27 research outputs found
The barium isotopic mixture for the metal-poor subgiant star HD140283
Current theory regarding heavy element nucleosynthesis in metal-poor
environments states that the r-process would be dominant. The star HD140283 has
been the subject of debate after it appeared in some studies to be dominated by
the s-process. We provide an independent measure of the Ba isotope mixture in
HD140283 using an extremely high quality spectrum and an extensive chi^2
analysis. We exploit hyperfine splitting of the BaII 4554 \AA\ and 4934 \AA\
resonance lines in an effort to constrain the isotope ratio in 1D LTE. Using
the code ATLAS in conjunction with KURUCZ06 model atmospheres we analyse 93 Fe
lines to determine the star's macroturbulence. With this information we
construct a grid of Ba synthetic spectra and, using a \chi^2 code, fit these to
our observed data to determine the isotopic ratio, fodd, which represents the
ratio of odd to even isotopes. We also analyse the Eu lines. We set a new upper
limit of the rotation of HD140283 at vsin{i}\leq3.9\kms, a new upper limit on
[Eu/H] < -2.80 and abundances [Fe/H] = -2.59\pm0.09, [Ba/H] = -3.46\pm0.11.
This leads to a new lower limit on [Ba/Eu] > -0.66. We find that, in the
framework of a 1D LTE analysis, the isotopic ratios of Ba in HD140283 indicate
fodd=0.02\pm0.06, a purely s-process signature. This implies that observations
and analysis do not validate currently accepted theory. We speculate that a 1D
code, due to simplifying assumptions, is not adequate when dealing with
observations with high levels of resolution and S/N because of the turbulent
motions associated with a 3D stellar atmosphere. New approaches to analysing
isotopic ratios, in particular 3D hydrodynamics, need to be considered when
dealing with the levels of detail required to properly determine them. However
published 3D results exacerbate the disagreement between theory and
observation.Comment: 16 pages, 10 figures, 7 tables, 1 online appendix Accepted by A&
Two chemically similar stellar overdensities on opposite sides of the plane of the Galaxy
Our Galaxy is thought to have undergone an active evolutionary history
dominated by star formation, the accretion of cold gas, and, in particular,
mergers up to 10 gigayear ago. The stellar halo reveals rich fossil evidence of
these interactions in the form of stellar streams, substructures, and
chemically distinct stellar components. The impact of dwarf galaxy mergers on
the content and morphology of the Galactic disk is still being explored. Recent
studies have identified kinematically distinct stellar substructures and moving
groups, which may have extragalactic origin. However, there is mounting
evidence that stellar overdensities at the outer disk/halo interface could have
been caused by the interaction of a dwarf galaxy with the disk. Here we report
detailed spectroscopic analysis of 14 stars drawn from two stellar
overdensities, each lying about 5 kiloparsecs above and below the Galactic
plane - locations suggestive of association with the stellar halo. However, we
find that the chemical compositions of these stars are almost identical, both
within and between these groups, and closely match the abundance patterns of
the Milky Way disk stars. This study hence provides compelling evidence that
these stars originate from the disk and the overdensities they are part of were
created by tidal interactions of the disk with passing or merging dwarf
galaxies.Comment: accepted for publication in Natur
Isotope shift calculations for atoms with one valence electron
This work presents a method for the ab initio calculation of isotope shift in
atoms and ions with one valence electron above closed shells. As a zero
approximation we use relativistic Hartree-Fock and then calculate correlation
corrections. The main motivation for developing the method comes from the need
to analyse whether different isotope abundances in early universe can
contribute to the observed anomalies in quasar absorption spectra. The current
best explanation for these anomalies is the assumption that the fine structure
constant, alpha, was smaller at early epoch. We test the isotope shift method
by comparing the calculated and experimental isotope shift for the alkali and
alkali-like atoms Na, MgII, K, CaII and BaII. The agreement is found to be
good. We then calculate the isotope shift for some astronomically relevant
transitions in SiII and SiIV, MgII, ZnII and GeII.Comment: 11 page
Measurement of isotope shift in Eu II
The isotope shift between singly-charged ^Eu and ^Eu in the
4f^7(^8S^o)6s^9S_4-4f^7(^8S^o)6p_1/2>(J=4) transition at 4129 A has been
measured using fast ion beam-laser technique. This Eu line has attracted
interest in connection with efforts of obtaining a cosmochronometer based on
observed Th/Eu abundance ratios. Knowledge of the isotope shift is of
importance in order to check that contaminations from line blends do not
