3,343 research outputs found
The First Galaxies: Clues from Element Abundances
It has recently become possible to measure directly the abundances of several
chemical elements in a variety of environments at redshifts up to z = 5. In
this review I summarise the latest observations of Lyman break galaxies, damped
Lyman alpha systems and the Lyman alpha forest with a view to uncovering any
clues which these data may offer to the first episodes of star formation. The
picture which is emerging is one where the universe at z = 3 already included
many of the components of today's galaxies--even at these early times we see
evidence for Populations I and II stars, while the `smoking gun' for Population
III objects may be hidden in the chemical composition of the lowest density
regions of the IGM, yet to be deciphered.Comment: 15 pages, LaTex, 8 Postscript Figures. To appear in the Philosophical
Transactions of The Royal Society, Series
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray
spallation in the ISM. Assuming that the cosmic-rays are globally transported
across the Galaxy, the beryllium production should be a widespread process and
its abundance should be roughly homogeneous in the early-Galaxy at a given
time. Thus, it could be useful as a tracer of time. In an investigation of the
use of Be as a cosmochronometer and of its evolution in the Galaxy, we found
evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate
into two components. One is consistent with predictions of evolutionary models
while the other is chemically indistinguishable from the thick-disk stars. This
is interpreted as a difference in the star formation history of the two
components and suggests that the local halo is not a single uniform population
where a clear age-metallicity relation can be defined. We also found evidence
that the star formation rate was lower in the outer regions of the thick disk,
pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
The Suspension of Treatments in ADAPT: Concerns beyond the Cardiovascular Safety of Celecoxib or Naproxen
The 'Forbidden' Abundance of Oxygen in the Sun
We reexamine closely the solar photospheric line at 6300 A, which is
attributed to a forbidden line of neutral oxygen, and is widely used in
analyses of other late-type stars.
We use a three-dimensional time-dependent hydrodynamical model solar
atmosphere which has been tested successfully against observed granulation
patterns and an array of absorption lines. We show that the solar line is a
blend with a Ni I line, as previously suggested but oftentimes neglected.
Thanks to accurate atomic data on the [O I] and Ni I lines we are able to
derive an accurate oxygen abundance for the Sun: log epsilon (O) = 8.69 +/-
0.05 dex, a value at the lower end of the distribution of previously published
abundances, but in good agreement with estimates for the local interstellar
medium and hot stars in the solar neighborhood. We conclude by discussing the
implication of the Ni I blend on oxygen abundances derived from the [O I] 6300
A line in disk and halo stars.Comment: 16 pages, 3 eps figures included; a more compact PostScript version
created using emulateapj.sty is available from
http://hebe.as.utexas.edu/recent_publi.html; to appear in ApJ
Instaseis: instant global seismograms based on a broadband waveform database
We present a new method and implementation (Instaseis) to store global Green's functions in a database which allows for near-instantaneous (on the order of milliseconds) extraction of arbitrary seismograms. Using the axisymmetric spectral element method (AxiSEM), the generation of these databases, based on reciprocity of the Green's functions, is very efficient and is approximately half as expensive as a single AxiSEM forward run. Thus, this enables the computation of full databases at half the cost of the computation of seismograms for a single source in the previous scheme and allows to compute databases at the highest frequencies globally observed. By storing the basis coefficients of the numerical scheme (Lagrange polynomials), the Green's functions are 4th order accurate in space and the spatial discretization respects discontinuities in the velocity model exactly. High-order temporal interpolation using Lanczos resampling allows to retrieve seismograms at any sampling rate. AxiSEM is easily adaptable to arbitrary spherically symmetric models of Earth as well as other planets. In this paper, we present the basic rationale and details of the method as well as benchmarks and illustrate a variety of applications. The code is open source and available with extensive documentation at www.instaseis.net
Instaseis: instant global seismograms based on a broadband waveform database
We present a new method and implementation (Instaseis) to store global Green's functions in a database which allows for near-instantaneous (on the order of milliseconds) extraction of arbitrary seismograms. Using the axisymmetric spectral element method (AxiSEM), the generation of these databases, based on reciprocity of the Green's functions, is very efficient and is approximately half as expensive as a single AxiSEM forward run. Thus, this enables the computation of full databases at half the cost of the computation of seismograms for a single source in the previous scheme and allows to compute databases at the highest frequencies globally observed. By storing the basis coefficients of the numerical scheme (Lagrange polynomials), the Green's functions are 4th order accurate in space and the spatial discretization respects discontinuities in the velocity model exactly. High-order temporal interpolation using Lanczos resampling allows to retrieve seismograms at any sampling rate. AxiSEM is easily adaptable to arbitrary spherically symmetric models of Earth as well as other planets. In this paper, we present the basic rationale and details of the method as well as benchmarks and illustrate a variety of applications. The code is open source and available with extensive documentation at www.instaseis.net
Tumour burden in early stage Hodgkin's disease: the single most important prognostic factor for outcome after radiotherapy.
