4,568 research outputs found
The Unimodal Distribution Of Blue Straggler Stars in M75 (NGC 6864)
We have used a combination of multiband high-resolution and wide-field
ground-based observations to image the Galactic globular cluster M75 (NGC
6864). The extensive photometric sample covers the entire cluster extension,
from the very central regions out to the tidal radius, allowing us to determine
the center of gravity and to construct the most extended star density profile
ever published for this cluster. We also present the first detailed star counts
in the very inner regions. The star density profile is well re-produced by a
standard King model with core radius r_c ~ 5.4" and intermediate-high
concentration c ~ 1.75. The present paper presents a detailed study of the BSS
population and its radial distribution. A total number of 62 bright BSSs (with
m_F255W < 21, corresponding to m_F555W < 20) has been identified, and they have
been found to be highly segregated in the cluster core. No significant upturn
in the BSS frequency has been observed in the outskirts of M75, in contrast to
several other clusters studied with the same technique. This observational fact
is quite similar to what has been found in M79 (NGC 1904) by Lanzoni et al.
(2007a). Indeed the BSS radial distributions in the two clusters is
qualitatively very similar, even if in M75 the relative BSS frequency seems to
decrease significantly faster than in M79: indeed it decreases by a factor of 5
(from 3.4 to 0.7) within 1 r_c. Such evidence indicate that the vast majority
of the cluster heavy stars (binaries) have already sunk to the core.Comment: ApJ accepted, 10 pages, 11 figures, 2 table
Electronic Hong-Ou-Mandel interferometry in two-dimensional topological insulators
The edge states of a two-dimensional topological insulator are characterized
by their helicity, a very remarkable property which is related to the
time-reversal symmetry and the topology of the underlying system. We
theoretically investigate a Hong-Ou-Mandel like setup as a tool to probe it.
Collisions of two electrons with the same spin show a Pauli dip, analogous to
the one obtained in the integer quantum Hall case. Moreover, the collisions
between electrons of opposite spin also lead to a dip, known as
dip, which is a direct consequence of the constraints imposed
by time-reversal symmetry. In contrast to the integer quantum Hall case, the
visibility of these dips is reduced by the presence of the additional edge
channels, and crucially depends on the properties of the quantum point contact.
As a unique feature of this system, we show the possibility of three-electron
interference, which leads to a total suppression of the noise independently of
the point contact configuration. This is assured by the peculiar interplay
between Fermi statistics and topology. This work intends to extend the domain
of applicability of electron quantum optics.Comment: 12 pages, 7 figure
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
Blue Straggler Stars: The Spectacular Population in M80
Using HST-WFPC2 observations in two ultraviolet (UV) filters (F225W and
F336W) of the central region of the high density Galactic Globular cluster
(GGC) M80 we have identified 305 Blue Straggler Stars (BSS) which represents
the largest and most concentrated population of BSS ever observed in a GGC. We
also identify the largest, clean sample of evolved BSS yet found. The high
stellar density alone cannot explain the BSS, and we suggest that in M80 we are
witnessing a transient dynamical state, during which stellar interactions are
delaying the core-collapse process leading to an exceptionally large population
of collisional-BSS.Comment: 15 pages, 5 figures, Astrophysical Journal Letters, in pres
UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams
We present GALEX data for 44 Galactic globular clusters obtained during 3
GALEX observing cycles between 2004 and 2008. This is the largest homogeneous
data set on the UV photometric properties of Galactic globular clusters ever
collected. The sample selection and photometric analysis are discussed, and
color-magnitude diagrams are presented. The blue and intermediate-blue
horizontal branch is the dominant feature of the UV color-magnitude diagrams of
old Galactic globular clusters. Our sample is large enough to display the
remarkable variety of horizontal branch shapes found in old stellar
populations. Other stellar types that are obviously detected are blue
stragglers and post core-He burning stars. The main features of UV
color-magnitude diagrams of Galactic globular clusters are briefly discussed.
