506 research outputs found
Impact of energetic particle orbits on long range frequency chirping of BGK modes
Long range frequency chirping of Bernstein-Greene-Kruskal modes, whose
existence is determined by the fast particles, is investigated in cases where
these particles do not move freely and their motion is bounded to restricted
orbits. An equilibrium oscillating potential, which creates different orbit
topologies of energetic particles, is included into the bump-on-tail
instability problem of a plasma wave. With respect to fast particles dynamics,
the extended model captures the range of particles motion (trapped/passing)
with energy and thus represents a more realistic 1D picture of the long range
sweeping events observed for weakly damped modes, e.g. global Alfven
eigenmodes, in tokamaks. The Poisson equation is solved numerically along with
bounce averaging the Vlasov equation in the adiabatic regime. We demonstrate
that the shape and the saturation amplitude of the nonlinear mode structure
depends not only on the amount of deviation from the initial eigenfrequency but
also on the initial energy of the resonant electrons in the equilibrium
potential. Similarly, the results reveal that the resonant electrons following
different equilibrium orbits in the electrostatic potential lead to different
rates of frequency evolution. As compared to the previous model [Breizman B.N.
2010 Nucl. Fusion 50 084014], it is shown that the frequency sweeps with lower
rates. The additional physics included in the model enables a more complete 1D
description of the range of phenomena observed in experiments.Comment: Submitted to Nuclear Fusion 25/01/201
RR Lyrae stars in Galactic globular clusters. VI. The Period-Amplitude relation
We compare theory and observations for fundamental RR Lyrae in the solar
neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic
globular clusters (GGCs). The distribution of cluster RR_ab in the PA_V plane
depends not only on the metal abundance, but also on the cluster Horizontal
Branch (HB) morphology. On average the observed k_puls parameter, connecting
the period to the visual amplitude, increases when moving from metal-poor to
metal-rich GGCs. However, this parameter shows marginal changes among OoI
clusters with intermediate to red HB types and iron abundances -1.8<= [Fe/H]
<=-1.1, whereas its value decreases in OoII clusters with the bluer HB
morphology. Moreover, at [Fe/H]=-1.7+-0.1 the OoI clusters present redder HB
types and larger values than the OoII clusters. The RR_ab variables in
Omega Cen and in the solar neighborhood further support the evidence that the
spread in [Fe/H], at fixed k_puls, is of the order of +-0.5 dex. Synthetic HB
simulations show that the PA_V plane can provide accurate cluster distance
estimates. The RR_ab variables in OoI and in OoII clusters with very blue HB
types obey a well-defined M_V(RR)-k_puls relation, while those in OoII clusters
with moderately blue HB types present a zero-point that is ~0.05 mag brighter.
Regarding field variables, we show that with [Fe/H]=> -1.0 a unique
M_V(RR)-k_puls relation can be adopted, independently of the parent HB
morphology. Current findings suggest that the PA_V distribution does not seem
to be a robust diagnostic for the metal abundance of RR_ab variables. However,
the same observables can be used to estimate the absolute magnitude of globular
cluster and field RR_ab variables. We show that over the metallicity range
-2.4<= [Fe/H] <= 0.0 the M_V(RR)-[Fe/H] relation shows a parabolic behavior.Comment: Paper accepted on A&A, 13 pages, 18 figure
Approximating RR Lyrae light curves using cubic polynomials
In this paper, we use cubic polynomials to approximate RR Lyrae light curves
and apply the method to HST data of RR Lyraes in the halo of M31. We compare
our method to the standard method of Fourier decomposition and find that the
method of cubic polynomials eliminates virtually all ringing effects and does
so with significantly fewer parameters than the Fourier technique. Further, for
RRc stars the parameters in the fit are all physical. Our study also reveals a
number of additional periodicites in this data not found previously: we find 23
RRc stars, 29 RRab stars and 3 multiperiodic stars.Comment: 6 pages, MNRAS accepte
Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions
We sing the praises of the central limit theorem. Having previously removed
all other possible causes of significant systematic error in the statistical
parallax determination of RR Lyrae absolute magnitudes, we investigate
systematic errors from two final sources of input data: apparent magnitudes and
extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the
statistical error. However, these are of opposite sign and so roughly cancel.
