1,421 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
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
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
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
Fingering Convection in Red Giants Revisited
Fingering (thermohaline) convection has been invoked for several years as a
possible extra-mixing which could occur in Red Giant stars due to the
modification of the chemical composition induced by nuclear reactions in the
hydrogen burning zone. Recent studies show however that this mixing is not
sufficient to account for the needed surface abundances. A new prescription for
fingering convection, based on 3D numerical simulations has recently been
proposed (BGS). The resulting mixing coefficient is larger than the ones
previously given in the literature. We compute models using this new
coefficient and compare them to previous studies. We use the LPCODE stellar
evolution code with the GNA generalized version of the mixing length theory to
compute Red Giant models and we introduce fingering convection using the BGS
prescription. The results show that, although the fingering zone now reaches
the outer dynamical convective zone, the efficiency of the mixing is not enough
to account for the observations. The fingering mixing coefficient should be
increased by two orders of magnitude for the needed surface abundances to be
reached. We confirm that fingering convection cannot be the mixing process
needed to account for surface abundances in RGB stars.Comment: Accepted for publication in Astronomy and Astrophysic
Lyman-alpha wing absorption in cool white dwarf stars
Kowalski & Saumon (2006) identified the missing absorption mechanism in the
observed spectra of cool white dwarf stars as the Ly-alpha red wing formed by
the collisions between atomic and molecular hydrogen and successfully explained
entire spectra of many cool DA-type white dwarfs. Owing to the important
astrophysical implications of this issue, we present here an independent
assessment of the process. For this purpose, we compute free-free
quasi-molecular absorption in Lyman-alpha due to collisions with H and H2
within the one-perturber, quasi-static approximation. Line cross-sections are
obtained using theoretical molecular potentials to describe the interaction
between the radiating atom and the perturber. The variation of the
electric-dipole transition moment with the interparticle distance is also
considered. Six and two allowed electric dipole transitions due to H-H and H-H2
collisions, respectively, are taken into account. The new theoretical
Lyman-alpha line profiles are then incorporated in our stellar atmosphere
program for the computation of synthetic spectra and colours of DA-type white
dwarfs. Illustrative model atmospheres and spectral energy distributions are
computed, which show that Ly-alpha broadening by atoms and molecules has a
significant effect on the white dwarf atmosphere models. The inclusion of this
collision-induced opacity significantly reddens spectral energy distributions
and affects the broadband colour indices for model atmospheres with Teff<5000
K. These results confirm those previously obtained by Kowalski & Saumon (2006).
Our study points out the need for reliable evaluations of H3 potential energy
surfaces covering a large region of nuclear configurations, in order to obtain
a better description of H-H2 collisions and a more accurate evaluation of their
influence on the spectrum of cool white dwarfs.Comment: 11 pages, 12 figures, 1 table, to be published in MNRA
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
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