6,424 research outputs found
Estimating the reproduction number of Ebola virus (EBOV) during the 2014 outbreak in West Africa
The 2014 Ebola virus (EBOV) outbreak in West Africa is the largest outbreak
of the genus Ebolavirus to date. To better understand the spread of infection
in the affected countries, it is crucial to know the number of secondary cases
generated by an infected index case in the absence and presence of control
measures, i.e., the basic and effective reproduction number. In this study, I
describe the EBOV epidemic using an SEIR
(susceptible-exposed-infectious-recovered) model and fit the model to the most
recent reported data of infected cases and deaths in Guinea, Sierra Leone and
Liberia. The maximum likelihood estimates of the basic reproduction number are
1.51 (95% confidence interval [CI]: 1.50-1.52) for Guinea, 2.53 (95% CI:
2.41-2.67) for Sierra Leone and 1.59 (95% CI: 1.57-1.60) for Liberia. The model
indicates that in Guinea and Sierra Leone the effective reproduction number
might have dropped to around unity by the end of May and July 2014,
respectively. In Liberia, however, the model estimates no decline in the
effective reproduction number by end-August 2014. This suggests that control
efforts in Liberia need to be improved substantially in order to stop the
current outbreak.Comment: Published version, PLOS Currents Outbreaks. 2014 Sep
Hydraulic Actuator System for Rotor Control
In the last ten years, several different types of actuators were developed and fabricated for active control of rotors. A special hydraulic actuator system capable of generating high forces to rotating shafts via conventional bearings is addressed. The actively controlled hydraulic force actuator features an electrohydraulic servo valve which can produce amplitudes and forces at high frequencies necessary for influencing rotor vibrations. The mathematical description will be given in detail. The experimental results verify the theoretical model. Simulations already indicate the usefulness of this compact device for application to a real rotor system
A nonadiabatic oscillation study of DB white dwarfs
A self-consistent abundance diffusion treatment in the evolution of cooling white dwarfs permitted a study of the effect of elemental segregation in nonadiabatic, nonradial stability computations. In particular, mode trapping manifesting itself in cyclically varying period separations behaved differently from its appearance in the damping/excitation rates. Another aspect of the investigation concerned the effect of heavy-element traces in homogeneous DB white-dwarf envelopes on their pulsational instability domain. The stellar models are computed with the CGM convection approach; the study can therefore be considered as a test of its performance in nonradial stability analyses.Fil: Gautschy, Alfred. Ferrachstrasse. 1; SuizaFil: Althaus, Leandro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; Argentin
Oscillatory secular modes: The thermal micropulses
Stars in the narrow mass range of about 2.5 and 3.5 solar masses can develop
a thermally unstable He-burning shell during its ignition phase. We study, from
the point of view secular stability theory, these so called thermal micropulses
and we investigate their properties; the thermal pulses constitute a convenient
conceptual laboratory to look thoroughly into the physical properties of a
helium-burning shell during the whole thermally pulsing episode. Linear
stability analyses were performed on a large number of 3 solar-mass star models
at around the end of their core helium-burning and the beginning of the
double-shell burning phase. The stellar models were not assumed to be in
thermal equilibrium. The thermal mircopulses, and we conjecture all other
thermal pulse episodes encountered by shell-burning stars, can be understood as
the nonlinear finite-amplitude realization of an oscillatory secular
instability that prevails during the whole thermal pulsing episode. Hence, the
cyclic nature of the thermal pulses can be traced back to a linear instability
concept.Comment: To be published - essentially footnote-free - in Astronomy &
Astrophysic
On the formation of hot DQ white dwarfs
We present the first full evolutionary calculations aimed at exploring the
origin of hot DQ white dwarfs. These calculations consistently cover the whole
evolution from the born-again stage to the white dwarf cooling track. Our
calculations provide strong support to the diffusive/convective-mixing picture
for the formation of hot DQs. We find that the hot DQ stage is a short-lived
stage and that the range of effective temperatures where hot DQ stars are found
can be accounted for by different masses of residual helium and/or different
initial stellar masses. In the frame of this scenario, a correlation between
the effective temperature and the surface carbon abundance in DQs should be
expected, with the largest carbon abundances expected in the hottest DQs. From
our calculations, we suggest that most of the hot DQs could be the cooler
descendants of some PG1159 stars characterized by He-rich envelopes markedly
smaller than those predicted by the standard theory of stellar evolution. At
least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an
evolutionary link between this star and the massive PG1159 star H1504+65 is
plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal
Letter
Asteroseismology of the Kepler V777 Her variable white dwarf with fully evolutionary models
DBV stars are pulsating white dwarfs with atmospheres rich in He.
Asteroseismology of DBV stars can provide valuable clues about the origin,
structure and evolution of hydrogen-deficient white dwarfs, and may allow to
study neutrino and axion physics. Recently, a new DBV star, KIC 8626021, has
been discovered in the field of the \emph{Kepler} spacecraft. It is expected
that further monitoring of this star in the next years will enable astronomers
to determine its detailed asteroseismic profile. We perform an
asteroseismological analysis of KIC 8626021 on the basis of fully evolutionary
DB white-dwarf models. We employ a complete set of evolutionary DB white-dwarf
structures covering a wide range of effective temperatures and stellar masses.
