16,681 research outputs found

    Improved V II log(gfgf) Values, Hyperfine Structure Constants, and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

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    New experimental absolute atomic transition probabilities are reported for 203 lines of V II. Branching fractions are measured from spectra recorded using a Fourier transform spectrometer and an echelle spectrometer. The branching fractions are normalized with radiative lifetime measurements to determine the new transition probabilities. Generally good agreement is found between this work and previously reported V II transition probabilities. Use of two spectrometers, independent radiometric calibration methods, and independent data analysis routines enables a reduction in systematic uncertainties, in particular those due to optical depth errors. In addition, new hyperfine structure constants are measured for selected levels by least squares fitting line profiles in the FTS spectra. The new V II data are applied to high resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to determine new, more accurate V abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. Very good agreement is found between our new solar photospheric V abundance, log {\epsilon}(V) = 3.95 from 15 V II lines, and the solar-system meteoritic value. In HD 84937, we derive [V/H] = -2.08 from 68 lines, leading to a value of [V/Fe] = 0.24.Comment: 32 pages, 7 tables (3 machine-readable), 8 figures; accepted for publication in ApJ

    A Linear Iterative Unfolding Method

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    A frequently faced task in experimental physics is to measure the probability distribution of some quantity. Often this quantity to be measured is smeared by a non-ideal detector response or by some physical process. The procedure of removing this smearing effect from the measured distribution is called unfolding, and is a delicate problem in signal processing, due to the well-known numerical ill behavior of this task. Various methods were invented which, given some assumptions on the initial probability distribution, try to regularize the unfolding problem. Most of these methods definitely introduce bias into the estimate of the initial probability distribution. We propose a linear iterative method, which has the advantage that no assumptions on the initial probability distribution is needed, and the only regularization parameter is the stopping order of the iteration, which can be used to choose the best compromise between the introduced bias and the propagated statistical and systematic errors. The method is consistent: "binwise" convergence to the initial probability distribution is proved in absence of measurement errors under a quite general condition on the response function. This condition holds for practical applications such as convolutions, calorimeter response functions, momentum reconstruction response functions based on tracking in magnetic field etc. In presence of measurement errors, explicit formulae for the propagation of the three important error terms is provided: bias error, statistical error, and systematic error. A trade-off between these three error terms can be used to define an optimal iteration stopping criterion, and the errors can be estimated there. We provide a numerical C library for the implementation of the method, which incorporates automatic statistical error propagation as well.Comment: Proceedings of ACAT-2011 conference (Uxbridge, United Kingdom), 9 pages, 5 figures, changes of corrigendum include

    Nucleosynthesis: Stellar and Solar Abundances and Atomic Data

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    Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy -- the progenitors of the halo stars -- responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis.Comment: 8 pages, 2 figures, 1 table. To appear in the Proceedings of the NASA Laboratory Astrophysics Workshop in Las Vegas, NV (February 2006

    A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. II. Multiwavelength Analyses of Sources in M51

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    We report the analysis of deep radio observations of the interacting galaxy system M51 from the Very Large Array, with the goal of understanding the nature of the population of compact radio sources in nearby spiral galaxies. We detect 107 compact radio sources, 64% of which have optical counterparts in a deep Hα\alpha Hubble Space Telescope image. Thirteen of the radio sources have X-ray counterparts from a {\em Chandra} observation of M51. We find that six of the associated Hα\alpha sources are young supernova remnants with resolved shells. Most of the SNRs exhibit steep radio continuum spectral indices onsistent with synchrotron emission. We detect emission from the Type Ic SN 1994I nearly a decade after explosion: the emission (160±22μ160\pm22 \muJy beam1^{-1} at 20 cm, 46±11μ46\pm11 \muJy beam1^{-1} at 6cm, α=1.02±0.28\alpha=-1.02\pm0.28) is consistent with light curve models for Type Ib/Ic supernovae. We detect X-ray emission from the supernova, however no optical counterpart is present. We report on the analysis of the Seyfert 2 nucleus in this galaxy, including the evidence for bipolar outflows from the central black hole.Comment: 22 pages, 8 figures (5 color) in separate files, AASTeX. Full resolution figures and preprint may be obtained by contacting [email protected]. AJ accepte

    Relativistic many-body calculations of the Stark-induced amplitude of the 6P1/2 -7P1/2 transition in thallium

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    Stark-induced amplitudes for the 6P1/2 - 7P1/2 transition in Tl I are calculated using the relativistic SD approximation in which single and double excitations of Dirac-Hartree-Fock levels are summed to all orders in perturbation theory. Our SD values alpha S = 368 a0 3 and beta S= 298 a 0 3 are in good agreement with the measurements alpha S=377(8) a 0 3$ and beta S = 313(8) a 0 3 by D. DeMille, D. Budker, and E. D. Commins [Phys. Rev. A 50, 4657 (1994)]. Calculations of the Stark shifts in the 6P1/2 - 7P1/2 and 6P1/2 - 7S1/2 transitions are also carried out. The Stark shifts predicted by our calculations agree with the most accurate measured values within the experimental uncertainties for both transitions
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