10,531 research outputs found
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XIX. A Chemical Tagging of the Multiple Stellar Populations Over the Chromosome Maps
The HST UV Survey of Globular Clusters (GCs) has investigated GCs and their
stellar populations. In previous papers of this series we have introduced a
pseudo two-color diagram, "chromosome map" (ChM), that maximises the separation
between the multiple populations. We have identified two main classes of GCs:
Type I (~83% of the objects) and Type II, both hosting two main groups of
stars, referred to in this series as first (1G) and second generation (2G).
Type II clusters exhibit two or more parallel sequences of 1G and 2G stars in
their ChMs. We exploit elemental abundances from literature to assign the
chemical composition to the distinct populations as identified on the ChMs of
29 GCs. We find that stars in different regions of the ChM have different
composition: 1G stars share the same light-element content as field stars,
while 2G stars are enhanced in N, Na and depleted in O. Stars enhanced in Al
and depleted in Mg populate the extreme regions of the ChM. We investigate the
color spread among 1G stars observed in many GCs, and find no evidence for
variations in light elements, whereas either a 0.1 dex Fe spread or a variation
in He remain to be verified. In the attempt of analysing the global properties
of the multiple populations, we have constructed a universal ChM, which
highlights that, though variegate, the phenomenon has some common pattern. The
universal ChM reveals a tight connection with Na, for which we have provided an
empirical relation. The additional ChM sequences typical of Type II GCs are
enhanced in metallicity and, often, in s elements. Omega Cen can be classified
as an extreme Type II GC, with a ChM displaying three main streams, each with
its own variations in chemical abundances. One of the most noticeable
differences is between the lower and upper streams, with the latter (associated
with higher He) having higher Fe and lower Li. We publicly release ChMs.Comment: 35 pages, 28 figures, 3 tables. Submitted to MNRA
A Generic Surface Sampler for Monte Carlo Simulations
We present an implementation of a Monte Carlo algorithm that generates points
randomly and uniformly on a set of arbitrary surfaces. The algorithm is
completely general and only requires the geometry modeling software to provide
the intersection points of an arbitrary line with the surface being sampled. We
demonstrate the algorithm using the Geant4 Monte Carlo simulation toolkit. The
efficiency of the sampling algorithm is discussed, along with various options
in the implementation and example applications
Charge pairing, superconducting transition and supersymmetry in high-temperature cuprate superconductors
We propose a model for high-T superconductors, valid for
, that includes both the spin fluctuations of the
Cu magnetic ions and of the O doped holes. Spin-charge separation
is taken into account with the charge of the doped holes being associated to
quantum skyrmion excitations (holons) of the Cu spin background. The
holon effective interaction potential is evaluated as a function of doping,
indicating that Cooper pair formation is determined by the competition between
the spin fluctuations of the Cu background and of spins of the O
doped holes (spinons). The superconducting transition occurs when the spinon
fluctuations dominate, thereby reversing the sign of the interaction. At this
point (), the theory is supersymmetric at short distances
and, as a consequence, the leading order results are not modified by radiative
corrections. The critical doping parameter for the onset of superconductivity
at T=0 is obtained and found to be a universal constant determined by the shape
of the Fermi surface. Our theoretical values for are in good
agreement with the experiment for both LSCO and YBCO.Comment: RevTex, 4 pages, no figure
Exact Asymptotic Behaviour of Fermion Correlation Functions in the Massive Thirring Model
We obtain an exact asymptotic expression for the two-point fermion
correlation functions in the massive Thirring model (MTM) and show that, for
, they reproduce the exactly known corresponding functions of the
massless theory, explicitly confirming the irrelevance of the mass term at this
point. This result is obtained by using the Coulomb gas representation of the
fermionic MTM correlators in the bipolar coordinate system.Comment: To appear in J. Phys. A: Math. Gen. 12 page
Global and non-global parameters of horizontal branch morphology of globular clusters
The horizontal branch (HB) morphology of globular clusters (GCs) is mainly
determined by metallicity. However, the fact that GCs with almost the same
metallicity exhibit different HB morphologies demonstrates that at least one
more parameter is needed to explain the HB morphology. It has been suggested
that one of these should be a global parameter that varies from GC to GC, and
the other a non-global parameter that varies within the GC. In this study we
provide empirical evidence corroborating this idea. We used the photometric
catalogs obtained with the Advanced Camera for Surveys (ACS) of the Hubble
Space Telescope (HST) and analyse the CMDs of 74 GCs. The HB morphology of our
sample of GCs has been investigated on the basis of the two new parameters L1
and L2 that measure the distance between the RGB and the coolest part of the
HB, and the color extension of the HB, respectively. We find that L1 correlates
with both metallicity and age, whereas L2 most strongly correlates with the
mass of the hosting GC. The range of helium abundance among the stars in a GC,
characterised by Delta Y and associated with the presence of multiple stellar
populations, has been estimated in a few GCs to date. In these GCs we find a
close relationship among Delta Y, GC mass, and L2. We conclude that age and
metallicity are the main global parameters while the range of helium abundance
within a GC is the main non-global parameter defining the HB morphology of
Galactic GCs.Comment: 34 pages, 13 figures, accepted for publication in Ap
Multiple stellar populations in Magellanic Cloud clusters. V. The split main sequence of the young cluster NGC1866
One of the most unexpected results in the field of stellar populations of the
last few years, is the discovery that some Magellanic-Cloud globular clusters
younger than ~400 Myr, exhibit bimodal main sequences (MSs) in their
color-magnitude diagrams (CMDs). Moreover, these young clusters host an
extended main sequence turn off (eMSTO) in close analogy with what is observed
in most ~1-2 Gyr old clusters of both Magellanic Clouds.