We establish the locus of post-core He burning stars in the UV color-magnitude
diagram and present a catalog of candidate AGB-manqu \'e, post early-AGB, and
post-AGB stars within our cluster sample.Comment: Accepted for publication by The Astronomical Journal. 46 pages,
including 21 Figures and 3 tables. All data will be made publicly available
by the time the article is published. In the meantime, please contact the
authors for data requests. Revised version fixed error with figure numbers
and caption
Horizontal-Branch Models and the Second-Parameter Effect. IV. The Case of M3 and Palomar 3
We present a detailed analysis of the "second-parameter pair" of globular
clusters M3 (NGC 5272) and Palomar 3. Our main results can be summarized as
follows: i) The horizontal-branch (HB) morphology of M3 is significantly bluer
in its inner regions (observed with the Hubble Space Telescope) than in the
cluster outskirts (observed from the ground), i.e., M3 has an internal second
parameter. Most plausibly the mass loss on the red giant branch (RGB) has been
more efficient in the inner than in the outer regions of the cluster. ii) The
dispersion in mass of the Pal 3 HB is found to be very small -- consistent with
zero -- and we argue that this is unlikely to be due to a statistical
fluctuation. It is this small mass dispersion that leads to the most apparent
difference in the HB morphologies of M3 and Pal 3. iii) The relative HB types
of M3 and Pal 3, as measured by mean colors or parameters involving the number
of blue, variable, and red HB stars, can easily be accounted for by a fairly
small difference in age between these clusters, of order 0.5-1 Gyr -- which is
in good agreement with the relative age measurement, based on the clusters'
turnoffs, by VandenBerg (2000).Comment: 20 pages, 12 figures, emulateapj5 style. The Astrophysical Journal,
in press. Figs. 1, 6, 9, 10 are in png format. The preprint (postscript
format) with full resolution (embedded) figures is available from
http://www.astro.virginia.edu/~mc6v
Polarized heat current generated by quantum pumping in two-dimensional topological insulators
We consider transport properties of a two dimensional topological insulator
in a double quantum point contact geometry in presence of a time-dependent
external field. In the proposed setup an external gate is placed above a single
constriction and it couples only with electrons belonging to the top edge. This
asymmetric configuration and the presence of an ac signal allow for a quantum
pumping mechanism, which, in turn, can generate finite heat and charge currents
in an unbiased device configuration. A microscopic model for the coupling with
the external time-dependent gate potential is developed and the induced finite
heat and charge currents are investigated. We demonstrate that in the
non-interacting case, heat flow is associated with a single spin component, due
to the helical nature of the edge states, and therefore a finite and polarized
heat current is obtained in this configuration. The presence of e-e
interchannel interactions strongly affects the current signal, lowering the
degree of polarization of the system. Finally, we also show that separate heat
and charge flows can be achieved, varying the amplitude of the external gate.Comment: 13 pages, 5 figure
Three-dimensional analytical magnetohydrostatic equilibria of rigidly rotating magnetospheres in cylindrical geometry
We present three-dimensional solutions of the magnetohydrostatic equations in
the co-rotating frame of reference outside a magnetized rigidly rotating
cylinder. We make no symmetry assumption for the magnetic field, but to be able
to make analytical progress we neglect outflows and specify a particular form
for the current density. The magnetohydrostatic equations can then be reduced
to a single linear partial differential equation for a pseudo-potential ,
from which the magnetic field can be calculated by differentiation. The
equation for can be solved by standard methods. The solutions can also be
used to determine the plasma pressure, density and temperature as functions of
all three spatial coordinates. Despite the obvious limitations of this
approach, it can for example be used as a simple tool to create
three-dimensional models for the closed field line regions of rotating
magnetospheres without rotational symmetry.Comment: 13 pages, 2 figures, accepted for publication by Geophysical and
Astrophysical Fluid Dynamic
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