1,795 research outputs found
An Investigation of the Large-scale Variability of the Apparently Single Wolf-Rayet Star WR 1
In recent years, much studies have focused on determining the origin of the
large-scale line-profile and/or photometric patterns of variability displayed
by some apparently single Wolf-Rayet stars, with the existence of an unseen
(collapsed?) companion or of spatially extended wind structures as potential
candidates. We present observations of WR 1 which highlight the unusual
character of the variations in this object. Our narrowband photometric
observations reveal a gradual increase of the stellar continuum flux amounting
to Delta v = 0.09 mag followed by a decline on about the same timescale (3-4
days). Only marginal evidence for variability is found during the 11 following
nights.
Strong, daily line-profile variations are also observed but they cannot be
easily linked to the photometric variations.
Similarly to the continuum flux variations, coherent time-dependent changes
are observed in 1996 in the centroid, equivalent width, and skewness of He II
4686. Despite the generally coherent nature of the variations, we do not find
evidence in our data for the periods claimed in previous studies. While the
issue of a cyclical pattern of variability in WR 1 is still controversial, it
is clear that this object might constitute in the future a cornerstone for our
understanding of the mechanisms leading to the formation of largely anisotropic
outflows in Wolf-Rayet stars.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Nuclear star cluster formation in energy-space
In a virialized stellar system, the mean-square velocity is a direct tracer
of the energy per unit mass of the system. Here, we exploit this to estimate
and compare root-mean-square velocities for a large sample of nuclear star
clusters and their host (late- or early-type) galaxies. Traditional
observables, such as the radial surface brightness and second-order velocity
moment profiles, are subject to short-term variations due to individual
episodes of matter infall and/or star formation. The total mass, energy and
angular momentum, on the other hand, are approximately conserved. Thus, the
total energy and angular momentum more directly probe the formation of galaxies
and their nuclear star clusters, by offering access to more fundamental
properties of the nuclear cluster-galaxy system than traditional observables.
We find that there is a strong correlation, in fact a near equality, between
the root-mean-square velocity of a nuclear star cluster and that of its host.
Thus, the energy per unit mass of a nuclear star cluster is always comparable
to that of its host galaxy. We interpret this as evidence that nuclear star
clusters do not form independently of their host galaxies, but rather that
their formation and subsequent evolution are coupled. We discuss how our
results can potentially be used to offer a clear and observationally testable
prediction to distinguish between the different nuclear star cluster formation
scenarios, and/or quantify their relative contributions.Comment: 12 pages, 3 figures, 4 tables; accepted for publication in MNRA
Chiral persistent currents and magnetic susceptibilities in the parafermion quantum Hall states in the second Landau level with Aharonov-Bohm flux
Using the effective conformal field theory for the quantum Hall edge states
we propose a compact and convenient scheme for the computation of the periods,
amplitudes and temperature behavior of the chiral persistent currents and the
magnetic susceptibilities in the mesoscopic disk version of the Z_k parafermion
quantum Hall states in the second Landau level. Our numerical calculations show
that the persistent currents are periodic in the Aharonov-Bohm flux with period
exactly one flux quantum and have a diamagnetic nature. In the high-temperature
regime their amplitudes decay exponentially with increasing the temperature and
the corresponding exponents are universal characteristics of non-Fermi liquids.
Our theoretical results for these exponents are in perfect agreement with those
extracted from the numerical data and demonstrate that there is in general a
non-trivial contribution coming from the neutral sector. We emphasize the
crucial role of the non-holomorphic factors, first proposed by Cappelli and
Zemba in the context of the conformal field theory partition functions for the
quantum Hall states, which ensure the invariance of the annulus partition
function under the Laughlin spectral flow.Comment: 14 pages, RevTeX4, 7 figures (eps
Prediction of sanding in subsurface hydrocarbon reservoirs.
Sand production in oil and gas wells can occur if the fluid velocity exceeds a
certain value. Due to drilling operations, the mechanical stresses can exceed the load bearing capacity of the rock. As the local stresses exceed certain level, a certain amount of rock is fractured into sand. Then, the sand is carried by the fluid through the wellbore depending on the flow rate. The amount of the solids can be less than a few grams per cubic meter of reservoir fluid or an essential amount. In the later case erosion of the rock and removing sufficient quantities of rock can occur. This can produce subsurface cavities which collapse and destroy the well.
