1,682 research outputs found
Improved Laboratory Transition Probabilities for Ce II, Application to the Cerium Abundances of the Sun and Five r-process Rich, Metal-Poor Stars, and Rare Earth Lab Data
Recent radiative lifetime measurements accurate to +/- 5% using laser-induced
fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of Ce II have
been combined with new branching fractions measured using a Fourier transform
spectrometer (FTS) to determine transition probabilities for 921 lines of Ce
II. This improved laboratory data set has been used to determine a new solar
photospheric Ce abundance, log epsilon = 1.61 +/- 0.01 (sigma = 0.06 from 45
lines), a value in excellent agreement with the recommended meteoritic
abundance, log epsilon = 1.61 +/- 0.02. Revised Ce abundances have also been
derived for the r-process-rich metal-poor giant stars BD+17 3248, CS 22892-052,
CS 31082-001, HD 115444 and HD 221170. Between 26 and 40 lines were used for
determining the Ce abundance in these five stars, yielding a small statistical
uncertainty of 0.01 dex similar to the Solar result. The relative abundances in
the metal-poor stars of Ce and Eu, a nearly pure r-process element in the Sun,
matches r-process only model predictions for Solar System material. This
consistent match with small scatter over a wide range of stellar metallicities
lends support to these predictions of elemental fractions. A companion paper
includes an interpretation of these new precision abundance results for Ce as
well as new abundance results and interpretations for Pr, Dy and Tm.Comment: 84 pages, 8 Figures, 14 Tables; To appear in the Astrophysical
Journal Supplemen
Quantitative estimates of discrete harmonic measures
A theorem of Bourgain states that the harmonic measure for a domain in
is supported on a set of Hausdorff dimension strictly less than
\cite{Bourgain}. We apply Bourgain's method to the discrete case, i.e., to the
distribution of the first entrance point of a random walk into a subset of , . By refining the argument, we prove that for all \b>0 there
exists \rho (d,\b)N(d,\b), any , and any | \{y\in\Z^d\colon \nu_{A,x}(y)
\geq n^{-\b} \}| \leq n^{\rho(d,\b)}, where denotes the
probability that is the first entrance point of the simple random walk
starting at into . Furthermore, must converge to as \b \to
\infty.Comment: 16 pages, 2 figures. Part (B) of the theorem is ne
The Hitting Times with Taboo for a Random Walk on an Integer Lattice
For a symmetric, homogeneous and irreducible random walk on d-dimensional
integer lattice Z^d, having zero mean and a finite variance of jumps, we study
the passage times (with possible infinite values) determined by the starting
point x, the hitting state y and the taboo state z. We find the probability
that these passages times are finite and analyze the tails of their cumulative
distribution functions. In particular, it turns out that for the random walk on
Z^d, except for a simple (nearest neighbor) random walk on Z, the order of the
tail decrease is specified by dimension d only. In contrast, for a simple
random walk on Z, the asymptotic properties of hitting times with taboo
essentially depend on the mutual location of the points x, y and z. These
problems originated in our recent study of branching random walk on Z^d with a
single source of branching
Atomic Transition Probabilities for Transitions of Si I and Si II and the Silicon Abundances of Several Very Metal-Poor Stars
We report new measurements of branching fractions for 20 UV and blue lines in
the spectrum of neutral silicon (Si I) originating in the 334
P, P and 33 D upper levels. Transitions studied include both strong, nearly pure LS
multiplets as well as very weak spin-forbidden transitions connected to these
upper levels. We also report a new branching fraction measurement of the
P - P intercombination lines in the
spectrum of singly-ionized silicon (Si II). The weak spin-forbidden lines of Si
I and Si II provide a stringent test on recent theoretical calculations, to
which we make comparison. The branching fractions from this study are combined
with previously reported radiative lifetimes to yield transition probabilities
and log()s for these lines. We apply these new measurements to abundance
determinations in five metal-poor stars.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series (26 pages, 6 figures, 5 tables; machine-readable versions of Tables 3
and 4 will be available from the publisher
Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052
First results are reported of a new abundance study of neutron-capture
elements in the ultra-metal-poor (UMP; [Fe/H] = -3.1) halo field giant star CS
22892-052. Using new high resolution, high signal-to-noise spectra, abundances
of more than 30 neutron-capture elements (Z>30) have been determined. Six
elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) have been detected
for the first time in a UMP star. Abundances are also derived for three of the
heaviest stable elements (Os, Ir, and Pb). A second transition of thorium,
Th{4086}, confirms the abundance deduced from the standard Th{4019} line, and
an upper limit to the abundance of uranium is established from the absence of
the U{3859} line. As found in previous studies, the abundances of the heavier
(Z>=56) stable neutron-capture elements in CS 22892-052 match well the scaled
solar system r-process abundance distribution. From the observed Th abundance,
an average age of ~= 16 +/- 4 Gyr is derived for cs22892-052, consistent with
the lower age limit of ~= 11 Gyr derived from the upper limit on the U
abundance. The concordance of scaled solar r-process and CS 22892-052
abundances breaks down for the lighter neutron-capture elements, supporting
previous suggestions that different r-process production sites are responsible
for lighter and heavier neutron-capture elements.Comment: To be published in the Astrophysical Journal Letter
Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars
Using near-ultraviolet spectra obtained with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in
three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17
3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second
r-process peak (A=130) associated with the N=82 neutron shell closure, and it
has not been detected previously in Galactic halo stars. The derived tellurium
abundances match the scaled solar system r-process distribution within the
uncertainties, confirming the predicted second peak r-process residuals. These
results suggest that tellurium is predominantly produced in the main component
of the r-process, along with the rare earth elements.Comment: Accepted for publication in the Astrophysical Journal Letters (5
pages, 2 figures
New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars
We use the Space Telescope Imaging Spectrograph on board the Hubble Space
Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom
wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive
abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been
detected previously in either star. Abundances derived for Ge I, Te I, Os II,
and Pt I confirm those derived from lines at longer wavelengths. We also derive
upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean
[As/Fe] ratio derived from these two stars and five others in the literature is
unchanged over the metallicity range -2.8 = +0.28
+/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two
stars and six others in the literature is also constant, = +0.16 +/-
0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to
a simple extrapolation of the iron-peak abundances to higher masses, suggesting
that this mass region (75 < A < 82) may be the point at which a different
nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich
freezeout of the iron peak. = +0.56 +/- 0.23 in HD 108317 and HD
128279, and we infer that lines of Cu I may not be formed in local
thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and
[Os/Fe] ratios are also derived from neutral and ionized species, and each
ratio pair agrees within the mutual uncertainties, which range from 0.15 to
0.52 dex.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 10
figure
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