1,720,993 research outputs found
Observational nuclear astrophysics: neutron-capture element abundances in old, metal-poor stars
Full text linkThe chemical abundances of metal-poor stars provide a great deal of information regarding the individual nucleosynthetic processes that created the observed elements and the overall process of chemical enrichment of the galaxy since the formation of the first stars. Here we review the abundance patterns of the neutron-capture elements (Z ≥ 38) in those metal-poor stars and our current understanding of the conditions and sites of their production at early times. We also review the relative contributions of these different processes to the build-up of these elements within the galaxy over time, and outline outstanding questions and uncertainties that complicate the interpretation of the abundance patterns observed in metal-poor stars. It is anticipated that future observations of large samples of metal-poor stars will help discriminate between different proposed neutron-capture element production sites and better trace the chemical evolution of the galaxy
Zirconium, barium, lanthanum, and europium abundances in open clusters
No full textWe present an analysis of the s-process elements Zr, Ba, and La and the r-process element Eu in a sample of 50 stars in 19 open clusters. Stellar abundances of each element are based on measures of a minimum of two lines per species via both equivalent width and spectrum synthesis techniques. We investigate cluster mean neutron-capture abundance trends as a function of cluster age and location in the Milky Way disk and compare them to results found in other studies in the literature. We find a statistically significant trend of increasing cluster [Ba/Fe] as a function of decreasing cluster age, in agreement with recent findings for other open cluster samples, supporting the increased importance of low-mass asymptotic giant branch stars to the generation of s-process elements. However, the other s-process elements, [La/Fe] and [Zr/Fe], do not show similar dependences, in contrast to theoretical expectations and the limited observational data from other studies. Conversely, cluster [Eu/Fe] ratios show a slight increase with increasing cluster age, although with marginal statistical significance. Ratios of [s/r]-process abundances, [Ba/Eu] and [La/Eu], however, show more clearly the increasing efficiency of s-process relative to r-process enrichment in open cluster chemical evolution, with significant increases among younger clusters. Last, cluster neutron-capture element abundances appear to be independent of Galactocentric distance. We conclude that a homogeneous analysis of a larger sample of open clusters is needed to resolve the apparent discrepant conclusions between different studies regarding s-process element abundance trends with age to better inform models of galactic chemical evolution.National Science Foundation (U.S.). Astronomy and Astrophysics Postdoctoral Fellowship (Award AST-0901919
CD −24°17504 REVISITED: A NEW COMPREHENSIVE ELEMENT ABUNDANCE ANALYSIS
Full text linkWith [Fe/H] ~ −3.3, CD −24°17504 is a canonical metal-poor main-sequence turn-off star. Though it has appeared in numerous literature studies, the most comprehensive abundance analysis for the star based on high-resolution, high signal-to-noise ratio (S/N) spectra is nearly 15 years old. We present a new detailed abundance analysis for 21 elements based on combined archival Keck-HIRES and Very Large Telescope-UVES spectra of the star that is higher in both spectral resolution and S/N than previous data. Our results are very similar to those of an earlier comprehensive study of the star, but we present for the first time a carbon abundance from the CH G-band feature as well as improved upper limits for neutron-capture species such as Y, Ba, and Eu. In particular, we find that CD −24°17504 has [Fe/H] = −3.41, [C/Fe] = +1.10, [Sr/H] = −4.68, and [Ba/H] ≤ −4.46, making it a carbon-enhanced metal-poor star with neutron-capture element abundances among the lowest measured in Milky Way halo stars.National Science Foundation (U.S.) (Grant AST-1255160
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
DERIVING STELLAR EFFECTIVE TEMPERATURES OF METAL-POOR STARS WITH THE EXCITATION POTENTIAL METHOD
Full text linkIt is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures ("excitation temperatures") of metal-poor stars based on a literature sample with –3.3 < [Fe/H] < –2.5. Excitation temperatures were determined from Fe I line abundances in high-resolution optical spectra in the wavelength range of ~3700-~7000 Å, although shorter wavelength ranges, up to 4750-6800 Å, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it brings the excitation temperatures in good agreement with photometrically derived values. The modified temperature also influences other stellar parameters, as the Fe I-Fe II ionization balance is simultaneously used to determine the surface gravity, while also forcing no abundance trend on the absorption line strengths to obtain the microturbulent velocity. As a result of increasing the temperature, the often too low gravities and too high microturbulent velocities in red giants become higher and lower, respectively. Our adjustment scheme thus continues to build on the advantage of deriving temperatures from spectroscopy alone, independent of reddening, while at the same time producing stellar chemical abundances that are more straightforwardly comparable to studies based on photometrically derived temperatures. Hence, our method may prove beneficial for comparing different studies in the literature as well as the many high-resolution stellar spectroscopic surveys that are or will be carried out in the next few years.Australian Research Council (Laureate Fellowship 0992131)Massachusetts Institute of Technology. Undergraduate Research Opportunities ProgramAustralia. Department of Education (Australian Prime Minister’s Endeavour Award Research Fellowship
NGC 7789: AN OPEN CLUSTER CASE STUDY
No full textWe have obtained high-resolution spectra of 32 giants in the open cluster NGC 7789 using the Wisconsin–Indiana–Yale–NOAO Hydra spectrograph. We explore differences in atmospheric parameters and elemental abundances caused by the use of the linelist developed for the Gaia-ESO Survey (GES) compared to one based on Arcturus used in our previous work. [Fe/H] values decrease when using the GES linelist instead of the Arcturus-based linelist; these differences are probably driven by systematically lower (~−0.1 dex) GES surface gravities. Using the GES linelist we determine abundances for 10 elements—Fe, Mg, Si, Ca, Ti, Na, Ni, Zr, Ba, and La. We find the clusterʼs average metallicity [Fe/H] = 0.03 ± 0.07 dex, in good agreement with literature values, and a lower [Mg/Fe] abundance than has been reported before for this cluster (0.11 ± 0.05 dex). We also find the neutron-capture element barium to be highly enhanced—[Ba/Fe] = +0.48 ± 0.08—and disparate from cluster measurements of neutron-capture elements La and Zr (−0.08 ± 0.05 and 0.08 ± 0.08, respectively). This is in accordance with recent discoveries of supersolar Ba enhancement in young clusters along with more modest enhancement of other neutron-capture elements formed in similar environments.United States. National Aeronautics and Space AdministrationNational Science Foundation (U.S.
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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