2,759 research outputs found
Branching ratio measurements of the 7.12-MeV state in 16O
Knowledge of the gamma-ray branching ratios of the 7.12-MeV state of 16O is
important for the extrapolation of the 12C(a,g)16O cross section to
astrophysical energies. Ground state transitions provide most of the
12C(a,g)16O total cross section while cascade transitions have contributions of
the order of 10-20%. Determining the 7.12-MeV branching ratio will result in a
better extrapolation of the cascade and E2 ground state cross section to low
energies. We report here on measurements on the branching ratio of the 7.12-MeV
level in 16O.Comment: 4 pages, 5 figures. Contribution to the Eigth International Symposium
on Nuclei in the Cosmo
Studying the Pulsation of Mira Variables in the Ultraviolet
We present results from an empirical study of the Mg II h & k emission lines
of selected Mira variable stars, using spectra from the International
Ultraviolet Explorer (IUE). The stars all exhibit similar Mg II behavior during
the course of their pulsation cycles. The Mg II flux always peaks after optical
maximum near pulsation phase 0.2-0.5, although the Mg II flux can vary greatly
from one cycle to the next. The lines are highly blueshifted, with the
magnitude of the blueshift decreasing with phase. The widths of the Mg II lines
are also phase-dependent, decreasing from about 70 km/s to 40 km/s between
phase 0.2 and 0.6. We also study other UV emission lines apparent in the IUE
spectra, most of them Fe II lines. These lines are much narrower and not nearly
as blueshifted as the Mg II lines. They exhibit the same phase-dependent flux
behavior as Mg II, but they do not show similar velocity or width variations.Comment: 26 pages, 12 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty;
to appear in Ap
Atomic data for neutron-capture elements I. Photoionization and recombination properties of low-charge selenium ions
We present multi-configuration Breit-Pauli AUTOSTRUCTURE calculations of
distorted-wave photoionization (PI) cross sections, and total and partial
final-state resolved radiative recombination (RR) and dielectronic
recombination (DR) rate coefficients for the first six ions of the trans-iron
element Se. These calculations were motivated by the recent detection of Se
emission lines in a large number of planetary nebulae. Se is a potentially
useful tracer of neutron-capture nucleosynthesis, but accurate determinations
of its abundance in photoionized nebulae have been hindered by the lack of
atomic data governing its ionization balance. Our calculations were carried out
in intermediate coupling with semi-relativistic radial wavefunctions. PI and
recombination data were determined for levels within the ground configuration
of each ion, and experimental PI cross-section measurements were used to
benchmark our results. For DR, we allowed dn=0 core excitations, which are
important at photoionized plasma temperatures. DR is the dominant recombination
process for each of these Se ions at temperatures representative of
photoionized nebulae (~10^4 K). To estimate the uncertainties of these data, we
compared results from three different configuration-interaction expansions for
each ion, and tested the sensitivity of the results to the radial scaling
factors in the structure calculations. We find that the internal uncertainties
are typically 30-50% for the direct PI cross sections and ~10% for the computed
RR rate coefficients, while those for low-temperature DR can be considerably
larger (from 15-30% up to two orders of magnitude) due to the unknown energies
of near-threshold autoionization resonances. The results are suitable for
incorporation into photoionization codes used to numerically simulate
astrophysical nebulae, and will enable robust determinations of nebular Se
abundances.Comment: 17 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Family Idoteidae
17 p. : ill. ; 26 cm.Includes bibliographical references (p. 15-17)."The valviferan isopods of the Gulf of California are reviewed, including eight species in four genera. One rediscovered species (Eusymmerus antennatus) and two new species (Colidotea findleyi and Erichsonella cortezi) are fully described. Two of the Gulf idoteids appear to be of subtropical or tropical origin (Eusymmerus antennatus and Colidotea findleyi), whereas the others appear to be cold or warm temperate derived species (Erichsonella cortezi, Idotea aculeata, I. urotoma, I. stenops, I. resecata, and I. wosnesenskii). These findings suggest, as other distributional data have, that the family Idoteidae is composed largely of cool to warm temperate-centered taxa, with few modern genera and species having evolved in the lower latitudes (e.g., Eusymmerus, Colidotea, and some species of Erichsonella). Members of this family, in the tropical and subtropical east Pacific, are generally found in association with littoral and shallow-water algae, especially species of Sargassum. Four species of Idotea are extremely wide-ranging forms, suggesting that members of this genus may be considerably more eurythermal than previously suspected. Idotea wosnesenskii and I. resecata range from the Gulf of California to Alaska; I. stenops ranges from Baja California (or possibly the Gulf of California) to Oregon; and I. urotoma ranges from the Gulf of California to Puget Sound"--P. [1]
Stellar Hydrodynamics in Radiative Regions
We present an analysis of the response of a radiative region to waves
generated by a convective region of the star; this wave treatment of the
classical problem of ``overshooting'' gives extra mixing relative to the
treatment traditionally used in stellar evolutionary codes. The interface
between convectively stable and unstable regions is dynamic and nonspherical,
so that the nonturbulent material is driven into motion, even in the absence of
``penetrative overshoot.'' These motions may be described by the theory of
nonspherical stellar pulsations, and are related to motion measured by
helioseismology. Multi-dimensional numerical simulations of convective flow
show puzzling features which we explain by this simplified physical model.
