nucleus, with Teff=35kK,
logQ<8 for the larger core. Results are supported by more sophisticated
calculations using evolutionary synthesis models. We assess the contribution
that Wolf-Rayet stars may make to highly ionized nebular lines (e.g. [OIV]).
From our Br(alpha) flux, the 2" nucleus contains the equivalent of
approximately 1000 O7V star equivalents and the starburst there is 2-3Myr old;
the 20" core contains about 2500 O7V star equivalents, with a representative
age of 5Myr. The Lyman ionizing flux of the nucleus is equivalent to the 30
Doradus region. These quantities are in good agreement with the observed mid-IR
dust luminosity of 7.8x10^8 L(sun) Since this structure of hot clusters
embedded in cooler emission may be common in dwarf starbursts, observing a
galaxy solely with a large aperture may result in confusion. Neglecting the
spatial distribution of nebular emission in NGC5253, implies `global' stellar
temperatures (or ages) of 36kK (4.8Myr) and 39kK (2.9 or 4.4Myr) from the
observed [NeIII/II] and [SIV/III] line ratios, assuming logQ=8.Comment: 16 pages, 7 figures, uses mn.sty, to appear in MNRA
Critical behavior of weakly interacting bosons: A functional renormalization group approach
We present a detailed investigation of the momentum-dependent self-energy
Sigma(k) at zero frequency of weakly interacting bosons at the critical
temperature T_c of Bose-Einstein condensation in dimensions 3<=D<4. Applying
the functional renormalization group, we calculate the universal scaling
function for the self-energy at zero frequency but at all wave vectors within
an approximation which truncates the flow equations of the irreducible vertices
at the four-point level. The self-energy interpolates between the critical
regime k > k_c, where k_c is the
crossover scale. In the critical regime, the self-energy correctly approaches
the asymptotic behavior Sigma(k) \propto k^{2 - eta}, and in the
short-wavelength regime the behavior is Sigma(k) \propto k^{2(D-3)} in D>3. In
D=3, we recover the logarithmic divergence Sigma(k) \propto ln(k/k_c)
encountered in perturbation theory. Our approach yields the crossover scale k_c
as well as a reasonable estimate for the critical exponent eta in D=3. From our
scaling function we find for the interaction-induced shift in T_c in three
dimensions, Delta T_c / T_c = 1.23 a n^{1/3}, where a is the s-wave scattering
length and n is the density, in excellent agreement with other approaches. We
also discuss the flow of marginal parameters in D=3 and extend our truncation
scheme of the renormalization group equations by including the six- and
eight-point vertex, which yields an improved estimate for the anomalous
dimension eta \approx 0.0513. We further calculate the constant lim_{k->0}
Sigma(k)/k^{2-eta} and find good agreement with recent Monte-Carlo data.Comment: 23 pages, 7 figure
Is continuity of primary care declining in England?:Practice-level longitudinal study from 2012 to 2017
An analysis of the Research Fellowship Scheme of the Royal College of Surgeons of England.
BACKGROUND: The Research Fellowship Scheme of the Royal College of Surgeons of England commenced in 1993 with the aim of exposing selected surgical trainees to research techniques and methodology, with the hope of having an impact on surgical research and increasing the cadre of young surgeons who might decide to pursue an academic career in surgery. Over 11 million pounds sterling (approximately US 20 million dollars) has been invested in 264 fellowships. The College wished to evaluate the impact of the Scheme on the careers of research fellows, surgical research, and patient care. As the 10th anniversary of the Scheme approached. STUDY DESIGN: Two-hundred and sixty research fellows whose current addresses were available were sent a questionnaire. Two-hundred and thirty-eight (91.5%) responded. RESULTS: Three-quarters of the research fellows conducted laboratory-based research, with most of the remainder conducting patient-based clinical research. One-third of the fellows who have reached consultant status have an academic component to their post. The total number of publications based on fellowship projects was 531, with a median impact factor of 3.5. Almost all fellows had been awarded a higher degree or were working toward this. Half of the fellows received subsequent funding for research, mostly awarded by national or international funding bodies. CONCLUSIONS: The Research Fellowship Scheme of the Royal College of Surgeons of England has successfully supported many trainee surgeons in the initial phase of their research career. It has helped surgical research by increasing the pool of surgeons willing to embark on an academic career. Indirectly, patient care has benefited by promoting an evidence-based culture among young surgeons. Such schemes are relevant to surgical training programs elsewhere if more young surgeons are to be attracted into academic surgery
Measurement of brain lactate during visual stimulation using a long TE semiâLASER sequence at 7 T
Estimation of metabolic changes during neuronal activation represents a challenge for in vivo MRS, especially for metabolites with low concentration and signal overlap, such as lactate. In this work, we aimed to evaluate the feasibility of detecting lactate during brain activation using a long urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0001 (144 ms) semiâLASER sequence at 7 T. urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0002 spectra were acquired on healthy volunteers ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0003) during a paradigm with 15 min of visual stimulation. Outerâvolume signals were further attenuated by the use of saturation slabs, and macromolecular signals in the vicinity of the inverted lactate peak were individually fitted with simulated Lorentzian peaks. All spectra were free of artefacts and highly reproducible across subjects. Lactate was accurately quantified with an average CramĂ©râRao lower bound of 8%. Statistically significant ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0004, oneâtailed urn:x-wiley:nbm:media:nbm4223:nbm4223-math-0005âtest) increases in lactate ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-000610%) and glutamate ( urn:x-wiley:nbm:media:nbm4223:nbm4223-math-00073%) levels during stimulation were detected in the visual cortex. Lactate and glutamate changes were consistent with previous measurements. We demonstrated that quantification of a clear and nonâcontaminated lactate peak obtained with a long TE sequence has the potential of improving the accuracy of functional MRS studies targeting nonâoxidative reaction pathways
tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: A 7T magnetic resonance spectroscopy study
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that alters cortical excitability in a polarity specific manner and has been shown to influence learning and memory. tDCS may have both on-line and after-effects on learning and memory, and the latter are thought to be based upon tDCS-induced alterations in neurochemistry and synaptic function. We used ultra-high-field (7 T) magnetic resonance spectroscopy (MRS), together with a robotic force adaptation and de-adaptation task, to investigate whether tDCS-induced alterations in GABA and Glutamate within motor cortex predict motor learning and memory. Note that adaptation to a robot-induced force field has long been considered to be a form of model-based learning that is closely associated with the computation and âsupervisedâ learning of internal âforwardâ models within the cerebellum. Importantly, previous studies have shown that on-line tDCS to the cerebellum, but not to motor cortex, enhances model-based motor learning. Here we demonstrate that anodal tDCS delivered to the hand area of the left primary motor cortex induces a significant reduction in GABA concentration. This effect was specific to GABA, localised to the left motor cortex, and was polarity specific insofar as it was not observed following either cathodal or sham stimulation. Importantly, we show that the magnitude of tDCS-induced alterations in GABA concentration within motor cortex predicts individual differences in both motor learning and motor memory on the robotic force adaptation and de-adaptation task
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