1,704 research outputs found
Regional Coalitions for Healthcare Improvement: Definition, Lessons, and Prospects
Outlines how regional quality coalitions can collaborate to help deliver evidence-based healthcare; improve care processes; and measure, report, and reward results. Includes guidelines for starting and running a coalition and summaries of NRHI coalitions
Poliovirus Replication: Interactions with Cytoplasmic Membranes and with Compounds Which Affect Virus Biosynthesis
Picornavirus multiplication is intimately associated with intracellular membranes and affects their production and distribution within the cell. We have shown that the density distribution of cytoplasmic membranes separated by isopycnic centrifugation in discontinuous sucrose gradients is progressively altered after infection of HeLa cells with poliovirus. The most striking change is a very large increase in the smooth microsomal membranes with which viral RNA synthesis is associated. These membranes begin to increase between 2.5 and 3.5 hours after infection, and continue to proliferate late in the virus growth cycle, at a time when cellular protein and nucleic acid syntheses are much reduced. These smooth membranes differ from the membranes in the corresponding fraction from uninfected cells in several ways. They have a higher phospholipid/protein ratio and the phospholipid composition is altered. We have confirmed that the viral RNA polymerase activity is firmly associated with the membranes in this fraction and that it may be localized in a complex with a sedimentation constant of 130 S after lysis of the membranes with deoxycholate
Density of States of GaAs-AlGaAs Heterostructures Deduced from Temperature Dependend Magnetocapacitance Measurements
Abstract We have analyzed the density of states of a two dimensional electron gas in a GaAs- AlGaAs hetereostructure by measuring the magnetocapacitance in magnetic fields up to 6 Tesla at temperatures below 10 K. The experimental data are well described by a Gaussian-like density of states where the linewidth à is proportional to B
Surface-effect corrections for solar-like oscillations using 3D hydrodynamical simulations
The space-borne missions have provided us with a wealth of high-quality
observational data that allows for seismic inferences of stellar interiors.
This requires the computation of precise and accurate theoretical frequencies,
but imperfect modeling of the uppermost stellar layers introduces systematic
errors. To overcome this problem, an empirical correction has been introduced
by Kjeldsen et al. (2008, ApJ, 683, L175) and is now commonly used for seismic
inferences. Nevertheless, we still lack a physical justification allowing for
the quantification of the surface-effect corrections. We used a grid of these
simulations computed with the COBOLD code to model the outer layers of
solar-like stars. Upper layers of the corresponding 1D standard models were
then replaced by the layers obtained from the horizontally averaged 3D models.
The frequency differences between these patched models and the 1D standard
models were then calculated using the adiabatic approximation and allowed us to
constrain the Kjeldsen et al. power law, as well as a Lorentzian formulation.
We find that the surface effects on modal frequencies depend significantly on
both the effective temperature and the surface gravity. We further provide the
variation in the parameters related to the surface-effect corrections using
their power law as well as a Lorentzian formulation. Scaling relations between
these parameters and the elevation (related to the Mach number) is also
provided. The Lorentzian formulation is shown to be more robust for the whole
frequency spectrum, while the power law is not suitable for the frequency
shifts in the frequency range above .Comment: 11 pages, 14 figures, 4 tables; accepted for publication in Astronomy
& Astrophysic
Probing the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler
We report for the first time a parametric fit to the pattern of the \ell = 1
mixed modes in red giants, which is a powerful tool to identify
gravity-dominated mixed modes. With these modes, which share the
characteristics of pressure and gravity modes, we are able to probe directly
the helium core and the surrounding shell where hydrogen is burning. We propose
two ways for describing the so-called mode bumping that affects the frequencies
of the mixed modes. Firstly, a phenomenological approach is used to describe
the main features of the mode bumping. Alternatively, a quasi-asymptotic
mixed-mode relation provides a powerful link between seismic observations and
the stellar interior structure. We used period \'echelle diagrams to emphasize
the detection of the gravity-dominated mixed modes. The asymptotic relation for
mixed modes is confirmed. It allows us to measure the gravity-mode period
spacings in more than two hundred red giant stars. The identification of the
gravity-dominated mixed modes allows us to complete the identification of all
major peaks in a red giant oscillation spectrum, with significant consequences
for the true identification of \ell = 3 modes, of \ell = 2 mixed modes, for the
mode widths and amplitudes, and for the \ell = 1 rotational splittings. The
accurate measurement of the gravity-mode period spacing provides an effective
probe of the inner, g-mode cavity. The derived value of the coupling
coefficient between the cavities is different for red giant branch and clump
stars. This provides a probe of the hydrogen-shell burning region that
surrounds the helium core. Core contraction as red giants ascend the red giant
branch can be explored using the variation of the gravity-mode spacing as a
function of the mean large separation.Comment: Accepted in A&
Seismic evidence for a weak radial differential rotation in intermediate-mass core helium burning stars
The detection of mixed modes that are split by rotation in Kepler red giants
has made it possible to probe the internal rotation profiles of these stars,
which brings new constraints on the transport of angular momentum in stars.
