138 research outputs found
Thermal Equilibration of 176-Lu via K-Mixing
In astrophysical environments, the long-lived (\T_1/2 = 37.6 Gy) ground state
of 176-Lu can communicate with a short-lived (T_1/2 = 3.664 h) isomeric level
through thermal excitations. Thus, the lifetime of 176-Lu in an astrophysical
environment can be quite different than in the laboratory. We examine the
possibility that the rate of equilibration can be enhanced via K-mixing of two
levels near E_x = 725 keV and estimate the relevant gamma-decay rates. We use
this result to illustrate the effect of K-mixing on the effective stellar
half-life. We also present a network calculation that includes the
equilibrating transitions allowed by K-mixing. Even a small amount of K-mixing
will ensure that 176-Lu reaches at least a quasi-equilibrium during an
s-process triggered by the 22-Ne neutron source.Comment: 9 pages, 6 figure
Properties of the 5- state at 839 keV in 176Lu and the s-process branching at A = 176
The s-process branching at mass number A = 176 depends on the coupling
between the high-K ground state and a low-lying low-K isomer in 176Lu. This
coupling is based on electromagnetic transitions via intermediate states at
higher energies. The properties of the lowest experimentally confirmed
intermediate state at 839 keV are reviewed, and the transition rate between
low-K and high-K states under stellar conditions is calculated on the basis of
new experimental data for the 839 keV state. Properties of further candidates
for intermediate states are briefly analyzed. It is found that the coupling
between the high-K ground state and the low-K isomer in 176Lu is at least one
order of magnitude stronger than previously assumed leading to crucial
consequences for the interpretation of the 176Lu/176Hf pair as an s-process
thermometer.Comment: 11 pages, 4 figures accepted for publication in Phys. Rev.
Detailed statistical analysis plan for the Danish Palliative care trial (DanPaCT)
Acknowledgements We wish to thank the students who sent out the questionnaires, who entered and compared all data, help with data management, made material blind to the investigators, and were/will be outcome assessors of interventions given. They were: Nicla Rohde Christensen, Ellen Lundorff, Marc Klee Olsen, Charlotte Lund Rasmussen, and Nete Skjødt. This work was funded by the Tryg Foundation [journal number 7-10-0838A] and the Danish Cancer Society [journal number R16-A695]. Other than funding the trial, the funding body had no role in the design, conduct, analysis, or reporting of the present trial.Peer reviewedPublisher PD
An Improved Metallicity Calibration with UBV Photometry
We used the data of 701 stars covering the colour index interval
0.32<B-V<=1.16, with metallicities -1.76<=[Fe/H]<=+0.40 dex, which were taken
from PASTEL catalogue and estimated metallicity dependent guillotine factors
which provide a more accurate metallicity calibration. We reduced the
metallicities of 11 authors to the metallicities of Valenti & Fischer (2005),
thus obtained a homogeneous set of data which increased the accuracy of the
calibration, i.e. [Fe/H]=-14.316*delta^{2}_{0.6}-3.557*delta_{0.6}+0.105.
Comparison of the metallicity residuals, for two sets of data, based on the
metallicity dependent guillotine factors with the ones obtained via metal free
guillotine factors, shows that metallicities estimated by means of new
guillotine factors are more accurate than the other ones. This advantage can be
used in the metallicity gradient investigation of the Galactic components, i.e.
thin disc, thick disc and halo.Comment: 12 pages, including 10 figures and 6 tables, accepted for publication
in PAS
Obscuration of Supersoft X-ray Sources by Circumbinary Material - A Way to Hide Type Ia Supernova Progenitors?
