177,582 research outputs found
The luminosity function and stellar mass-to-light ratio of the massive globular cluster NGC 2419★
No full textWe used archival Hubble Space Telescope Wide Field Camera 3 (WFC3) images to obtain the luminosity function of the remote globular cluster NGC 2419 from 2 mag above the horizontal branch level down to ≃ 3.0 mag below the turn-off point (to M I≃ 6.4), approximately covering the range of initial stellar masses The completeness-corrected luminosity function does not display any change of shape over the radial range covered by the WFC3 data, out to ≃ 6 core radii (r c), or, equivalently, to ≃ 2 half-light radii. The luminosity function in this radial range is also identical to that obtained from ground-based data at much larger distances from the cluster centre (), in the magnitude range in which the two distributions overlap (M I≤ 4.0). These results support the conclusion by Dalessandro et al. that there is no significant mass segregation among cluster stars; hence, the stellar mass-to-light ratio remains constant with distance from the cluster centre. We fitted the observed luminosity function with theoretical counterparts with the proper age and metallicity from different sets of stellar evolution models, and we consistently derive a total V-band mass-to-light ratio by extrapolating to the hydrogen-burning limit, with a best-fitting value M/L V= 1.5 ± 0.1. On the other hand, assuming that there are no cluster stars with m≤ 0.3M ⊙, we establish a robust lower limit M/L V > 0.8. These estimates provide useful constraints for dynamical models of the cluster that were forced to consider the stellar mass-to-light ratio as a (nearly) free parameter
SECURITY, SAFETY E POWER QUALITY NEI SISTEMI INDUSTRIALI A MEDIA TENSIONE: POSSIBILI VANTAGGI DELL'ESERCIZIO CON NEUTRO A TERRA CON BOBINA PETERSEN
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Massa molecolare e composizione glicosidica di pectine ed emicellulose di pareti cellulari ottenute da radici di frumento duro sottoposte a stress idrico
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A preliminary report on the Montepiano (Central Italy) landslide stabilization using regrading and drainage.
No full textModelling the observed stellar mass function and its radial variation in galactic globular clusters
Get PDFWe 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
Slowly Cooling White Dwarfs in NGC 6752
Get PDFRecently, a new class of white dwarfs (“slowly cooling WDs”) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with ∼70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches ∼300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster.Fil: Chen, Jianxing. Universidad de Bologna; ItaliaFil: Ferraro, Francesco R.. Universidad de Bologna; ItaliaFil: Cadelano, Mario. Universidad de Bologna; ItaliaFil: Salaris, Maurizio. Istituto Nazionale di Astrofisica; ItaliaFil: 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: Cassisi, Santi. Istituto Nazionale di Astrofisica; ItaliaFil: Dalessandro, Emanuele. Osservatorio di Astrofisica e Scienza dello Spazio di Bologna; Itali
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
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