1,721,156 research outputs found
Pixel lensing observations towards globular cluster
No full textIt has been suggested that a monitoring program employing the pixel lensing method to search for microlensing events towards galactic globular clusters may increase the statistics and discriminate among different halo models. Stimulated by this proposal, we evaluate an upper limit to the pixel lensing event rate for such a survey. Four different dark halo models have been considered changing both the flattening and the slope of the mass density profile. The lens mass function has been modelled as a homogenous power – law for and both the mass limits and the slope of the mass function have been varied to investigate their effect on the rate. The target globular clusters have been selected in order to minimize the disk contribution to the event rate. We find that a pixel lensing survey towards globular clusters is unable to discriminate among different halo models since the number of detectable events is too small to allow any reliable statistical analysis.
New Optical and Near-Infrared Surface Brightness Fluctuation Models: A Primary Distance Indicator Ranging from Globular Clusters to Distant Galaxies?
No full text
New Optical and Near-Infrared Surface Brightness Fluctuation Models. II. Young and Intermediate-Age Stellar Populations
No full textWe present theoretical surface brightness fluctuation (SBF) amplitudes for single-burst stellar populations of young and intermediate age (25 Myr <= t <= 5 Gyr) and metallicities Z = 0.0003, 0.001, 0.004, 0.008, 0.01, 0.02, and 0.04. The fluctuation magnitudes and colors as expected in the Johnson-Cousins (UBVRIJHK) photometric system are provided. We pay attention to the contribution of thermally pulsating asymptotic giant
branch (TP-AGB) stars. The sensitivity of the predicted SBF to changes in the mass-loss scenario along the TP-AGB phase is examined. Below 0.6-1 Gyr both optical and near-IR SBF models exhibit a strong dependence on age and mass loss. We also evaluate SBF amplitudes using Monte Carlo techniques to
reproduce the random variation in the number of stars experiencing bright and fast evolutionary phases (red giant branch, AGB, TP-AGB). On these grounds we provide constraints on the faintest integrated flux of real stellar populations required to derive reliable and meaningful SBF measurements. We analyze a technique for deriving SBF amplitudes of star
clusters from the photometry of individual stars and estimate the
uncertainty due to statistical effects, which may impinge on the
procedure. The first optical SBF measurements for 11 Large Magellanic Cloud (LMC) star-rich clusters - with ages ranging from a few megayears to several gigayears - are derived using Hubble Space Telescope observations.
The measurements are compared to our SBF predictions, providing a good agreement with models of metallicity Z = 0.0001-0.01. Our results suggest that, for TP-AGB stars, a mass loss as a power-law function of the star luminosity is required in order to properly reproduce the optical SBF data
of the LMC clusters. Finally, near-IR models have been compared toavailable data, thus showing that the general trend is well fitted. We suggest how to overcome the general problem of SBF models in reproducing the details of the near-IR SBF measurements of the Magellanic Cloud star clusters
New Optical and Near-Infrared Surface Brightness Fluctuation Models: A Primary Distance Indicator Ranging from Globular Clusters to Distant Galaxies?
No full textA closer look at supernovae as seeds for galactic magnetization
Full text linkContext. Explaining the currently observed magnetic fields in galaxies requires relatively strong seeding in the early Universe. One of the current theories proposes that magnetic seeds on the order of μG were expelled by supernova (SN) explosions after primordial fields of nG strength or weaker were amplified in stellar interiors. Aims. In this work, we take a closer look at this theory and calculate the maximum magnetic energy that can be injected in the interstellar medium by a stellar cluster of mass Mcl based on what is currently known about stellar magnetism. Methods. We consider early-type stars and adopt either a Salpeter or a top-heavy initial mass function. For their magnetic fields, we adopt either a Gaussian or a bimodal distribution. The Gaussian model assumes that all massive stars are magnetized with 103 < B∗ 104 G, while the bimodal, consistent with observations of Milky Way stars, assumes only 5-10% of OB stars have 103 < B∗ 104 G, while the rest have 10 < B∗ 102 G. We ignore the effect of magnetic diffusion and assume no losses of magnetic energy. Results. We find that the maximum magnetic energy that can be injected by a stellar population is between 10-10 and 10-7 times the total SN energy. The highest end of these estimates is about five orders of magnitude lower than what is usually employed in cosmological simulations, where about 10-2 of the SN energy is injected as magnetic. Conclusions. Pure advection of the stellar magnetic field by SN explosions is a good candidate for seeding a dynamo, but not enough to magnetize galaxies. Assuming SNe as the main mechanism for galactic magnetization, the magnetic field cannot exceed an intensity of 10-7 G in the best-case scenario for a population of 105 solar masses in a superbubble of 300 pc radius, while more typical values are between 10-10 and 10-9 G. Therefore, other scenarios for galactic magnetization at high redshift need to be explored
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
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