534 research outputs found
The central Blue Straggler population in four outer-halo globular clusters
Using HST/WFPC2 data, we have performed a comparative study of the Blue
Straggler Star (BSS) populations in the central regions of the globular
clusters AM 1, Eridanus, Palomar 3, and Palomar 4. Located at distances RGC >
50 kpc from the Galactic Centre, these are (together with Palomar 14 and NGC
2419) the most distant clusters in the Halo. We determine their
colour-magnitude diagrams and centres of gravity. The four clusters turn out to
have similar ages (10.5-11 Gyr), significantly smaller than those of the
inner-Halo globulars, and similar metallicities. By exploiting wide field
ground based data, we build the most extended radial density profiles from
resolved star counts ever published for these systems. These are well
reproduced by isotropic King models of relatively low concentration. BSSs
appear to be significantly more centrally segregated than red giants in all
globular clusters, in agreement with the estimated core and half-mass
relaxation times which are smaller than the cluster ages. Assuming that this is
a signature of mass segregation, we conclude that AM 1 and Eridanus are
slightly dynamically more evolved than Pal 3 and Pal 4.Comment: Accepted for publication in ApJ, references update
Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters
We measure how the slope of the stellar mass function (MF) changes
as a function of clustercentric distance in five Galactic globular clusters
and compare to predictions from direct -body star cluster
simulations. Theoretical studies predict that (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
. 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.Comment: 12 pages, 9 figures, Accepted for publication in MNRA
Measurement of Electron Trapping in the CESR Storage Ring
The buildup of low-energy electrons has been shown to affect the performance
of a wide variety of particle accelerators. Of particular concern is the
persistence of the cloud between beam bunch passages, which can impose
limitations on the stability of operation at high beam current. We have
obtained measurements of long-lived electron clouds trapped in the field of a
quadrupole magnet in a positron storage ring, with lifetimes much longer than
the revolution period. Based on modeling, we estimate that about 7% of the
electrons in the cloud generated by a 20-bunch train of 5.3 GeV positrons with
16-ns spacing and population survive longer than 2.3 s in a
quadrupole field of gradient 7.4 T/m. We have observed a non-monotonic
dependence of the trapping effect on the bunch spacing. The effect of a witness
bunch on the measured signal provides direct evidence for the existence of
trapped electrons. The witness bunch is also observed to clear the cloud,
demonstrating its effectiveness as a mitigation technique.Comment: 6 pages, 9 figures, 28 citation
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
Numerical Computation of Resonances and Pseudospectra in Acoustic Scattering
Acoustic scattering is a well-known physical phenomenon which arises in a wide range of fields: when acoustic waves propagating in a medium impinge on a localised non-uniformity, such as a density fluctuation or an external obstacle, their trajectories are deviated and scattered waves are generated. A key role in scattering theory is played by resonances; these are particular scatterer-dependent non-physical âcomplexâ frequencies at which acoustic scattering exhibits exceptional behaviour. The study of acoustic resonances for a particular scatterer provides an insight in the behaviour that the acoustic scattering assumes at the near physical ârealâ frequencies, and it is a fundamental step in many applications. Yet, the numerical computation of resonances and pseudospectra - a mathematical tool which can be used to study the influence of resonances on physical frequencies - remains very expensive. With the present Thesis we want to address this particular problem, by proposing numerical algorithms based on the Boundary Element Method (BEM) for computing resonances and pseudospectra and by analysing their efficiency and performance. Finally, we apply such algorithms to half a dozen of physically relevant scatterer, inspired from different fields where acoustic scattering plays a relevant role
Radio timing and optical photometry of the black widow system PSR J1518+0204C in the globular cluster M5
We report on the determination of astrometric, spin and orbital parameters
for PSR J1518+0204C, a "black widow" binary millisecond pulsar in the globular
cluster M5. The accurate position and orbital parameters obtained from radio
timing allowed us to search for the optical companion. By using WFC3/HST images
we identified a very faint variable star (m_F390W > 24.8, m_F606W > 24.3,
m_F814W > 23.1) located at only 0.25" from the pulsar's timing position. Due to
its strong variability, this star is visible only in a sub-sample of images.
However, the light curve obtained folding the available data with the orbital
parameters of the pulsar shows a maximum at the pulsar inferior conjunction and
a possible minimum at the pulsar superior conjunction. Furthermore, the shape
of the optical modulation indicates a heating process possibly due to the
pulsar wind. This is the first identification of an optical companion to a
black widow pulsar in the dense stellar environment of a globular cluster.Comment: Accepted for publication by ApJ; 24 Pages, 5 Figures, 1 Tabl
Discovery of three new millisecond pulsars in Terzan 5
We report on the discovery of three new millisecond pulsars (namely
J1748-2446aj, J1748-2446ak and J1748-2446al) in the inner regions of the dense
stellar system Terzan 5. These pulsars have been discovered thanks to a method,
alternative to the classical search routines, that exploited the large set of
archival observations of Terzan 5 acquired with the Green Bank Telescope over 5
years (from 2010 to 2015). This technique allowed the analysis of stacked power
spectra obtained by combining ~206 hours of observation. J1748-2446aj has a
spin period of ~2.96 ms, J1748-2446ak of ~1.89 ms (thus it is the fourth
fastest pulsar in the cluster) and J1748-2446al of ~5.95 ms. All the three
millisecond pulsars are isolated and currently we have timing solutions only
for J1748-2446aj and J1748-2446ak. For these two systems, we evaluated the
contribution to the measured spin-down rate of the acceleration due to the
cluster potential field, thus estimating the intrinsic spin-down rates, which
are in agreement with those typically measured for millisecond pulsars in
globular clusters. Our results increase to 37 the number of pulsars known in
Terzan 5, which now hosts 25% of the entire pulsar population identified, so
far, in globular clusters.Comment: 10 pages, 6 figures. ApJ (accepted
The surface density profile of NGC 6388: a good candidate for harboring an intermediate-mass black hole
We have used a combination of high resolution (HST ACS-HRC, ACS-WFC, and
WFPC2) and wide-field (ESO-WFI) observations of the galactic globular cluster
NGC 6388 to derive its center of gravity, projected density profile, and
central surface brightness profile. While the overall projected profiles are
well fit by a King model with intermediate concentration (c=1.8) and sizable
core radius (rc=7"), a significant power law (with slope \alpha=-0.2) deviation
from a flat core behavior has been detected within the inner 1 arcsecond. These
properties suggest the presence of a central intermediate mass black hole. The
observed profiles are well reproduced by a multi-mass isotropic, spherical
model including a black hole with a mass of ~5.7x10^3 Msol.Comment: ApJ Letter in pres
Modeling meander morphodynamics over self-formed heterogeneous floodplains
This work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
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