Mosser et al. (2012) have measured the rotation rates in the central regions of
intermediate-mass core helium burning stars (secondary clump stars). Our aim
was to exploit& the rotational splittings of mixed modes to estimate the amount
of radial differential rotation in the interior of secondary clump stars using
Kepler data, in order to place constraints on angular momentum transport in
intermediate-mass stars. We selected a subsample of Kepler secondary clump
stars with mixed modes that are clearly rotationally split. By applying a
thorough statistical analysis, we showed that the splittings of both
gravity-dominated modes (trapped in central regions) and p-dominated modes
(trapped in the envelope) can be measured. We then used these splittings to
estimate the amount of differential rotation by using inversion techniques and
by applying a simplified approach based on asymptotic theory (Goupil et al.
2013). We obtained evidence for a weak radial differential rotation for six of
the seven targets that were selected, with the central regions rotating
to times faster than the envelope. The last target was
found to be consistent with a solid-body rotation. This demonstrates that an
efficient redistribution of angular momentum occurs after the end of the main
sequence in the interior of intermediate-mass stars, either during the
short-lived subgiant phase, or once He-burning has started in the core. In
either case, this should bring constraints on the angular momentum transport
mechanisms that are at work.Comment: 16 pages, 8 figures, accepted in A&
Stellar granulation as seen in disk-integrated intensity. II. Theoretical scaling relations compared with observations
A large set of stars observed by CoRoT and Kepler shows clear evidence for
the presence of a stellar background, which is interpreted to arise from
surface convection, i.e., granulation. These observations show that the
characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness
fluctuations (sigma) associated with the granulation scale as a function of the
peak frequency (nu_max) of the solar-like oscillations. We aim at providing a
theoretical background to the observed scaling relations based on a model
developed in the companion paper. We computed for each 3D model the theoretical
power density spectrum (PDS) associated with the granulation as seen in
disk-integrated intensity on the basis of the theoretical model. For each PDS
we derived tau_eff and sigma and compared these theoretical values with the
theoretical scaling relations derived from the theoretical model and the Kepler
measurements. We derive theoretical scaling relations for tau_eff and sigma,
which show the same dependence on nu_max as the observed scaling relations. In
addition, we show that these quantities also scale as a function of the
turbulent Mach number (Ma) estimated at the photosphere. The theoretical
scaling relations for tau_eff and sigma match the observations well on a global
scale. Our modelling provides additional theoretical support for the observed
variations of sigma and tau_eff with nu_m max. It also highlights the important
role of Ma in controlling the properties of the stellar granulation. However,
the observations made with Kepler on a wide variety of stars cannot confirm the
dependence of our scaling relations on Ma. Measurements of the granulation
background and detections of solar-like oscillations in a statistically
sufficient number of cool dwarf stars will be required for confirming the
dependence of the theoretical scaling relations with Ma.Comment: 12 pages, 6 figures,accepted for publication in A&
The CoRoT target HD 49933: 2- Comparison of theoretical mode amplitudes with observations
From the seismic data obtained by CoRoT for the star HD 49933 it is possible,
as for the Sun, to constrain models of the excitation of acoustic modes by
turbulent convection. We compare a stochastic excitation model described in
Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star
that is rather metal poor and significantly hotter than the Sun. Using the mode
linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation
rates computed in Paper I, we derive the expected surface velocity amplitudes
of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic
approximation relating the mode amplitudes in intensity to those in velocity,
we derive the expected values of the mode amplitude in intensity. Our amplitude
calculations are within 1-sigma error bars of the mode surface velocity
spectrum derived with the HARPS spectrograph. The same is found with the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other
hand, at high frequency, our calculations significantly depart from the CoRoT
and HARPS measurements. We show that assuming a solar metal abundance rather
than the actual metal abundance of the star would result in a larger
discrepancy with the seismic data. Furthermore, calculations that assume the
``new'' solar chemical mixture are in better agreement with the seismic data
than those that assume the ``old'' solar chemical mixture. These results
validate, in the case of a star significantly hotter than the Sun and Alpha Cen
A, the main assumptions in the model of stochastic excitation. However, the
discrepancies seen at high frequency highlight some deficiencies of the
modelling, whose origin remains to be understood.Comment: 8 pages, 3 figures (B-W and color), accepted for publication in
Astronomy & Astrophysics. Corrected typo in Eq. (4). Updated references.
