34,326 research outputs found
Element Abundance Determination in Hot Evolved Stars
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class
PG1159 is probably caused by a (very) late helium-shell flash or a AGB final
thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden
intershell region. Thus, the photospheric element abundances of these stars
allow us to draw conclusions about details of nuclear burning and mixing
processes in the precursor AGB stars. We compare predicted element abundances
to those determined by quantitative spectral analyses performed with advanced
non-LTE model atmospheres. A good qualitative and quantitative agreement is
found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for
others (P, S, Fe) point at shortcomings in stellar evolution models for AGB
stars. Almost all of the chemical trace elements in these hot stars can only be
identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer
and the Hubble Space Telescope played a crucial role for this research.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical,
Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009,
Kodaikanal, India (Springer
Magnitude Uncertainties Impact Seismic Rate Estimates, Forecasts and Predictability Experiments
The Collaboratory for the Study of Earthquake Predictability (CSEP) aims to
prospectively test time-dependent earthquake probability forecasts on their
consistency with observations. To compete, time-dependent seismicity models are
calibrated on earthquake catalog data. But catalogs contain much observational
uncertainty. We study the impact of magnitude uncertainties on rate estimates
in clustering models, on their forecasts and on their evaluation by CSEP's
consistency tests. First, we quantify magnitude uncertainties. We find that
magnitude uncertainty is more heavy-tailed than a Gaussian, such as a
double-sided exponential distribution, with scale parameter nu_c=0.1 - 0.3.
Second, we study the impact of such noise on the forecasts of a simple
clustering model which captures the main ingredients of popular short term
models. We prove that the deviations of noisy forecasts from an exact forecast
are power law distributed in the tail with exponent alpha=1/(a*nu_c), where a
is the exponent of the productivity law of aftershocks. We further prove that
the typical scale of the fluctuations remains sensitively dependent on the
specific catalog. Third, we study how noisy forecasts are evaluated in CSEP
consistency tests. Noisy forecasts are rejected more frequently than expected
for a given confidence limit. The Poisson assumption of the consistency tests
is inadequate for short-term forecast evaluations. To capture the
idiosyncrasies of each model together with any propagating uncertainties, the
forecasts need to specify the entire likelihood distribution of seismic rates.Comment: 35 pages, including 15 figures, agu styl
Comment on "Analysis of the Spatial Distribution between Successive Earthquakes" by Davidsen and Paczuski
By analyzing a southern California earthquake catalog, Davidsen and Paczuski
[Phys. Rev. Lett. 94, 048501 (2005)] claim to have found evidence contradicting
the theory of aftershock zone scaling in favor of scale-free statistics. We
present four elements showing that Davidsen and Paczuski's results may be
insensitive to the existence of physical length scales associated with
aftershock zones or mainshock rupture lengths, so that their claim is
unsubstantiated. (i) Their exponent smaller than 1 for a pdf implies that the
power law statistics they report is at best an intermediate asymptotic; (ii)
their power law is not robust to the removal of 6 months of data around Landers
earthquake within a period of 17 years; (iii) the same analysis for Japan and
northern California shows no evidence of robust power laws; (iv) a statistical
model of earthquake triggering that explicitely obeys aftershock zone scaling
can reproduce the observed histogram of Davidsen and Paczuski, demonstrating
that their statistic may not be sensitive to the presence of characteristic
scales associated with earthquake triggering
A deterministic model for the occurrence and dynamics of multiple mutations in hierarchically organized tissues
We model a general, hierarchically organized tissue by a multi compartment
approach, allowing any number of mutations within a cell. We derive closed
solutions for the deterministic clonal dynamics and the reproductive capacity
of single clones. Our results hold for the average dynamics in a hierarchical
tissue characterized by an arbitrary combination of proliferation parameters.Comment: 4 figures, to appear in Royal Society Interfac
Deposing the Cool Corona of KPD 0005+5106
The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf
KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner
& Barstow (1993). It defied the best non-LTE modelling attempts in terms of
photospheric emission from He-dominated atmospheres including C, N and O and
was instead interpreted as the first evidence for a coronal plasma around a
white dwarf. We show here that a recent high resolution Chandra LETGS spectrum
has more structure than expected from a thermal bremsstrahlung continuum and
lacks the narrow lines of H-like and He-like C expected from a coronal plasma.
Moreover, a coronal model requires a total luminosity more than two orders of
magnitude larger than that of the star itself. Instead, the observed 20-80 AA
flux is consistent with photospheric models containing trace amounts of heavier
elements such as Fe. The soft X-ray flux is highly sensitive to the adopted
metal abundance and provides a metal abundance diagnostic. The weak X-ray
emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from
the photosphere and requires alternative explanations. We echo earlier
speculation that such emission arises in a shocked wind. Despite the presence
of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O
VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be
trapped by resonant scattering within the emitting plasma and destroyed by
photoelectric absorption.Comment: 15 Pages, 4 figures. Accepted for the Astrophysical Journa
A combination of soluble helper factors bypasses the requirement for stimulator cells and induces nonspecific cytotoxic T cell responses
The specificity of cytotoxic T lymphocyte (CTL) responses generated in the presence of lymphokines was studied. Thymic responder cells were activated in the presence of stimulator cells that differed in their metabolic activity. After 5 days of culture, the cytotoxic response was estimated in a 4-h 51Cr-release test. Coculture of thymic responders with irradiated splenic stimulator cells in the presence of interleukin 2(IL 2) led to preferential cytolysis of target cells that expressed the same histocompatibility antigens as the cells used for sensitization. Addition of T cell cytotoxicity-inducing factor 1 (TCF1), however, to those cultures made the presence of stimulator cells unnecessary and induced cytotoxic responses against all target cells tested, including target cells syngeneic to the responder cells. This activation was neither due to contaminating mitogen nor to the effect of heterologous serum in the assay system. The conclusion of these findings was that either polyclonal activation of CTL was induced by TCF1 or that some specific CTL clones differentiated into unrestricted killer cells under the influence of TCF1
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