Gravity waves generated at the interface are dissipated, resulting in slow
circulation and mixing seen outside the formal convection zone. The approach
may be extended to deal with rotation and composition gradients. Tests of this
description in the stellar evolution code TYCHO produce carbon stars on the
asymptotic giant branch (AGB), an isochrone age for the Hyades and three young
clusters with lithium depletion ages from brown dwarfs, and lithium and
beryllium depletion consistent with observations of the Hyades and Pleiades,
all without tuning parameters. The insight into the different contributions of
rotational and hydrodynamic mixing processes could have important implications
for realistic simulation of supernovae and other questions in stellar
evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa
Angular momentum transport by internal gravity waves III - Wave excitation by core convection and the Coriolis effect
This is the third in a series of papers that deal with angular momentum
transport by internal gravity waves. We concentrate on the waves excited by
core convection in a 3Msun, Pop I main sequence star. Here, we want to examine
the role of the Coriolis acceleration in the equations of motion that describe
the behavior of waves and to evaluate its impact on angular momentum transport.
We use the so-called traditional approximation of geophysics, which allows
variable separation in radial and horizontal components. In the presence of
rotation, the horizontal structure is described by Hough functions instead of
spherical harmonics. The Coriolis acceleration has two main effects on waves.
It transforms pure gravity waves into gravito-inertial waves that have a larger
amplitude closer to the equator, and it introduces new waves whose restoring
force is mainly the conservation of vorticity. Taking the Coriolis acceleration
into account changes the subtle balance between prograde and retrograde waves
in non-rotating stars. It also introduces new types of waves that are either
purely prograde or retrograde. We show in this paper where the local deposition
of angular momentum by such waves is important.Comment: 9 pages, 10 figures, accepted for publication by A&
A near-infrared spectroscopic screening of the red giant populations in omega Centauri
Near-infrared spectra of 21 giants in omega Centauri, spanning the whole
range of metallicities observed in this cluster, are presented. This work is
part of a coordinated photometric and spectroscopic campaign in the optical and
in the infrared, aimed at studying the complex stellar population of omega
Centauri and understanding its formation and chemical evolution. By analyzing
the several CO and OH molecular bands and atomic lines in the spectra of the
selected giants, metal abundances and abundance ratios have been obtained. The
existence of three major metallicity regimes at [Fe/H]=-1.6, -1.2 and
[Fe/H]<-0.5 has been confirmed. The most metal-rich stars in our sample show a
lower (if any) alpha-enhancement when compared to the more metal-poor
components, suggesting that they should have formed in a medium significantly
polluted by type Ia supernova ejecta. Isotopic carbon abundances have been also
inferred, providing an average 13C/12C=4, which clearly indicates that
extra-mixing processes occurred in the stellar interiors during the ascent on
the Red Giant Branch.Comment: 22 pages, 7 .ps figures. aastex. Accepted for pubilcation in the
Astrophysical Journa
The roots of "Western European societal evolution". A concept of Europe by JenĆ SzƱcs
JenĆ SzƱcs wrote his essay entitled Sketch on the three regions of Europe in the early 1980s in Hungary. During these years, a historically well-argued opinion emphasising a substantial difference between Central European and Eastern European societies was warmly received in various circles of the political opposition. In a wider European perspective SzƱcs used the old âliberty toposâ which claims that the history of Europe is no other than the fulfillment of liberty. In his Sketch, SzƱcs does not only concentrate on questions concerning the Middle Ages in Western Europe. Yet it is this stream of thought which brought a new perspective to explaining European history. His picture of the Middle Ages represents well that there is a way to integrate all typical Western motifs of post-war self-definition into a single theory. Mainly, the âliberty motifâ, as a sign of âEuropeanismâ â in the interpretation of BibĂłâs concept, Anglo-saxon Marxists and Weberâs social theory â, developed from medieval concepts of state and society and from an analysis of economic and social structures. SzƱcsâs historical aspect was a typical intellectual product of the 1980s: this was the time when a few Central European historians started to outline non-Marxist aspects of social theory and categories of modernisation theories, but concealing them with Marxist terminology
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