Galactic Cosmochronometry from Radioactive Elements in the Spectra of Very Old Metal-Poor Stars

In a short review of neutron-capture elemental abundances in Galactic halo stars, emphasis is placed on the use of these elements to estimate the age of the Galactic halo. Two prominent characteristics of neutron-capture elements in halo stars are their large star-to-star scatter in the overall abundance level with respect to lighter elements, and the dominance of r-process abundance patterns at lowest stellar metallicities. The r-process abundance signature potentially allows the direct determination of the age of the earliest Galactic halo nucleosynthesis events, but further developments in r-process theory, high resolution spectroscopy of very metal-poor stars, and in basic atomic data are needed to narrow the uncertainties in age estimates. Attention is brought to the importance of accurate transition probabilities in neutron-capture element cosmochronometry. Recent progress in the transition probabilities of rare earth elements is discussed, along with suggestions for future work on other species.Comment: 19 pages, 5 figures; To appear in Physica Script

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

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

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 H$\alpha$ 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 H$\alpha$ 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 ($160\pm22 \mu$Jy beam$^{-1}$ at 20 cm, $46\pm11 \mu$Jy beam$^{-1}$ at 6cm, $\alpha=-1.02\pm0.28$) 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

Number development and children with Specific Language Impairment.

The aims of this chapter is to investigate whether the number skills of children with specific language impairment (SLI) differ from those of their typically developing peers, matched in nonverbal reasoning, and a group of younger typically developing children matched on language comprehension. It assesses whether small number quantification accuracy accounts for additional variation in number tasks beyond the other influences. How children develop competence with numbers and why they differ so much in their progress are important questions whether one is concerned with numeracy, the skills and knowledge for dealing with numerical information in everyday life, or mathematics, the sciences dealing with the logic of quantity, shape, and arrangement. The study of number development in children with SLI has the potential to contribute both to the understanding of the factors that influence children's progress generally and to the knowledge base for professionals working with these children. Previous work on number in children with SLI had indicated selective impairments: children with SLI showed impaired procedural skills, particularly in counting, from an early age but less impaired understanding of number, for example counting principles. An investigation of children with SLI provided ample evidence of the continuing deficit in counting and calculation