The progenitors of supernovae (SNe) type Ia are usually assumed to be either
a single white dwarf (WD) accreting from a non-degenerate companion (the SD
channel) or the result of two merging WDs (DD channel). However, no consensus
currently exists as to which progenitor scenario is the correct one, or whether
the observed SN Ia rate is produced by a combination of both channels. Unlike a
DD progenitor a SD progenitor is expected to emit supersoft X-rays for a
prolonged period of time (~1 Myr) as a result of the burning of accreted matter
on the surface of the WD. An argument against the SD channel as a significant
producer of SNe type Ia has been the lack of observed supersoft X-ray sources
(SSS) and the lower-than-expected integrated soft X-ray flux from elliptical
galaxies.
We wish to determine if it is possible to obscure the supersoft X-ray
emission from a nuclear burning white dwarf in an accreting single degenerate
binary system. In case of obscured systems we wish to determine their general
observational characteristics.
We examine the emergent X-ray emission from a canonical SSS system surrounded
by a spherically symmetric configuration of material, assuming a black body
spectrum with T_BB=50 eV and L=10^38 erg/s. The circumbinary material is
assumed to be of solar chemical abundances, and we leave the mechanism behind
the mass loss into the circumbinary region unspecified.
If steadily accreting, nuclear burning WDs are canonical SSS our analysis
suggests that they can be obscured by relatively modest circumbinary mass loss
rates. This may explain the discrepancy of SSS compared to the SN Ia rate
inferred from observations if the SD progenitor scenario contributes
significantly to the SN Ia rate. Recycled emissions from obscured systems may
be visible in other wavebands than X-rays. It may also explain the lack of
observed SSS in symbiotic binary systems.Comment: 10 pages, 4 figures, accepted A&
The Stellar Composition of the Star Formation Region CMa R1 -- III. A new outburst of the Be star component in Z CMa
We report on a recent event in which, after more than a decade of slowly
fading, the visual brightness of the massive young binary Z CMa suddenly
started to rise by about 1 magnitude in December 1999, followed by a rapid
decline to its previous brightness over the next six months. This behaviour is
similar to that exhibited by this system around its eruption in February 1987.
A comparison of the intrinsic luminosities of the system with recent
evolutionary calculations shows that Z CMa may consist of a 16 M_sun B0 IIIe
primary star and a ~ 3 M_sun FUOr secondary with a common age of ~ 3 x 10^5 yr.
We also compare new high-resolution spectra obtained in Jan. and Feb. 2000,
during the recent rise in brightness, with archive data from 1991 and 1996. The
spectra are rich in emission lines, which originate from the envelope of the
early B-type primary star. The strength of these emission lines increased
strongly with the brightness of Z CMa. We interpret the collected spectral data
in terms of an accretion disc with atmosphere around the Herbig B0e component
of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high
resolution profile of the 6300 A [O I] emission line, obtained by us in March
2002 shows an increase in flux and a prominent blue shoulder to the feature
extending to ~ -700 km/s, which was much fainter in the pre-outburst spectra.
We propose that this change in profile is a result of a strong change in the
collimation of a jet, as a result of the outburst at the start of this century.Comment: 22 pages, 12 figures, accepted for publication in MNRA
Equation of state for Universe from similarity symmetries
In this paper we proposed to use the group of analysis of symmetries of the
dynamical system to describe the evolution of the Universe. This methods is
used in searching for the unknown equation of state. It is shown that group of
symmetries enforce the form of the equation of state for noninteracting scaling
multifluids. We showed that symmetries give rise the equation of state in the
form and energy density
, which
is commonly used in cosmology. The FRW model filled with scaling fluid (called
homological) is confronted with the observations of distant type Ia supernovae.
