804 research outputs found
The "UV-route" to search for Blue Straggler Stars in Globular Clusters: first results from the HST UV Legacy Survey
We used data from the HST UV Legacy Survey of Galactic Globular Clusters to
select the Blue Straggler Star (BSS) population in four intermediate/high
density systems (namely NGC 2808, NGC 6388, NGC 6541 and NGC 7078) through a
"UV-guided search". This procedure consists in using the F275W images in each
cluster to construct the master list of detected sources, and then force it to
the images acquired in the other filters. Such an approach optimizes the
detection of relatively hot stars and allows the detection of complete sample
of BSSs even in the central region of high-density clusters, because the light
from the bright cool giants, which dominates the optical emission in old
stellar systems, is sensibly reduced at UV wavelengths. Our UV-guided
selections of BSSs have been compared to the samples obtained in previous,
optical-driven surveys, clearly demonstrating the efficiency of the UV
approach. In each cluster we also measured the parameter A+, defined as the
area enclosed between the cumulative radial distribution of BSSs and that of a
reference population, which traces the level of BSS central segregation and the
level of dynamical evolution suffered by the system. The values measured for
the four clusters studied in this paper nicely fall along the dynamical
sequence recently presented for a sample of 25 clusters.Comment: 30 pages, 9 figures, 4 tables; accepted for publication in Ap
The optical companion to the binary millisecond pulsar J1824-2452H in the globular cluster M28
We report on the optical identification of the companion star to the
eclipsing millisecond pulsar PSR J1824-2452H in the galactic globular cluster
M28 (NGC 6626). This star is at only 0.2" from the nominal position of the
pulsar and it shows optical variability (~ 0.25 mag) that nicely correlates
with the pulsar orbital period. It is located on the blue side of the cluster
main sequence, ~1.5 mag fainter than the turn-off point. The observed light
curve shows two distinct and asymmetric minima, suggesting that the companion
star is suffering tidal distortion from the pulsar. This discovery increases
the number of non-degenerate MSP companions optically identified so far in
globular clusters (4 out of 7), suggesting that these systems could be a common
outcome of the pulsar recycling process, at least in dense environments where
they can be originated by exchange interactions.Comment: accepted for publication on ApJ, 17 pages, 5 figure
Equilibrium Cross Section of River Channels With Cohesive Erodible Banks
Predicting the equilibrium cross section of natural rivers has been widely investigated in fluvial morphology. Several approaches have been developed to meet this aim, starting from regime equations to the empirical formulations of Parker et al. (2007) and Wilkerson and Parker (2011), who proposed quasi-universal relations for describing bankfull conditions in sand and gravel bed rivers. Nevertheless, a general physics-based framework is still missing, and it remains an open issue to better clarify the basic mechanisms whereby a river selects its width. In this contribution we focus our attention on lowland rivers with cohesive banks, whose resistance to erosion is crucial to control the river width. In particular, we formulate a theoretical model that evaluates the equilibrium width of river cross sections modeling the interaction between the core flow in the central part of the section and the boundary layer that forms in the vicinity of the cohesive banks. The model computes the cross-section equilibrium configuration by which the shear stresses on the banks equal a critical threshold value. These stresses are computed by partitioning the total shear stress into an effective grain roughness component and a form component (Kean and Smith, 2006a). The model is applied to a large data set, concerning both sand and gravel bed rivers, and it is used to determine the relations expressing the channel width and the bankfull flow depth to the bankfull discharge, which appear to provide a unitary description of bankfull hydraulic geometry
A Comparison between Non-Localized Post-Activation Performance Enhancements Following Resistance Exercise for the Upper and the Lower Body
