1,721,139 research outputs found
Evidences of tidal distortion and mass loss from the old open cluster NGC 6791
No full textWe present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We find that the 2D density map shows a clear elongation and an irregular distribution starting from ~ 300'' from the cluster center and two tails extending in opposite directions beyond the tidal radius. These features are aligned to both the absolute proper motion and to the Galactic centre directions. Accordingly we find that both the surface brightness and star count density profiles reveal a departure from a King model starting from ~ 600''. These observational evidences suggest that NGC 6791 is currently undergoing mass-loss likely due to gravitational shocking and interactions with the tidal field of the Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactions and we estimate the initial cluster mass to be Mini = (1.5 - 4.0) × 105 M ⊙
ANOTHER BRICK IN UNDERSTANDING CHEMICAL AND KINEMATICAL PROPERTIES OF BSSs: NGC 6752
Full text linkWe used high-resolution spectra acquired with the multifiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to investigate the chemical and kinematical properties of a sample of 22 blue straggler stars (BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752. We measured radial and rotational velocities and Fe, O, and C abundances. According to radial velocities, metallicity, and proper motions, we identified 18 BSSs as likely cluster members. We found that all the BSSs rotate slowly (less than 40 km s-1), similar to the findings in 47 Tucanae, NGC 6397, and M30. The Fe abundance analysis reveals the presence of three BSSs affected by radiative levitation (showing [Fe/H] significantly higher than that measured in "normal" cluster stars), confirming that element transport mechanisms occur in the photosphere of BSSs hotter than ≃8000 K. Finally, BSS C and O abundances are consistent with those measured in dwarf stars. No C and O depletion ascribable to mass transfer processes has been found on the atmospheres of the studied BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the collisional origin for BSSs in NGC 6752 or that the CO depletion is a transient phenomenon
Multiple stellar populations in the globular cluster M3 (NGC 5272): a Strömgren perspective
No full textWe present Str\"omgren photometry of the Galactic Globular Cluster M3 to
study its multiple generations phenomenon. The use of different
colour-magnitude diagrams and especially of the notoriously efficient c_y index
allowed us to detect a double Red Giant Branch in the cluster CMD. After
decontamination from fore- and background sources, the two sequences turned out
to be equally populated. The two components also show a bimodal radial
distribution well corresponding to that predicted by numerical simulations for
clusters living in an intermediate dynamical evolutive state and with a
population with modified chemical composition that was born more centrally
concentrated than the primordial. The analysis of high-resolution spectra
quantitatively demonstrates that the two detected sequences correspond to the
first (Na-poor) generation and the second (Na-rich) generation, thus confirming
the importance of synergy between photometry and spectroscopy
THE WFPC2 ULTRAVIOLET SURVEY: THE BLUE STRAGGLER POPULATION IN NGC 5824
Full text linkWe have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations, in ultraviolet and optical bands, to study the blue straggler star population of the massive outer halo globular cluster NGC 5824 over its entire radial extent. We have computed the center of the cluster and constructed the radial density profile from detailed star counts. The profile is well reproduced by a Wilson model with a small core (rc ~= 4.''4) and a concentration parameter c ~= 2.74. We also present the first age determination for this cluster. From a comparison with isochrones, we find t = 13 ± 0.5 Gyr. We discuss this result in the context of the observed age-metallicity relation of Galactic globular clusters. A total of 60 bright blue stragglers has been identified. Their radial distribution is found to be bimodal, with a central peak, a well-defined minimum at r ~ 20'', and an upturn at large radii. In the framework of the dynamical clock recently defined by Ferraro et al., this feature suggests that NGC 5824 is a cluster of intermediate dynamical age
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
Slowly cooling white dwarfs in M13 from stable hydrogen burning
Full text linkWhite dwarfs (WDs) are the final evolutionary product of the vast majority of stars in the Universe. They are electron-degenerate structures characterized by no stable thermonuclear activity, and their evolution is generally described as a pure cooling process. Their cooling rate is adopted as cosmic chronometer to constrain the age of several Galactic populations, including the disk, globular and open clusters. By analysing high-resolution photometric data of two very similar Galactic globular clusters (M3 and M13), we find a clear-cut and unexpected overabundance of bright WDs in M13. Theoretical models suggest that, consistent with the horizontal branch morphology, this overabundance is due to a slowing down of the cooling process in ~70% of the WDs in M13, caused by stable thermonuclear burning in their residual hydrogen-rich envelope. The presented observational evidence of quiescent thermonuclear activity occurring in cooling WDs brings new attention on the use of the WD cooling rate as cosmic chronometer for low-metallicity environments.Fil: Chen, Jianxing. Universidad de Bologna; ItaliaFil: Ferraro, Francesco R.. Universidad de Bologna; ItaliaFil: Cadelano, Mario. Astrophysics and Space Science Observatory of Bologna; ItaliaFil: Salaris, Maurizio. Liverpool John Moores University; Reino UnidoFil: Lanzoni, Barbara. Universidad de Bologna; ItaliaFil: Pallanca, Cristina. Universidad de Bologna; ItaliaFil: Althaus, Leandro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Dalessandro, Emanuele. Astrophysics and Space Science Observatory of Bologna; Itali
Modelling the observed stellar mass function and its radial variation in galactic globular clusters
Full text linkWe measure how the slope of the stellar mass function (MF) changes as a
function of clustercentric distance r in five Galactic globular clusters and compare
(r) to predictions from direct N-body star cluster simulations. Theoretical studies
predict that (r) (which traces the degree of mass segregation in a cluster) should
steepen with time as a cluster undergoes two-body relaxation and that the amount by
which the global MF can evolve from its initial state due to stellar escape is directly
linked to (r). We find that the amount of mass segregation in M10, NGC 6218, and
NGC 6981 is consistent with their dynamical ages, but only the global MF of M10 is
consistent with its degree of mass segregation as well. NGC 5466 and NGC 6101 on
the other hand appear to be less segregated than their dynamical ages would indicate.
Furthermore, despite the fact that the escape rate of stars in non-segregated clusters
is independent of stellar mass, both NGC 5466 and NGC 6101 have near-flat MFs. We
discuss various mechanisms which could produce non-segregated clusters with near-
flat MFs, including higher mass-loss rates and black hole retention, but argue that for
some clusters (NGC 5466 and NGC 6101) explaining the present-day properties might
require either a non-universal IMF or a much more complex dynamical history
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