1,168 research outputs found
The Brown-dwarf Atmosphere Monitoring (BAM) Project II: Multi-epoch monitoring of extremely cool brown dwarfs
With the discovery of Y dwarfs by the WISE mission, the population of field
brown dwarfs now extends to objects with temperatures comparable to those of
Solar System planets. To investigate the atmospheres of these newly identified
brown dwarfs, we have conducted a pilot study monitoring an initial sample of
three late T-dwarfs (T6.5, T8 and T8.5) and one Y-dwarf (Y0) for infrared
photometric variability at multiple epochs. With J-band imaging, each target
was observed for a period of 1.0h to 4.5h per epoch, which covers a significant
fraction of the expected rotational period. These measurements represent the
first photometric monitoring for these targets. For three of the four targets
(2M1047, Ross 458C and WISE0458), multi-epoch monitoring was performed, with
the time span between epochs ranging from a few hours to ~2 years. During the
first epoch, the T8.5 target WISE0458 exhibited variations with a remarkable
min-to-max amplitude of 13%, while the second epoch light curve taken ~2 years
later did not note any variability to a 3% upper limit. With an effective
temperature of ~600 K, WISE0458 is the coldest variable brown dwarf published
to-date, and combined with its high and variable amplitude makes it a
fascinating target for detailed follow-up. The three remaining targets showed
no significant variations, with a photometric precision between 0.8% and 20.0%,
depending on the target brightness. Combining the new results with previous
multi-epoch observations of brown dwarfs with spectral types of T5 or later,
the currently identified variables have locations on the colour-colour diagram
better matched by theoretical models incorporating cloud opacities rather than
cloud-free atmospheres. This preliminary result requires further study to
determine if there is a definitive link between variability among late-T dwarfs
and their location on the colour-colour diagram.Comment: 9 pages, 6 figures, 3 tables, accepted for publication in MNRA
Intense field stabilization in circular polarization: 3D time-dependent dynamics
We investigate the stabilization of a hydrogen atom in circularly polarized
laser fields. We use a time-dependent, fully three dimensional approach to
study the quantum dynamics of the hydrogen atom subject to high intensity,
short wavelength laser pulses. We find enhanced survival probability as the
field is increased under fixed envelope conditions. We also confirm wavepacket
dynamics seen in prior time-dependent computations restricted to two
dimensions.Comment: 4 pages, 3 figures, submitte
Existence criteria for stabilization from the scaling behaviour of ionization probabilities
We provide a systematic derivation of the scaling behaviour of various
quantities and establish in particular the scale invariance of the ionization
probability. We discuss the gauge invariance of the scaling properties and the
manner in which they can be exploited as consistency check in explicit
analytical expressions, in perturbation theory, in the Kramers-Henneberger and
Floquet approximation, in upper and lower bound estimates and fully numerical
solutions of the time dependent Schroedinger equation. The scaling invariance
leads to a differential equation which has to be satisfied by the ionization
probability and which yields an alternative criterium for the existence of
atomic bound state stabilization.Comment: 12 pages of Latex, one figur
Referrer Graph: A cost-effective algorithm and pruning method for predicting web accesses
This paper presents the Referrer Graph (RG) web prediction algorithm and a pruning method for the associated
graph as a low-cost solution to predict next web users accesses. RG is aimed at being used in a real
web system with prefetching capabilities without degrading its performance. The algorithm learns from
users accesses and builds a Markov model. These kinds of algorithms use the sequence of the user accesses
to make predictions. Unlike previous Markov model based proposals, the RG algorithm differentiates
dependencies in objects of the same page from objects of different pages by using the object URI and the
referrer in each request. Although its design permits us to build a simple data structure that is easier to
handle and, consequently, needs lower computational cost in comparison with other algorithms, a pruning
mechanism has been devised to avoid the continuous growing of this data structure. Results show
that, compared with the best prediction algorithms proposed in the open literature, the RG algorithm
achieves similar precision values and page latency savings but requiring much less computational and
memory resources. Furthermore, when pruning is applied, additional and notable resource consumption
savings can be achieved without degrading original performance. In order to reduce further the resource
consumption, a mechanism to prune de graph has been devised, which reduces resource consumption of
the baseline system without degrading the latency savings.
2013 Elsevier B.V. All rights reserved.This work has been partially supported by Spanish Ministry of Science and Innovation under Grant TIN2009-08201. The authors would also like to thank the technical staff of the School of Computer Science at the Polytechnic University of Valencia for providing us recent and customized trace files logged by their web server.De La Ossa Perez, BA.; Gil Salinas, JA.; Sahuquillo Borrás, J.; Pont Sanjuan, A. (2013). Referrer Graph: A cost-effective algorithm and pruning method for predicting web accesses. Computer Communications. 36(8):881-894. https://doi.org/10.1016/j.comcom.2013.02.005S88189436
Stellar population synthesis of post-AGB stars: the s-process in MACHO47.2496.8
The low-metallicity RV Tauri star MACHO47.2496.8, recently discovered in the
Large Magellanic Cloud, is highly enriched in carbon and heavy elements
produced by the slow neutron capture process (s-process), and is most probably
a genuine post-C(N-type) asymptotic giant branch (AGB) star. We use the
analysis of the abundances of MACHO47.2496.8 to constrain free parameters in
AGB models. We test which values of the free parameters describing uncertain
physical mechanisms in AGB stars, namely the third dredge-up and the features
of the 13C neutron source, produce models that better match the abundances
observed in MACHO47.2496.8. We carry out stellar population synthesis coupled
with s-process nucleosynthesis using a synthetic stellar evolution code. The
s-process ratios observed in MACHO47.2496.8 can be matched by the same models
that explain the s-process ratios of Galactic AGB and post-AGB stars of
metallicity > Z_sun/10, except for the choice of the effectiveness of 13C as a
neutron source, which has to be lower by roughly a factor of 3 to 6. The less
effective neutron source for lower metallicities is also required when
comparing population synthesis results to observations of Galactic halo
-enhanced stars, such as Pb stars. The 12C/13C ratio in MACHO47.2496.8
cannot be matched simultaneously and requires the occurrence of extra-mixing
processes. The confirmed trend of the decreased efficiency of the 13C neutron
source with metallicity requires an explanation from AGB s-process models. The
present work is to date the first comparison between theoretical models and the
detailed abundances of an extragalactic post-AGB star.Comment: accepted for publication on Astronomy & Astrophysics Letter
Can higher order curvature theories explain rotation curves of galaxies?
Higher order curvature gravity has recently received a lot of attention due
to the fact that it gives rise to cosmological models which seem capable of
solving dark energy and quintessence issues without using "ad hoc" scalar
fields. In this letter, a gravitational potential is obtained which differs
from the Newtonian one because of a repulsive correction increasing with
distance. We evaluate the rotation curve of our Galaxy and compare it with the
observed data in order both to test the viability of these theories and to
estimate the scalelength of the correction. It is remarkable that the Milky Way
rotation curve is well fitted without the need of any dark matter halo and a
similar result tentatively holds also for other galaxies.Comment: 8 pages, to appear in Phys. Lett.
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