1,720,958 research outputs found
DR 1: a WO3 star in IC 1613 and its surrounding nebula, S3
Get PDFWe present an analysis of the WO3 star, DR 1, which is located in the dwarf irregular galaxy IC 1613, and is surrounded by an HII region which shows nebular HeII 4686 A in emission, a rare phenomenon in nebulae surrounding Wolf-Rayet stars. We have derived E(B-V)=0.07 via a comparison of the observed Balmer line ratios to those predicted by theory, using the electron temperature of T_e_=17100K derived from our nebular analysis. We find O/H=4.99x10^-5^ by number for the nebula, in agreement with the O/H ratios found for other emission-line regions in IC 1613. We derive the following nebular mass fractions: X=0.761, Y=0.238 and Z=0.00091. After allowance for the contribution by the nebular continuum, we have derived a stellar absolute magnitude of M_V_=-3.6 for DR 1, a stellar effective temperature of T_*_=75000K via a HI and HeII Zanstra analysis, and a stellar luminosity of 10^6^Lsun_. A terminal wind velocity of vinfinity_=2850km/s is derived for DR 1 from the width of the strongest stellar emission lines. We also performed an abundance analysis of the stellar wind via a recombination theory analysis of the stellar emission-line features, and derive X(C)=0.48, X(O)=0.27 and X(He)=0.25. These values are within the range found for other WO stars by Kingsburgh et al. (1994) and agree with those predicted by the Z=0.004 massive star evolutionary models of Meynet et al. (1994), but not with their Z=0.001 models. Our observations confirm the prediction that WO stars in low-metallicity galaxies should be much more luminous than their counterparts in higher metallicity galaxies
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Distances for galactic planetary nebulae using mean forbidden [O II] doublet ratio electron densities
Get PDFWe present [O II| 3726, 3729-Å doublet ratios and electron densities for 68 galactic planetary nebulae (PN). For 45 of the objects, the doublet ratios represent integrations over the whole of the nebula. Distances have been derived for the majority of the nebulae, using calibrations recently derived from Magellanic Cloud PN. For PN which are optically thin in the hydrogen Lyman continuum, we have derived distances using a variant of the Shklovsky method (constant ionized hydrogen mass) which uses the mean [O II] electron density and the measured radio flux and which does not require knowledge of the filling factor or nebular angular radius. For PN which are optically thick in the Lyman continuum, the constant Hβ flux method was used to derive distances. The typical [O II] density at the transition point between an optically thick and thin nebula is 4500 cm−3. Since the optically thin and thick methods both over-estimate the distance when applied to inappropriate nebulae, the smaller of the two distance estimates is adopted for each nebula. An extensive comparison is made between the distances derived here and previously published distances and distance scales. It is shown that the present distances, based on Magellanic Cloud calibrations, give consistency with independent distance estimates. They also yield much greater self-consistency between central star masses derived from luminosity versus Teff comparisons on the one hand, and from absolute magnitude versus evolutionary age comparisons on the other hand. For the PN in our sample, rms electron densities, filling factors and absolute radii have also been derived. The derived filling factors are found to decrease with increasing absolute angular radius, but we argue that this effect can be attributed entirely to the effects of measurement uncertainties in the adopted angular radii
Elemental abundances for a sample of southern galctic planetary nebulae
Get PDFWe present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with UV data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredgedup primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H II regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O > 0.8. In the current sample, 11 nebulae having N/O > 0.8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H=(4.93±2.22)×10−4 by number for 42 non-Type I PN and O/H=(4.42±1.44)×10−4 for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H II regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN.
The helium abundances derived for the non-Type I PN are in accord with those predicted by Becker&Iben for the first and third dredge-up phases. A comparison between the nitrogen abundances in the PN and the carbon+ nitrogen abundances in galactic H II regions indicates that roughly 36 per cent of the initial carbon is converted into nitrogen in the case of the non-Type I PN, consistent with predictions for the first dredge-up by Becker&Iben. In order to explain the high nitrogen abundances derived for the Type I PN, however, envelope-burning of dredged-up carbon into nitrogen, following the third dredge-up, is definitely required. Total C + N + O abundances are found to be correlated with C/H for the combined nonType I and Type I sample; the carbon has been enhanced by He-burning processed material brought up by the third dredge-up
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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