2,079 research outputs found
AGB subpopulations in the nearby globular cluster NGC 6397
It has been well established that Galactic Globular clusters (GCs) harbour
more than one stellar population, distinguishable by the anti-correlations of
light element abundances (C-N, Na-O, and Mg-Al). These studies have been
extended recently to the asymptotic giant branch (AGB). Here we investigate the
AGB of NGC 6397 for the first time. We have performed an abundance analysis of
high-resolution spectra of 47 RGB and 8 AGB stars, deriving Fe, Na, O, Mg and
Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich
AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB
and RGB in NGC 6397 are identical, within uncertainties. This agrees with
expectations from stellar theory. This GC acts as a control for our earlier
work on the AGB of M 4 (with contrasting results), since the same tools and
methods were used.Comment: 10 pages, 7 figures, 8 tables (2 online-only). Accepted for
publication in MNRA
Revealing the reality of undergraduate GP teaching in UK medical curricula: a cross-sectional questionnaire study
BACKGROUND: Time in general practice offers medical students opportunities to learn a breadth of clinical knowledge and skills relevant to their future clinical practice. Undergraduate experiences shape career decisions and current recommendations are that 25% of undergraduate curriculum time should be focused on general practice. However, previous work demonstrated that GP teaching had plateaued or reduced in UK medical schools. Therefore, an up-to-date description of undergraduate GP teaching is timely. AIM: To describe the current picture of UK undergraduate GP teaching, including the amount of time and resources allocated to GP teaching. DESIGN AND SETTING: A cross-sectional questionnaire study across 36 UK medical schools. METHOD: The questionnaire was designed based on a previous survey performed in 2011–2013, with additional questions on human and financial support allocated to GP teaching. The questionnaire was piloted and revised prior to distribution to leads of undergraduate GP teaching in UK medical schools. RESULTS: The questionnaire response rate was 100%. GP teaching constituted an average of 9.2% of medical curricula; this was lower than previous figures, though the actual number of GP sessions has remained static. The majority (n = 23) describe plans to increase GP teaching in their local curricula over the next 5 years. UK-wide average payment was 55.60 GBP/student/session of in-practice teaching, falling well below estimated costs to practices. Allocation of human resources was varied. CONCLUSION: Undergraduate GP teaching provision has plateaued since 2000 and falls short of national recommendations. Chronic underinvestment in GP teaching persists at a time when teaching is expected to increase. Both aspects need to be addressed to facilitate high-quality undergraduate GP teaching and promotion of the expert medical generalist role
A new method for the spectroscopic identification of stellar non-radial pulsation modes. II. Mode identification of the Delta Scuti star FG Virginis
We present a mode identification based on new high-resolution time-series
spectra of the non-radially pulsating Delta Scuti star FG~Vir (HD 106384, V =
6.57, A5V). From 2002 February to June a global Delta Scuti Network (DSN)
campaign, utilizing high-resolution spectroscopy and simultaneous photometry
has been conducted for FG~Vir in order to provide a theoretical pulsation
model. In this campaign we have acquired 969 Echelle spectra covering 147 hours
at six observatories. The mode identification was carried out by analyzing line
profile variations by means of the Fourier parameter fit method, where the
observational Fourier parameters across the line are fitted with theoretical
values. This method is especially well suited for determining the azimuthal
order m of non-radial pulsation modes and thus complementary with the method of
Daszynska-Daszkiewicz (2002) which does best at identifying the degree l. 15
frequencies between 9.2 and 33.5 c/d were detected spectroscopically. We
determined the azimuthal order m of 12 modes and constrained their harmonic
degree l. Only modes of low degree (l <= 4) were detected, most of them having
