1,721,145 research outputs found
Chemical evolution of damped Lyα systems
No full textHigh redshift DLA systems suggest that the relative abundances of elements might be roughly solar, although with absolute abundances of more than two orders of magnitude below solar. The result comes from observations of the [SII/ZnII] ratio, which is a reliable diagnostic of the true abundance, and from DLA absorbers with small dust depletion and negligible HII contamination. In particular, in two DLA systems nitrogen is detected and at remarkably high levels (Vladilo et al. 1995 (Proc. ESO Workshop on QSO absorption lines, ed. G. Meylan, Springer Verlag, 103 ), Molaro et al. (1996A&A...308....1M), Green et al. 1995 (Proc. ESO Workshop on QSO absorption lines, ed. G. Meylan, Springer Verlag, 85), Kulkarni et al. (1996MNRAS.279..197K)). Here we compare the predictions from chemical evolution models of galaxies of different morphological type with the abundances and abundance ratios derived for such systems. We conclude that solar ratios and relatively high nitrogen abundances can be obtained in the framework of a chemical evolution model assuming short but intense bursts of star formation, which in turn trigger enriched galactic winds, and a primary origin for nitrogen in massive stars. Such a model is the most successful in describing the chemical abundances of dwarf irregular galaxies and in particular of the peculiar galaxy IZw18. Thus, solar ratios at very low absolute abundances, if confirmed, seem to favour dwarf galaxies rather than spirals as the progenitors of at least some of the DLA systems
7Li evolution in the thin and thick discs of the Milky Way
Full text linkRecent detection of the isotope 7Be (which decays into 7Li) in the outbursts of classical novae confirms the suggestion made in the 1970s that novae could make 7Li. We reconsidered the role of novae as producers of 7Li by means of a detailed model of the chemical evolution of the Milky Way. We showed that novae could be the Galactic 7Li source accounting for the observed increase of Li abundances in the thin disc with stellar metallicity. The best agreement with the upper envelope of the observed Li abundances is obtained for a delay time of ≈1 Gyr for nova production and an effective 7Li yield of 1.8(± 0.6)×10−5 M⊙ over the whole nova lifetime. Lithium in halo stars is depleted by ≈0.35 dex, assuming the pristine abundance from standard big-bang nucleosynthesis. We elaborate a model that matches the pristine stellar abundances, assuming that all stars are depleted by 0.35 dex. In this case, the delay remains the same, but the Li yields are doubled compared with the previous case. This model also has the merit of matching the Li abundance in meteorites and young T Tauri stars. A thick disc model, adopting the parameters derived for the thin disc, can also explain the absence of an increase of Li abundance in its stars. The thick disc is old, but formed and evolved in a time shorter than that required by novae to contribute significantly to 7Li. Therefore, no 7Li enhancement is expected in thick disc stars. We show that the almost constant Li abundance in the thick disc results from the compensation of stellar astration by spallation processes
Pre-MS depletion, accretion and primordial 7Li
Full text linkWe reconsider the role of pre-main sequence (pre-MS) Li depletion on the basis of new observational and theoretical evidence: i) new observations of Halpha emissions in young clusters show that mass accretion could be continuing till the first stages of the MS, ii) theoretical implications from helioseismology suggest large overshooting values below the bottom of the convective envelopes. We argue here that a significant pre-MS 7Li destruction, caused by efficient overshoot mixing, could be followed by a matter accretion after 7Li depletion has ceased on MS thus restoring Li almost to the pristine value. As a test case we show that a halo dwarf of 0.85 Msun with an extended overshooting envelope starting with an initial abundance of A(Li) = 2.74 would burn Li completely, but an accretion rate of the type
1 × 10-8 exp{-t/3× 10^6} Msun yr-1 would restore Li to end with an A(Li) = 2.31. A self-regulating process is required to produce similar final values in a range of different stellar masses to explain the PopII Spite plateau. However, this framework could explain why open cluster stars have lower Li abundances than the pre-solar nebula , the absence of Li in the most metal poor dwarfs and a number of other features which lack of a satisfactory explanation
Lithium and Beryllium in the Gaia-Enceladus Galaxy
Full text linkData from Gaia DR2 and APOGEE surveys revealed a relatively new component in
the inner Galactic halo, which is likely the dynamical remnant of a disrupted
dwarf galaxy named Gaia-Enceladus that collided with the Milky Way about 10
Gyrs ago. This merging event offers an extraordinary opportunity to study
chemical abundances of elements in a dwarf galaxy, since they are generally
hampered in external galaxies. Here, we focus on Li and Be in dwarf stars which
are out of reach even in Local Group galaxies.Searching in GALAH, Gaia-ESO
survey and in literature, we found several existing 7Li abundance
determinations of stars belonging to the Gaia-Enceladus galaxy. The 7Li
abundances of stars at the low metallicity end overlap with those of the
Galactic halo.These are effective extragalactic 7Li measurements, which suggest
that the {\it Spite plateau} is universal, as is the cosmological Li problem.
We found a Li-rich giant out of 101 stars, which suggests a small percentage
similar to that of the Milky Way. We also collect 9Be abundances for a
subsample of 25 Gaia-Enceladus stars from literature. Their abundances share
the Galactic [Be/H] values at the low metallicity end but grow slower with
[Fe/H] and show a reduced dispersion. This suggests that the scatter observed
in the Milky Way could reflect the different \beix\ evolution patterns of
different stellar components which are mixed-up in the Galactic halo
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
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