When sanding is unavoidable it is necessary to estimate the characteristics of the process. Our aim was to generate a simple one-dimensional local model, which predicts the volume of sanding, the radius and the porosity of the yielded zone. Such model will help the company in the development of complex 3D models
ROLE OF THE CYCLIC ADENOSINE MONOPHOSPHATE SYSTEM IN THE EFFECT EXERTED BY SUBSTANCES WITH SELECTIVE INFLUENCE ON MYONEURAL SYNAPSES
No abstrac
Chemo-dynamical evolution of Globular Cluster Systems
We studied the relation between the ratio of rotational velocity to velocity
dispersion and the metallicity (/\sigma_{v}-metallicity relation) of
globular cluster systems (GCS) of disk galaxies by comparing the relation
predicted from simple chemo-dynamical models for the formation and evolution of
disk galaxies with the observed kinematical and chemical properties of their
GCSs. We conclude that proto disk galaxies underwent a slow initial collapse
that was followed by a rapid contraction and derive that the ratio of the
initial collapse time scale to the active star formation time scale is \sim 6
for our Galaxy and \sim 15 for M31. The fundamental formation process of disk
galaxies was simulated based on simple chemo-dynamical models assuming the
conservation of their angular momentum. We suggest that there is a typical
universal pattern in the /\sigma_{v}-metallicity relation of the GCS
of disk galaxies. This picture is supported by the observed properties of GCSs
in the Galaxy and in M31. This relation would deviate from the universal
pattern, however, if large-scale merging events took major role in
chemo-dynamical evolution of galaxies and will reflect the epoch of such
merging events. We discuss the properties of the GCS of M81 and suggest the
presence of past major merging event.Comment: 25 pages, 8 figures, Accepted for publication in the Astrophysical
Journa
Timing performance of a double layer diamond detector
In order to improve the time precision of detectors based on diamonds sensors we have built a detector with two scCVD layers connected in parallel to the same amplifier. This work describes the design and the first measurements of such a prototype performed on a particle beam at CERN. With this different configuration we have obtained an improvement larger than a factor of 1.6-1.7 for the timing precision of the measurement when compared to a one layer scCVD diamond detector.Peer reviewe
Gradient-free quantum optimization on NISQ devices
Variational Quantum Eigensolvers (VQEs) have recently attracted considerable
attention. Yet, in practice, they still suffer from the efforts for estimating
cost function gradients for large parameter sets or resource-demanding
reinforcement strategies. Here, we therefore consider recent advances in
weight-agnostic learning and propose a strategy that addresses the trade-off
between finding appropriate circuit architectures and parameter tuning. We
investigate the use of NEAT-inspired algorithms which evaluate circuits via
genetic competition and thus circumvent issues due to exceeding numbers of
parameters. Our methods are tested both via simulation and on real quantum
hardware and are used to solve the transverse Ising Hamiltonian and the
Sherrington-Kirkpatrick spin model.Comment: 13 pages, 6 figures, comments welcome
Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations
We present the results of a spectroscopic study of the intermediate age (approximately 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at 3839 and 4300 Angstroms respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of 5 stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3
Evidence for multiple populations in intermediate age cluster Lindsay 1 in the SMC
Lindsay 1 is an intermediate age (≈8 Gyr) massive cluster in the Small Magellanic Cloud (SMC). Using VLT FORS2 spectra of 16 probable cluster members on the lower RGB of the cluster, we measure CN and CH band strengths (at ≃ 3883 and 4300 Å respectively), along with carbon and nitrogen abundances and find that a sub-population of stars has significant nitrogen enrichment. A lack of spread in carbon abundances excludes evolutionary mixing as the source of this enrichment, so we conclude that this is evidence of multiple populations. Therefore, L1 is the youngest cluster to show such variations, implying that the process triggering the onset of multiple populations must operate until at least redshift ∼1
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