Language improvement
The CoRoT target HD175726: an active star with weak solar-like oscillations
Context. The CoRoT short runs give us the opportunity to observe a large
variety of late-type stars through their solar-like oscillations. We report
observations of the star HD175726 that lasted for 27 days during the first
short run of the mission. The time series reveals a high-activity signal and
the power spectrum presents an excess due to solar-like oscillations with a low
signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to
extract as much information as possible from the power density spectrum.
Methods. The most productive method appears to be the autocorrelation of the
time series, calculated as the spectrum of the filtered spectrum. This method
is efficient, very rapid computationally, and will be useful for the analysis
of other targets, observed with CoRoT or with forthcoming missions such as
Kepler and Plato. Results. The mean large separation has been measured to be
97.2+-0.5 microHz, slightly below the expected value determined from solar
scaling laws.We also show strong evidence for variation of the large separation
with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial
modes, which is 1.7 times less than expected. Due to the low signal-to-noise
ratio, mode identification is not possible for the available data set of
HD175726. Conclusions. This study shows the possibility of extracting a seismic
signal despite a signal-to-noise ratio of only 0.37. The observation of such a
target shows the efficiency of the CoRoT data, and the potential benefit of
longer observing runs.Comment: 8 pages. Accepted in A&
Seismic and spectroscopic characterization of the solar-like pulsating CoRoT target HD 49385
The star HD 49385 is the first G-type solar-like pulsator observed in the
seismology field of the space telescope CoRoT. The satellite collected 137 days
of high-precision photometric data on this star, confirming that it presents
solar-like oscillations. HD 49385 was also observed in spectroscopy with the
NARVAL spectrograph in January 2009. Our goal is to characterize HD 49385 using
both spectroscopic and seismic data. The fundamental stellar parameters of HD
49385 are derived with the semi-automatic software VWA, and the projected
rotational velocity is estimated by fitting synthetic profiles to isolated
lines in the observed spectrum. A maximum likelihood estimation is used to
determine the parameters of the observed p modes. We perform a global fit, in
which modes are fitted simultaneously over nine radial orders, with degrees
ranging from l=0 to l=3 (36 individual modes). Precise estimates of the
atmospheric parameters (Teff, [M/H], log g) and of the vsini of HD 49385 are
obtained. The seismic analysis of the star leads to a clear identification of
the modes for degrees l=0,1,2. Around the maximum of the signal (nu=1013
microHz), some peaks are found significant and compatible with the expected
characteristics of l=3 modes. Our fit yields robust estimates of the
frequencies, linewidths and amplitudes of the modes. We find amplitudes of
about 5.6 +/- 0.8 ppm for radial modes at the maximum of the signal. The
lifetimes of the modes range from one day (at high frequency) to a bit more
than two days (at low frequency). Significant peaks are found outside the
identified ridges and are fitted. They are attributed to mixed modes.Comment: 13 pages, 14 figures, accepted in A&
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