2,744 research outputs found
Extraction of the atmospheric neutrino fluxes from experimental event rate data
The precise knowledge of the atmospheric neutrino fluxes is a key ingredient
in the interpretation of the results from any atmospheric neutrino experiment.
In the standard atmospheric neutrino data analysis, these fluxes are
theoretical inputs obtained from sophisticated numerical calculations. In this
contribution we present an alternative approach to the determination of the
atmospheric neutrino fluxes based on the direct extraction from the
experimental data on neutrino event rates. The extraction is achieved by means
of a combination of artificial neural networks as interpolants and Monte Carlo
methods.Comment: 6 pages, 2 figs, to appear in the proceedings of the 2nd
International Conference on Quantum Theories and Renormalization Group in
Gravity and Cosmology, Barcelona, July 200
Giant planets around two intermediate-mass evolved stars and confirmation of the planetary nature of HIP67851 c
Precision radial velocities are required to discover and characterize planets
orbiting nearby stars. Optical and near infrared spectra that exhibit many
hundreds of absorption lines can allow the m/s precision levels required for
such work. However, this means that studies have generally focused on
solar-type dwarf stars. After the main-sequence, intermediate-mass stars
(former A-F stars) expand and rotate slower than their progenitors, thus
thousands of narrow absorption lines appear in the optical region, permitting
the search for planetary Doppler signals in the data for these types of stars.
We present the discovery of two giant planets around the intermediate-mass
evolved star HIP65891 and HIP107773. The best Keplerian fit to the HIP65891 and
HIP107773 radial velocities leads to the following orbital parameters: P=1084.5
d; msin = 6.0 M; =0.13 and P=144.3 d; msin = 2.0
M; =0.09, respectively. In addition, we confirm the planetary nature
of the outer object orbiting the giant star HIP67851. The orbital parameters of
HIP67851c are: P=2131.8 d, msin = 6.0 M and =0.17. With
masses of 2.5 M and 2.4 M HIP65891 and HIP107773 are two of the
most massive stars known to host planets. Additionally, HIP67851 is one of five
giant stars that are known to host a planetary system having a close-in planet
( 0.7 AU). Based on the evolutionary states of those five stars, we
conclude that close-in planets do exist in multiple systems around subgiants
and slightly evolved giants stars, but probably they are subsequently destroyed
by the stellar envelope during the ascent of the red giant branch phase. As a
consequence, planetary systems with close-in objects are not found around
horizontal branch stars.Comment: Accepted for publication in A&
An eccentric companion at the edge of the brown dwarf desert orbiting the 2.4 Msun giant star HIP67537
We report the discovery of a substellar companion around the giant star
HIP67537. Based on precision radial velocity measurements from CHIRON and FEROS
high-resolution spectroscopic data, we derived the following orbital elements
for HIP67537: msin = 11.1 M,
= 4.9 AU and = 0.59. Considering
random inclination angles, this object has 65% probability to be
above the theoretical deuterium-burning limit, thus it is one of the few known
objects in the planet to brown-dwarf transition region. In addition, we
analyzed the Hipparcos astrometric data of this star, from which we derived a
minimum inclination angle for the companion of 2 deg. This value
corresponds to an upper mass limit of 0.3 M, therefore the
probability that HIP67537 is stellar in nature is 7%. The large
mass of the host star and the high orbital eccentricity makes HIP67537 a
very interesting and rare substellar object. This is the second candidate
companion in the brown dwarf desert detected in the sample of intermediate-mass
stars targeted by the EXPRESS radial velocity program, which corresponds to a
detection fraction of = 1.6%. This value is larger than the
fraction observed in solar-type stars, providing new observational evidence of
an enhanced formation efficiency of massive substellar companions in massive
disks. Finally, we speculate about different formation channels for this
object.Comment: Accepted for publication to A&
Four new planets around giant stars and the mass-metallicity correlation of planet-hosting stars
CONTEXT. Exoplanet searches have demonstrated that giant planets are
preferentially found around metal-rich stars and that their fraction increases
with the stellar mass. AIMS. During the past six years, we have conducted a
radial velocity follow-up program of 166 giant stars, to detect substellar
companions, and characterizing their orbital properties. Using this
information, we aim to study the role of the stellar evolution in the orbital
parameters of the companions, and to unveil possible correlations between the
stellar properties and the occurrence rate of giant planets. METHODS. Using
FEROS and CHIRON spectra, we have computed precision radial velocities and we
have derived atmospheric and physical parameters for all of our targets.