contribute to the line intensity of Eu II. The measured value of the isotope
shift -0.1527(2) cm-1 (= -4578 MHz) is consistent with the old spectroscopic
value of Krebs and Winkler -0.1503(25) cm-1 using a Fabry Perot interferometer,
while the accuracy is improved substantially.Comment: 12 pages, in press for Physica Scripta, in swete
Scattering in flatland: Efficient representations via wave atoms
This paper presents a numerical compression strategy for the boundary
integral equation of acoustic scattering in two dimensions. These equations
have oscillatory kernels that we represent in a basis of wave atoms, and
compress by thresholding the small coefficients to zero. This phenomenon was
perhaps first observed in 1993 by Bradie, Coifman, and Grossman, in the context
of local Fourier bases \cite{BCG}. Their results have since then been extended
in various ways. The purpose of this paper is to bridge a theoretical gap and
prove that a well-chosen fixed expansion, the nonstandard wave atom form,
provides a compression of the acoustic single and double layer potentials with
wave number as -by- matrices with
nonnegligible entries, with a constant that depends on the relative
accuracy \eps in an acceptable way. The argument assumes smooth, separated,
and not necessarily convex scatterers in two dimensions. The essential features
of wave atoms that enable to write this result as a theorem is a sharp
time-frequency localization that wavelet packets do not obey, and a parabolic
scaling wavelength (essential diameter). Numerical experiments
support the estimate and show that this wave atom representation may be of
interest for applications where the same scattering problem needs to be solved
for many boundary conditions, for example, the computation of radar cross
sections.Comment: 39 page
The Hamburg/ESO R-process Enhanced Star survey (HERES) III. HE 0338-3945 and the formation of the r+s stars
We have derived abundances of 33 elements and upper limits for 6 additional
elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE 0338-3945 from
high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor
of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ..). It
is also heavily enriched in Eu, which is generally considered an r-element, and
in other similar elements. It is less enriched, by about a factor of 10, in the
lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a
somewhat lesser degree, N and O. These abundance estimates are subject to
severe uncertainties due to NLTE and thermal inhomogeneities which are not
taken into detailed consideration. However, an interesting result, which is
most probably robust in spite of these uncertainties, emerges: the abundances
derived for this star are very similar to those of other stars with an overall
enhancement of all elements beyond the iron peak.
We have defined criteria for this class of stars, r+s stars, and discuss nine
different scenarios to explain their origin. None of these explanations is
found to be entirely convincing. The most plausible hypotheses involve a binary
system in which the primary component goes through its giant branch and
asymptotic giant branch phases and produces CNO and s-elements which are dumped
onto the observed star. Whether the r-element Eu is produced by supernovae
before the star was formed (perhaps triggering the formation of a low-mass
binary), by a companion as it explodes as a supernova (possibly triggered by
mass transfer), or whether it is possibly produced in a high-neutron-density
version of the s-process is still unclear. Several suggestions are made on how
to clarify this situation.Comment: Accepted for A&A; 22 pages, 9 figures, 2 tables. Table 2 is in
electronic form and available at http://www.astro.uu.se/~karin/table2.dat
with description at http://www.astro.uu.se/~karin/jonsellReadMe
Efficient transform coding of two-channel audio signals by means of complex-valued stereo prediction
Traditional MDCT-based perceptual audio coding schemes employ mid/side and intensity stereo techniques to allow efficient joint coding of the two channels of a stereophonic signal. These techniques, however, provide only little coding gain for critical stereo signals characterized by spectral components with a distinct level or phase difference between the channels. To overcome this deficiency, we propose an extension to the mid/side coding paradigm that utilizes complex-valued inter-channel linear prediction in the MDCT spectral domain. The required imaginary spectrum (MDST) is calculated in a computationally efficient manner without additional algorithmic delay. A formal listening test conducted in the course of the ISO/MPEG standardization of the unified speech and audio codec USAC illustrates that the proposed stereo prediction approach provides significant improvements in coding efficiency and shows that at 96 kb/s, excellent quality can be obtained even for critical signals