One hundred and forty-two patients with Hodgkin's disease PS I or II were treated with total or subtotal nodal irradiation as part of a prospective randomized trial in the Danish National Hodgkin Study during the period 1971-83. They were followed till death or--at the time of this analysis--from 15 to 146 months after initiation of therapy. The initial tumour burden of each patient was assessed, combining tumour size of each involved region and number of regions involved. Tumour burden thus assessed proved to be the single most important prognostic factor with regard to disease free survival. Other known prognostic factors such as number of involved regions, mediastinal size, pathological stage, systemic symptoms, and ESR were related to tumour burden and lost their prognostic significance in a multivariate analysis. The only other factors of independent significance were histologic subtype and, to a lesser extent, sex. Combining tumour burden and histologic subtype made it possible to single out a group of patients with a very poor disease free survival. These patients also had a poorer survival from Hodgkin's disease and thus clearly candidates for additional initial treatment
Dynamic earthquake rupture modelled with an unstructured 3-D spectral element method applied to the 2011 M9 Tohoku earthquake
An important goal of computational seismology is to simulate dynamic earthquake rupture and strong ground motion in realistic models that include crustal heterogeneities and complex fault geometries. To accomplish this, we incorporate dynamic rupture modelling capabilities in a spectral element solver on unstructured meshes, the 3-D open source code SPECFEM3D, and employ state-of-the-art software for the generation of unstructured meshes of hexahedral elements. These tools provide high flexibility in representing fault systems with complex geometries, including faults with branches and non-planar faults. The domain size is extended with progressive mesh coarsening to maintain an accurate resolution of the static field. Our implementation of dynamic rupture does not affect the parallel scalability of the code. We verify our implementation by comparing our results to those of two finite element codes on benchmark problems including branched faults. Finally, we present a preliminary dynamic rupture model of the 2011 M_w 9.0 Tohoku earthquake including a non-planar plate interface with heterogeneous frictional properties and initial stresses. Our simulation reproduces qualitatively the depth-dependent frequency content of the source and the large slip close to the trench observed for this earthquake
Dynamic earthquake rupture modelled with an unstructured 3-D spectral element method applied to the 2011 M9 Tohoku earthquake
An important goal of computational seismology is to simulate dynamic earthquake rupture and strong ground motion in realistic models that include crustal heterogeneities and complex fault geometries. To accomplish this, we incorporate dynamic rupture modelling capabilities in a spectral element solver on unstructured meshes, the 3-D open source code SPECFEM3D, and employ state-of-the-art software for the generation of unstructured meshes of hexahedral elements. These tools provide high flexibility in representing fault systems with complex geometries, including faults with branches and non-planar faults. The domain size is extended with progressive mesh coarsening to maintain an accurate resolution of the static field. Our implementation of dynamic rupture does not affect the parallel scalability of the code. We verify our implementation by comparing our results to those of two finite element codes on benchmark problems including branched faults. Finally, we present a preliminary dynamic rupture model of the 2011 Mw 9.0 Tohoku earthquake including a non-planar plate interface with heterogeneous frictional properties and initial stresses. Our simulation reproduces qualitatively the depth-dependent frequency content of the source and the large slip close to the trench observed for this earthquak
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