axisymmetric character mainly due to the relatively low projected rotational
velocity of FG Vir. The detected non-axisymmetric modes have azimuthal orders
between -2 and 1. We derived an inclination of 19 degrees, which implies an
equatorial rotational rate of 66 km/s.Comment: 14 pages, 26 figure
On the AGB stars of M 4: A robust disagreement between spectroscopic observations and theory
Several recent spectroscopic investigations have presented conflicting
results on the existence of Na-rich asymptotic giant branch (AGB) stars in the
Galactic globular cluster M4 (NGC6121). The studies disagree on whether or not
Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of
previously published HER- MES/AAT AGB and RGB stellar spectra we present a
re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al
abundances; we also present CN band strengths for this sample, derived from
low-resolution AAOmega spectra. Following a detailed literature comparison, we
find that the AGB samples of all studies consistently show lower abundances of
Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar
to recent observations of AGB stars in NGC 6752 and M 62. In an attempt to
explain this result, we present new theoretical stellar evolutionary models for
M 4; however, these predict that all stars, including Na-rich RGB stars, evolve
onto the AGB. We test the robustness of our abundance results using a variety
of atmospheric models and spectroscopic methods; however, we do not find
evidence that systematic modelling uncertainties can explain the apparent lack
of Na- rich AGB stars in M4. We conclude that an unexplained, but robust,
discordance between observations and theory remains for the AGB stars in M 4.Comment: 25 pages, 21 figures, 15 tables, accepted for publication in MNRA
Long-term study of VOCs measured with PTR-MS at a rural site in New Hampshire with urban influences
A long-term, high time-resolution volatile organic compound (VOC) data set from a ground site that experiences urban, rural, and marine influences in the Northeastern United States is presented. A proton-transfer-reaction mass spectrometer (PTR-MS) was used to quantify 15 VOCs: a marine tracer dimethyl sulfide (DMS), a biomass burning tracer acetonitrile, biogenic compounds (monoterpenes, isoprene), oxygenated VOCs (OVOCs: methyl vinyl ketone (MVK) plus methacrolein (MACR), methanol, acetone, methyl ethyl ketone (MEK), acetaldehyde, and acetic acid), and aromatic compounds (benzene, toluene, C<sub>8</sub> and C<sub>9</sub> aromatics). Time series, overall and seasonal medians, with 10th and 90th percentiles, seasonal mean diurnal profiles, and inter-annual comparisons of mean summer and winter diurnal profiles are shown. Methanol and acetone exhibit the highest overall median mixing ratios 1.44 and 1.02 ppbv, respectively. Comparing the mean diurnal profiles of less well understood compounds (e.g., MEK) with better known compounds (e.g., isoprene, monoterpenes, and MVK + MACR) that undergo various controls on their atmospheric mixing ratios provides insight into possible sources of the lesser known compounds. The constant diurnal value of ~0.7 for the toluene:benzene ratio in winter, may possibly indicate the influence of wood-based heating systems in this region. Methanol exhibits an initial early morning release in summer unlike any other OVOC (or isoprene) and a dramatic late afternoon mixing ratio increase in spring. Although several of the OVOCs appear to have biogenic sources, differences in features observed between isoprene, methanol, acetone, acetaldehyde, and MEK suggest they are produced or emitted in unique ways
Role of the antisymmetric exchange in quantum spin liquids
The quantum critical state of organic quantum spin liquids (QSL) exhibits large sensitivity even to weak perturbations. For example, the antisymmetric exchange, the Dzyaloshinskii-Moriya (DM) interaction, which is present in all spin systems without inversion symmetry, could result in a phase transition from the quantum critical phase to an antiferromagnetic phase already at moderate magnetic fields. Using the combination of multi-frequency Electron Spin Resonance spectroscopy (ESR) in the 1-500 GHz frequency range and muon spin rotation (mSR), we studied the influence of the DM interaction in two-dimensional and quasi-one-dimensional organic QSL candidates.