They have been obtained on the basis of a complete treatment of the
evolutionary history of progenitors stars. We compute g-mode adiabatic
pulsation periods for this set of models and compare them with the pulsation
properties exhibited by KIC 8626021. On the basis of the mean period spacing of
the star, we found that the stellar mass should be substantially larger than
spectroscopy indicates. From period-to-period fits we found an
asteroseismological model characterized by an effective temperature much higher
than the spectroscopic estimate. In agreement with a recent asteroseismological
analysis of this star by other authors, we conclude that KIC 8626021 is located
near the blue edge of the DBV instability strip, contrarily to spectroscopic
predictions. We also conclude that the mass of KIC 8626021 should be
substantially larger than thought.Comment: 7 pages, 5 figures, 3 tables. To be published in Astronomy and
Astrophysic
Evolution of DA white dwarfs in the context of a new theory of convection
In this study we compute the structure and evolution of carbon-oxygen DA
white dwarfs by means of a detailed and updated evolutionary code. We treat the
energy transport by convection within the formalism of the full spectrum
turbulence theory, as given by the Canuto, Goldman and Mazzitelli (CGM) model.
We explore the effect of various hydrogen layer masses on both the surface
gravity and the hydrogen burning. Convective mixing at low luminosities is also
considered. One of our main interests in this work has been to study the
evolution of ZZ Ceti models, with the aim of comparing the CGM and mixing
length theory (MLT) predictions. In this connection, we find that the
temperature profile given by the CGM model is markedly different from that of
the ML1 and ML2 versions of MLT. We have also computed approximate effective
temperatures for the theoretical blue edge of the DA instability strip by using
thermal timescale arguments for our evolving models. In this context, we found
that the CGM theory leads to blue edges that are cooler than the observed ones.
However, because the determination of the atmospheric parameters of ZZ Ceti
stars is dependent on the assumed convection description in model atmosphere
calculation, observed blue edges computed considering the CGM theory are
required in order to perform a sef consistent comparison of our results with
observations. Finally, detailed pulsation calculations of ZZ Ceti models
considering the CGM convection would be necessary to place the results found in
this paper on a firmer basis.Comment: 11 pages, 11 figures. Uses mn.st
Outer boundary conditions for evolving cool white dwarfs
White dwarf evolution is essentially a gravothermal cooling process,
which,for cool white dwarfs, sensitively depends on the treatment of the outer
boundary conditions. We provide detailed outer boundary conditions appropriate
for computing the evolution of cool white dwarfs employing detailed non-gray
model atmospheres for pure H composition. We also explore the impact on the
white dwarf cooling times of different assumptions for energy transfer in the
atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are
computed taken into account non-ideal effects in the gas equation of state and
chemical equilibrium, collision-induced absorption from molecules, and the
Lyman alpha quasi-molecular opacity. Our results show that the use of detailed
outer boundary conditions becomes relevant for effective temperatures lower
than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively.
Detailed model atmospheres predict ages that are up to approx 10% shorter at
log L/L_sun=-4 when compared with the ages derived using Eddington-like
approximations at tau_Ross=2/3. We also analyze the effects of various
assumptions and physical processes of relevance in the calculation of outer
boundary conditions. In particular, we find that the Ly_alpha red wing
absorption does not affect substantially the evolution of white dwarfs. White
dwarf cooling timescales are sensitive to the surface boundary conditions for
T_eff < 6000K. Interestingly enough, non-gray effects have little consequences
on these cooling times at observable luminosities. In fact, collision-induced
absorption processes, which significantly affect the spectra and colors of old
white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in
their cooling rates, except throughout the Rosseland mean opacity.Comment: 6 pages, 9 figures, to be published in Astronomy and Astrophysic
Grids of white dwarf evolutionary models with masses from M= 0.1 to 1.2 Ms
We present detailed evolutionary calculations for carbon
- oxygen - and helium - core white dwarf (WD) models with masses ranging from
M= 0.1 to M= 1.2 solar masses and for metallicities Z= 0.001 and Z= 0. The
sequences cover a wide range of hydrogen envelopes as well. We employed a
detailed WD evolutionary code. In particular, the energy transport by
convectcion is treated within the formalism of the full spectrum turbulence
theory. The set of models presented here is very detailed and should be
valuable for the interpretation of the observational data on low - mass WDs
recently discovered in numerous binary configurations and also for the general
problem of determining the theoretical luminosity function for WDs. In this
context, we compare our cooling sequences with the observed WD luminosity
function recently improved by Leggett, Ruiz and Bergeron (1998) and we obtain
an age for the Galactic disc of approximately 8 Gyr. Finally, we applied the
results of this paper to derive stellar masses of a sample of low - mass white
dwarfs.Comment: 9 pages, 8 figures; accepted for publication in MNRAS; replaced with
minor corrections to tex
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