Additionally, velocities computed from UCLES spectra are presented here. By
studying the periodic radial velocity signals, we have detected the presence of
several substellar companions. RESULTS. We present four new planetary systems
around the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally,
we find that giant planets are more frequent around metal-rich stars, reaching
a peak in the detection of = 16.7% around stars with
[Fe/H] 0.35 dex. Similarly, we observe a positive correlation of the
planet occurrence rate with the stellar mass, between M 1.0 -2.1
M, with a maximum of = 13.0%, at M = 2.1
M. CONCLUSIONS. We conclude that giant planets are preferentially
formed around metal-rich stars. Also, we conclude that they are more
efficiently formed around more massive stars, in the mass range of M
1.0 - 2.1 M. These observational results confirm previous
findings for solar-type and post-MS hosting stars, and provide further support
to the core-accretion formation model.Comment: Accepted for publication in A&
Negative Electron-electron Drag Between Narrow Quantum Hall Channels
Momentum transfer due to Coulomb interaction between two parallel,
two-dimensional, narrow, and spatially separated layers, when a current
I_{drive} is driven through one layer, is studied in the presence of a
perpendicular magnetic field B. The current induced in the drag layer,
I_{drag}, is evaluated self-consistently with I_{drive} as a parameter.
I_{drag} can be positive or negative depending on the value of the filling
factor \nu of the highest occupied bulk Landau level (LL). For a fully occupied
LL, I_{drag} is negative, i.e., it flows opposite to I_{drive}, whereas it is
positive for a half-filled LL. When the circuit is opened in the drag layer, a
voltage \Delta V_{drag} develops in it; it is negative for a half-filled LL and
positive for a fully occupied LL. This positive \Delta V_{drag}, expressing a
negative Coulomb drag, results from energetically favored near-edge inter-LL
transitions that occur when the highest occupied bulk LL and the LL just above
it become degenerate.Comment: Text file in Latex/Revtex/preprint format, 7 separate PS figures,
Physical Review B, in pres
Detection of protection benefits for predatory fishes depends on census methodology
Marine protected areas (MPAs) are used as fisheries management and conservation tools. Well-enforced no-take zones allow the rebuilding of natural populations of exploited species; however, there is still controversy on the role of buffer zones. The effectiveness of MPAs could be underestimated, as fish population assessments depend largely on traditional methodologies that have difficulties in detecting predatory fish because of their low abundances, their patchy distribution, and their reaction to the presence of divers. The performance of different census methods was compared in assessing the protection benefits for large predatory fishes under different protection levels (i.e. no-take and buffer zones) in five Mediterranean MPAs. Specifically, conventional strip transects (CSTs, 50 × 5 m2) and tracked roaming transects combined with distance sampling (TRT + DS, variable lengths) were compared, including a series of TRT-derived estimators with variable transect lengths and fixed widths of 20, 10, and 6 m (TRT20, TRT10, and TRT6, respectively). Additionally, the effectiveness of the MPAs studied and protection levels for conserving large predatory species was evaluated. Transects covering larger areas (i.e. TRT + DS and TRT20) allowed the detection of a greater number of species and yielded more accurate estimates of density and biomass than transects of narrower fixed widths, particularly the CSTs, which were associated with the lowest richness detection capability, accuracy, and precision. On average, both no-take zones and buffer zones appeared effective for the conservation of predatory fishes, indicating that multiple protection areas were ecologically effective. Differences between MPAs were also observed, however, probably arising from both local environmental and management factors. We suggest the implementation of methodologies with larger transects for the study of large predatory fish, combined with CSTs for the rest of the fish community, in order to avoid biases in predatory population assessments, which are key indicators of MPA effectiveness
Anomalous Dimension and Spatial Correlations in a Point-Island Model
We examine the island size distribution function and spatial correlation
function of a model for island growth in the submonolayer regime in both 1 and
2 dimensions. In our model the islands do not grow in shape, and a fixed number
of adatoms are added, nucleate, and are trapped at islands as they diffuse.
We study the cases of various critical island sizes for nucleation as a
function of initial coverage. We found anomalous scaling of the island size
distribution for large . Using scaling, random walk theory, a version of
mean-field theory we obtain a closed form for the spatial correlation function.
Our analytic results are verified by Monte Carlo simulations
Characterization of the material response in the granular ratcheting
The existence of a very special ratcheting regime has recently been reported
in a granular packing subjected to cyclic loading \cite{alonso04}. In this
state, the system accumulates a small permanent deformation after each cycle.
After a short transient regime, the value of this permanent strain accumulation
becomes independent on the number of cycles. We show that a characterization of
the material response in this peculiar state is possible in terms of three
simple macroscopic variables. They are defined that, they can be easily
measured both in the experiments and in the simulations. We have carried out a
thorough investigation of the micro- and macro-mechanical factors affecting
these variables, by means of Molecular Dynamics simulations of a polydisperse
disk packing, as a simple model system for granular material. Biaxial test
boundary conditions with a periodically cycling load were implemented. The
effect on the plastic response of the confining pressure, the deviatoric stress
and the number of cycles has been investigated. The stiffness of the contacts
and friction has been shown to play an important role in the overall response
of the system. Specially elucidating is the influence of the particular
hysteretical behavior in the stress-strain space on the accumulation of
permanent strain and the energy dissipation.Comment: 13 pages, 20 figures. Submitted to PR
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