We use high-precision Hubble-Space-Telescope photometry to study the young
star cluster NGC1866 in the Large Magellanic Cloud. We discover an eMSTO and a
split MS. The analysis of the CMD reveals that (i) the blue MS is the less
populous one, hosting about one-third of the total number of MS stars; (ii)
red-MS stars are more centrally concentrated than blue-MS stars; (iii) the
fraction of blue-MS stars with respect to the total number of MS stars drops by
a factor of ~2 in the upper MS with F814W <~19.7.
The comparison between the observed CMDs and stellar models reveals that the
observations are consistent with ~200 Myr old highly-rotating stars on the
red-MS, with rotation close to critical value, plus a non-rotating stellar
population spanning an age interval between ~140 and 220 Myr, on the blue-MS.
Noticeable, neither stellar populations with different ages only, nor coeval
stellar models with different rotation rates, properly reproduce the observed
split MS and eMSTO. We discuss these results in the context of the eMSTO and
multiple MS phenomenon.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Probing the AGN Unification Model at redshift z 3 with MUSE observations of giant Ly nebulae
A prediction of the classic active galactic nuclei (AGN) unification model is
the presence of ionisation cones with different orientations depending on the
AGN type. Confirmations of this model exist for present times, but it is less
clear in the early Universe. Here, we use the morphology of giant Ly
nebulae around AGNs at redshift z3 to probe AGN emission and therefore
the validity of the AGN unification model at this redshift. We compare the
spatial morphology of 19 nebulae previously found around type I AGNs with a new
sample of 4 Ly nebulae detected around type II AGNs. Using two
independent techniques, we find that nebulae around type II AGNs are more
asymmetric than around type I, at least at radial distances ~physical kpc
(pkpc) from the ionizing source. We conclude that the type I and type II AGNs
in our sample show evidence of different surrounding ionising geometries. This
suggests that the classical AGN unification model is also valid for
high-redshift sources. Finally, we discuss how the lack of asymmetry in the
inner parts (r30 pkpc) and the associated high values of the HeII to
Ly ratios in these regions could indicate additional sources of (hard)
ionizing radiation originating within or in proximity of the AGN host galaxies.
This work demonstrates that the morphologies of giant Ly nebulae can be
used to understand and study the geometry of high redshift AGNs on
circum-nuclear scales and it lays the foundation for future studies using much
larger statistical samples.Comment: 15 pages, 13 figures, accepted for publication in MNRA
First prototype of a silicon tracker using an artificial retina for fast track finding
We report on the R\&D for a first prototype of a silicon tracker based on an
alternative approach for fast track finding. The working principle is inspired
from neurobiology, in particular by the processing of visual images by the
brain as it happens in nature. It is based on extensive parallelisation of data
distribution and pattern recognition. In this work we present the design of a
practical device that consists of a telescope based on single-sided silicon
detectors; we describe the data acquisition system and the implementation of
the track finding algorithms using available digital logic of commercial FPGA
devices. Tracking performance and trigger capabilities of the device are
discussed along with perspectives for future applications.Comment: 9 pages, 7 figures, Technology and Instrumentation in Particle
Physics 2014 (TIPP 2014), conference proceeding
A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752
Multi-band Hubble Space Telescope photometry reveals that the main sequence,
sub-giant, and the red giant branch of the globular cluster NGC6752 splits into
three main components in close analogy with the three distinct segments along
its horizontal branch stars. These triple sequences are consistent with three
stellar groups: a stellar population with a chemical composition similar to
field halo stars (population a), a population (c) with enhanced sodium and
nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y
~0.03), and a population (b) with an intermediate (between population a and c)
chemical composition and slightly helium enhanced (Delta Y ~0.01). These
components contain ~25% (population a), ~45% (population b), and ~30%
(population c) of the stars. No radial gradient for the relative numbers of the
three populations has been identified out to about 2.5 half mass radii.Comment: 42 pages, 24 figures, accepted for publication in Ap
Simulation and performance of an artificial retina for 40 MHz track reconstruction
We present the results of a detailed simulation of the artificial retina
pattern-recognition algorithm, designed to reconstruct events with hundreds of
charged-particle tracks in pixel and silicon detectors at LHCb with LHC
crossing frequency of . Performances of the artificial retina
algorithm are assessed using the official Monte Carlo samples of the LHCb
experiment. We found performances for the retina pattern-recognition algorithm
comparable with the full LHCb reconstruction algorithm.Comment: Final draft of WIT proceedings modified according to JINST referee's
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