The apparent magnitude system that we previously adopted from Layden et al. was
calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we
show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were
previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V
should rather be based on new COBE/IRAS dust-emission map of Schlegel,
Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find
M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or
M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from
716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen.
These are 2 and 3 sigma fainter than recent determinations of M_V from main
sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid
and Gratton et al. Since statistical parallax is being cleared of systematic
errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we
conclude that these brighter determinations may be in error. In the course of
three papers, we have corrected 6 systematic errors whose absolute values total
0.20 mag. Had these, contrary to the expectation of the central limit theorem,
all lined up one way, they could have resolved the conflict in favor of the
brighter determinations. In fact, the net change was only 0.06 mag.Comment: submitted to ApJ, 21 pages, 2 tables, 4 figure
The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
We present time-series VI photometry of the metal-rich ([Fe/H] = -0.53)
globular cluster NGC 6441. Our color-magnitude diagram shows that the extended
blue horizontal branch seen in Hubble Space Telescope data exists in the
outermost reaches of the cluster. The red clump slopes nearly parallel to the
reddening vector. A component of this slope is due to differential reddening,
but part is intrinsic. The blue horizontal branch stars are more centrally
concentrated than the red clump stars. We have discovered about 50 new variable
stars near NGC 6441, among them eight or more RR Lyrae stars which are very
probably cluster members. Comprehensive period searches over the range 0.2-1.0
days yielded unusually long periods (0.5-0.9 days) for the fundamental
pulsators compared with field RR Lyrae of the same metallicity. Three similar
long-period RR Lyrae are known in other metal-rich globulars. With over ten
examples in hand, it seems that a distinct sub-class of RR Lyrae is emerging.
The observed properties of the horizontal branch stars are in reasonable
agreement with recent models which invoke deep mixing to enhance the
atmospheric helium abundance, while they conflict with models which assume high
initial helium abundance. The light curves of the c-type RR Lyrae seem to have
unusually long rise times and sharp minima. Reproducing these light curves in
stellar pulsation models may provide another means of constraining the physical
variables responsible for the anomalous blue horizontal branch extension and
sloped red clump observed in NGC 6441.Comment: 30 pages plus 6 EPS and 6 JPEG figures; uses AAS TeX. Accepted by the
Astronomical Journal. Minor changes include computing He abundance,
modifications to Figs 1 and 8, and expansion on idea that blue HB stars may
be produced in binarie
Individual risk factors associated with exertional heat illness: A systematic review
This is the final version. Available on open access from Wiley via the DOI in this record. Despite the widespread knowledge of exertional heat illness (EHI) and clear guidance for its
prevention, the incidence of EHI remains high. We carried out a systematic review of available
literature evaluating the scientific evidence underpinning the risk factors associated with EHI.
Medline, PsycINFO, SportDiscus and Embase were searched from inception to January 2019 with
no date limitation, with supplementary searches also being performed. Search terms included
permutations of risk and heat illness, with only studies in English included. Study selection,
data extraction and quality assessment, using the QUALSYST tool, were performed by two
independent reviewers. Of 8898 articles identified by the searches, 42 were included in the
systematic review as primary evidence demonstrating a link between a risk factor and EHI. The
quality scores ranged from 57.50 to 100%, and studies were generally considered to be of strong
quality. The majority of risks attributable to EHI were categorized as those associated with lifestyle factors. The findings from the systematic review suggest complex manifestation of EHI through multiple risk factors rather than any one factor in isolation. Further research is needed
to explore the accumulation of risk factors to help in development of effective preventative measures
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