In the triangular lattice QSL, k-(ET)2Ag2(CN)3 (J’/J=0.94, J=175 K), our ESR measurements found a static staggered moment of 6×10-3 mB at T=1.5 K and at B=15 T [1]. The magnetic field dependence of the ESR linewidth, which measures the spectral density of the antiferromagnetic fluctuations, proves that this staggered moment stems from the DM interaction (DM0=4 K) in a perfectly crystalline two-dimensional structure. In a new quasi-one-dimensional QSL candidate, (EDT-TTF-CONH2)2+BABCO-, which is a weak Mott insulator with a distorted triangular lattice (J’/J=3, J=360 K), our combined ESR and mSR study confirmed the absence of magnetic ordering down to 20 mK [2]. This remarkable observation is partially attributed to a unique structural motif of the (EDT-TTF-CONH2)2+BABCO- salt. Here, the (EDT-TTF-CONH2)2+ conducting layers are separated by the highly disordered BABCO- molecular rotors. Importantly, despite the presence of a sizable DM interaction (DM0=0.6 K), the staggered moment is smaller than 4×10-4 mB at T=1.5 K and B=15 T. The magnetic field dependence of the ESR linewidth does not show the effect of the DM interaction. Instead, the linear dependence is indicative of the presence of fast spin fluctuations, which is supported by longitudinal-field mSR measurements that reveal the spin excitations to possess one-dimensional diffusive character. The quenching of the effect of the DM interaction is explained by the strong disorder introduced by the anion layer.
Despite the fact that the magnitude of the DM interaction is 2 to 3 orders of magnitude weaker than the symmetric exchange, it can substantially alter the phase diagram of QSLs. Our work gives a novel explanation to the field-induced phase transitions, and it demonstrates that high-frequency ESR is a powerful technique to study the spin dynamics of QSLs
Geometrically Frustrated Crystals: Elastic Theory and Dislocations
Elastic theory of ring-(or cylinder-)shaped crystals is constructed and the
generation of edge dislocations due to geometrical frustration caused by the
bending is studied. The analogy to superconducting (or superfluid) vortex state
is pointed out and the phase diagram of the ring-crystal, which depends on
radius and thickness, is discussed.Comment: 4 pages, 3 figure
The Asymmetric Wind in M82
We have obtained detailed imaging Fabry-Perot observations of the nearby
galaxy M82, in order to understand the physical association between the
high-velocity outflow and the starburst nucleus. The observed velocities of the
emitting gas in M82 reveal a bipolar outflow of material, originating from the
bright starburst regions in the galaxy's inner disk, but misaligned with
respect to the galaxy spin axis. The deprojected outflow velocity increases
with radius from 525 to 655 km/s. Spectral lines show double components in the
centers of the outflowing lobes, with the H-alpha line split by ~300 km/s over
a region almost a kiloparsec in size. The filaments are not simple surfaces of
revolution, nor is the emission distributed evenly over the surfaces. We model
these lobes as a composite of cylindrical and conical structures, collimated in
the inner ~500 pc but expanding at a larger opening angle of ~25 degrees beyond
that radius. We compare our kinematic model with simulations of
starburst-driven winds in which disk material surrounding the source is
entrained by the wind. The data also reveal a remarkably low [NII]/H-alpha
ratio in the region of the outflow, indicating that photoionization by the
nuclear starburst may play a significant role in the excitation of the optical
filament gas, particularly near the nucleus.Comment: 42 pages AASTeX with 16 figures; accepted for publication in ApJ;
figures reformatted for better printin
Variational Multiscale Stabilization and the Exponential Decay of Fine-scale Correctors
This paper addresses the variational multiscale stabilization of standard
finite element methods for linear partial differential equations that exhibit
multiscale features. The stabilization is of Petrov-Galerkin type with a
standard finite element trial space and a problem-dependent test space based on
pre-computed fine-scale correctors. The exponential decay of these correctors
and their localisation to local cell problems is rigorously justified. The
stabilization eliminates scale-dependent pre-asymptotic effects as they appear
for standard finite element discretizations of highly oscillatory problems,
e.g., the poor approximation in homogenization problems or the pollution
effect in high-frequency acoustic scattering
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