The aim of the present investigation was to compare the acute non-localized post-activation performance enhancement (PAPE) of an exercise protocol involving either the upper or the lower body muscles. Twenty-four resistance trained men participated in the present study and were randomly assigned to an upper body (UB) or to a lower body (LB) group. Both groups tested for upper and lower body power (bench press throw (BPT) and countermovement jump power (CMJP) tests). Participants in the UB group were tested pre and post a high-intensity (HI) and a high-power (POW) bench press protocol while participants in the LB group performed a HI squat and a jump session (POW). A significant group × time interaction was found for CMJP in HI (p = 0.012). Post hoc tests revealed that CMJP was elevated in UB group only (+1.6%; p = 0.025). No other significant interactions were detected. Results of this study indicate that a non-localized PAPE on the lower body may be induced by a HI bench press protocol while a HI squat protocol may not increase upper body power. In particular, the squat protocol performed in the present study (5 sets of 1 rep) may be too demanding to produce a non-localized PAPE
Radio Timing and Optical Photometry of the Black Widow System PSR J1953+1846A in the Globular Cluster M71
We report on the determination of the astrometric, spin and orbital
parameters for PSR J1953+1846A, a "black widow" binary millisecond pulsar in
the globular cluster M71. By using the accurate position and orbital parameters
obtained from radio timing, we identified the optical companion in ACS/Hubble
Space Telescope images. It turns out to be a faint (m_F606W>=24, m_F814W>=23)
and variable star located at only ~0.06" from the pulsar timing position. The
light curve shows a maximum at the pulsar inferior conjunction and a minimum at
the pulsar superior conjunction, thus confirming the association with the
system. The shape of the optical modulation suggests that the companion star is
heated, likely by the pulsar wind. The comparison with the X-ray light curve
possibly suggests the presence of an intra-binary shock due to the interaction
between the pulsar wind and the material released by the companion. This is the
second identification (after COM-M5C) of an optical companion to a black widow
pulsar in a globular cluster. Interestingly, the two companions show a similar
light curve and share the same position in the color magnitude diagram.Comment: Accepted for publication by ApJ; 33 Pages, 10 Figures, 3 Table
First evidence of fully spatially mixed first and second generations in globular clusters: the case of NGC 6362
We present the first evidence of multiple populations in the Galactic
globular cluster NGC 6362. We used optical and near-UV Hubble Space Telescope
and ground based photometry, finding that both the sub giant and red giant
branches are split in two parallel sequences in all color magnitude diagrams
where the F336W filter (or U band) is used. This cluster is one of the least
massive globulars (M_tot~5x10^4 M_sun) where multiple populations have been
detected so far. Even more interestingly and at odds with any previous finding,
we observe that the two identified populations share the same radial
distribution all over the cluster extension. NGC 6362 is the first system where
stars from different populations are found to be completely spatially mixed.
Based on N-body and hydrodynamical simulations of multiple stellar generations,
we argue that, to reproduce these findings, NGC 6362 should have lost up to the
80% of its original massComment: Accepted for publication by ApJ Letters; 6 pages, 5 figure
Ultra-deep GEMINI near-infrared observations of the bulge globular cluster NGC 6624
We used ultra-deep and images secured with the near-infrared GSAOI
camera assisted by the multi-conjugate adaptive optics system GeMS at the
GEMINI South Telescope in Chile, to obtain a (, ) color-magnitude
diagram (CMD) for the bulge globular cluster NGC 6624. We obtained the deepest
and most accurate near-infrared CMD from the ground for this cluster, by
reaching 21.5, approximately 8 magnitudes below the horizontal
branch level. The entire extension of the Main Sequence (MS) is nicely sampled
and at 20 we detected the so-called MS "knee" in a purely
near-infrared CMD. By taking advantage of the exquisite quality of the data, we
estimated the absolute age of NGC 6624 ( = 12.0 0.5 Gyr), which
turns out to be in good agreement with previous studies in the literature. We
also analyzed the luminosity and mass functions of MS stars down to M
0.45 M finding evidence of a significant increase of low-mass stars
at increasing distances from the cluster center. This is a clear signature of
mass segregation, confirming that NGC 6624 is in an advanced stage of dynamical
evolution.Comment: Accepted for publication by ApJ. 39 pages, 19 figures, 1 tabl
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