We found the class of model parameters admissible by the statistical analysis
of SNIa data. We showed that the model with scaling fluid fits well to
supernovae data. We found that and (), which can correspond to (hyper) phantom fluid, and to a
high density universe. However if we assume prior that
then the favoured model is close to concordance
CDM model. Our results predict that in the considered model with
scaling fluids distant type Ia supernovae should be brighter than in
CDM model, while intermediate distant SNIa should be fainter than in
CDM model. We also investigate whether the model with scaling fluid is
actually preferred by data over CDM model. As a result we find from
the Akaike model selection criterion prefers the model with noninteracting
scaling fluid.Comment: accepted for publication versio
Stellar structure and compact objects before 1940: Towards relativistic astrophysics
Since the mid-1920s, different strands of research used stars as "physics
laboratories" for investigating the nature of matter under extreme densities
and pressures, impossible to realize on Earth. To trace this process this paper
is following the evolution of the concept of a dense core in stars, which was
important both for an understanding of stellar evolution and as a testing
ground for the fast-evolving field of nuclear physics. In spite of the divide
between physicists and astrophysicists, some key actors working in the
cross-fertilized soil of overlapping but different scientific cultures
formulated models and tentative theories that gradually evolved into more
realistic and structured astrophysical objects. These investigations culminated
in the first contact with general relativity in 1939, when J. Robert
Oppenheimer and his students George Volkoff and Hartland Snyder systematically
applied the theory to the dense core of a collapsing neutron star. This
pioneering application of Einstein's theory to an astrophysical compact object
can be regarded as a milestone in the path eventually leading to the emergence
of relativistic astrophysics in the early 1960s.Comment: 83 pages, 4 figures, submitted to the European Physical Journal
Effects of photon escape on diagnostic diagrams for HII regions
In this article we first outline the mounting evidence that a significant
fraction of the ionizing photons emitted by OB stars within HII regions escape
from their immediate surroundings and explain how an HII region structure
containing high density contrast in homogeneities facilitates this escape. Next
we describe sets of models containing inhomogeneities which are used to predict
tracks in the commonly used diagnostic diagrams (based on ratios of emission
lines) whose only independent variable is the photon escape fraction, xi. We
show that the tracks produced by the models in two of the most cited of these
diagrams conform well to the distribution of observed data points, with the
models containing optically thick inhomogeneities ("CLUMPY" models) yielding
somewhat better agreement than those with optically thin inhomogeneities ("FF"
models). We show how variations in the ionization parameter U, derived from
emission line ratios, could be due to photon escape. Using a rather wide range
of assumptions about the filling factor of dense clumps we find, for a selected
set of regions observed in M51 photon escape fraction ranging between 30% and
50%. We show, using oxygen as the test element, that models with different
assumptions about the gas inhomogeneity will give variations in the abundance
values derived from diagnostic diagrams, but do not claim here to have a fully
developed set of diagnostic tools to improve abundance determinations made in
this way. We finally propose a combination of line ratios with the absolute
Halpha luminosity of a given HII region, which allows us to determine the
photon escape fraction, and hence resolve the degeneracy between U and xi.Comment: 13 pages, 15 figures. Accepted for publication in A&
A probable stellar solution to the cosmological lithium discrepancy
The measurement of the cosmic microwave background has strongly constrained
the cosmological parameters of the Universe. When the measured density of
baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis
calculations, the amounts of hydrogen, helium and lithium produced shortly
after the Big Bang can be predicted with unprecedented precision. The predicted
primordial lithium abundance is a factor of two to three higher than the value
measured in the atmospheres of old stars. With estimated errors of 10 to 25%,
this cosmological lithium discrepancy seriously challenges our understanding of
stellar physics, Big Bang nucleosynthesis or both. Certain modifications to
nucleosynthesis have been proposed, but found experimentally not to be viable.
Diffusion theory, however, predicts atmospheric abundances of stars to vary
with time, which offers a possible explanation of the discrepancy. Here we
report spectroscopic observations of stars in the metalpoor globular cluster
NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage
for various elements. These element-specific trends are reproduced by
stellar-evolution models with diffusion and turbulent mixing. We thus conclude
that diffusion is predominantly responsible for the low apparent stellar
lithium abundance in the atmospheres of old stars by transporting the lithium
deep into the star.Comment: 10 pages, 3 two-panel figures, 2 tables, includes all Supplementary
Information otherwise accessible online via www.nature.co
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