3,157 research outputs found
A refined analysis of the low-mass eclipsing binary system T-Cyg1-12664
The observational mass-radius relation of main sequence stars with masses
between ~0.3 and 1.0 Msun reveals deviations between the stellar radii
predicted by models and the observed radii of stars in detached binaries. We
generate an accurate physical model of the low-mass eclipsing binary
T-Cyg1-12664 in the Kepler mission field to measure the physical parameters of
its components and to compare them with the prediction of theoretical stellar
evolution models. We analyze the Kepler mission light curve of T-Cyg1-12664 to
accurately measure the times and phases of the primary and secondary eclipse.
In addition, we measure the rotational period of the primary component by
analyzing the out-of-eclipse oscillations that are due to spots. We accurately
constrain the effective temperature of the system using ground-based absolute
photometry in B, V, Rc, and Ic. We also obtain and analyze V, Rc, Ic
differential light curves to measure the eccentricity and the orbital
inclination of the system, and a precise Teff ratio. From the joint analysis of
new radial velocities and those in the literature we measure the individual
masses of the stars. Finally, we use the PHOEBE code to generate a physical
model of the system. T-Cyg1-12664 is a low eccentricity system, located
d=360+/-22 pc away from us, with an orbital period of P=4.1287955(4) days, and
an orbital inclination i=86.969+/-0.056 degrees. It is composed of two very
different stars with an active G6 primary with Teff1=5560+/-160 K,
M1=0.680+/-0.045 Msun, R1=0.799+/-0.017 Rsun, and a M3V secondary star with
Teff2=3460+/-210 K, M2=0.376+/-0.017 Msun, and R2=0.3475+/-0.0081 Rsun. The
primary star is an oversized and spotted active star, hotter than the stars in
its mass range. The secondary is a cool star near the mass boundary for fully
convective stars (M~0.35 Msun), whose parameters appear to be in agreement with
low-mass stellar model.Comment: 18 pages, 15 figures, 15 table
Non perturbative renormalisation group and momentum dependence of -point functions (I)
We present an approximation scheme to solve the Non Perturbative
Renormalization Group equations and obtain the full momentum dependence of the
-point functions. It is based on an iterative procedure where, in a first
step, an initial ansatz for the -point functions is constructed by solving
approximate flow equations derived from well motivated approximations. These
approximations exploit the derivative expansion and the decoupling of high
momentum modes. The method is applied to the O() model. In leading order,
the self energy is already accurate both in the perturbative and the scaling
regimes. A stringent test is provided by the calculation of the shift in the transition temperature of the weakly repulsive Bose gas, a quantity
which is particularly sensitive to all momentum scales. The leading order
result is in agreement with lattice calculations, albeit with a theoretical
uncertainty of about 25%.Comment: 48 pages, 15 figures A few minor corrections. A reference adde
A versatile magnetic refrigeration demonstrator
Trabajo presentado a la "6th International Conference on Magnetic Refrigeration at Room Temperature" celebrada en Victoria (Canadá) del 7 al 10 de septiembre de 2014.A versatile room temperature reciprocating magnetic refrigeration demonstrator has been designed, built and tested in order to check suitable magnetocaloric materials for magnetic refrigeration. Test experiments have been done with 31 g of Gd spheres of 0.2 – 0.4 mm diameter as refrigerant material, because it is a well-known benchmark material for magnetic refrigeration. The magnetic field is provided by a Halbach Nd2Fe14B permanent magnet with a slot of 10 mm width and a maximum field of 1.4 T. At optimized values of frequency (f = 0.7 Hz) and utilization factor (U = 0.19), the demonstrator achieves a maximum no load temperature span of 19.3 K. A maximum cooling power of 6 W at zero temperature span was obtained at optimized values f = 0.31 Hz and U = 1.1. Different thermodynamic cycles have been studied looking for the optimized parameters.Peer Reviewe
Josephson current in strongly correlated double quantum dots
We study the transport properties of a serial double quantum dot (DQD)
coupled to two superconducting leads, focusing on the Josephson current through
the DQD and the associated 0- transitions which result from the subtle
interplay between the superconductivity, the Kondo physics, and the inter-dot
superexchange interaction. We examine the competition between the
superconductivity and the Kondo physics by tuning the relative strength
of the superconducting gap and the Kondo temperature
, for different strengths of the superexchange coupling determined by the
interdot tunneling relative to the dot level broadening . We find
strong renormalization of , a significant role of the superexchange coupling
, and a rich phase diagram of the 0 and -junction regimes. In
particular, when both the superconductivity and the exchange interaction are in
close competion with the Kondo physics (), there appears
an island of -phase at large values of the superconducting phase
difference.Comment: 4 pages, 4 figure
Hawking Radiation for Scalar and Dirac Fields in Five Dimensional Dilatonic Black Hole via Anomalies
We study massive scalar fields and Dirac fields propagating in a five
dimensional dilatonic black hole background. We expose that for both fields the
physics can be describe by a two dimensional theory, near the horizon. Then, in
this limit, by applying the covariant anomalies method we find the Hawking flux
by restoring the gauge invariance and the general coordinate covariance, which
coincides with the flux obtained from integrating the Planck distribution for
fermions.Comment: 10 page
Enriching gender in PER: A binary past and a complex future
In this article, we draw on previous reports from physics, science education,
and women's studies to propose a more nuanced treatment of gender in physics
education research (PER). A growing body of PER examines gender differences in
participation, performance, and attitudes toward physics. We have three
critiques of this work: (1) it does not question whether the achievements of
men are the most appropriate standard, (2) individual experiences and student
identities are undervalued, and (3) the binary model of gender is not
questioned. Driven by these critiques, we propose a conception of gender that
is more up-to-date with other fields and discuss gender-as-performance as an
extended example. We also discuss work on the intersection of identities [e.g.,
gender with race and ethnicity, socioeconomic status, lesbian, gay, bisexual,
and transgender (LGBT) status], much of which has been conducted outside of
physics. Within PER, some studies examine the intersection of gender and race,
and identify the lack of a single identity as a key challenge of "belonging" in
physics. Acknowledging this complexity enables us to further critique what we
term a binary gender deficit model. This framework, which is implicit in much
of the gender-based PER, casts gender as a fixed binary trait and suggests that
women are deficient in characteristics necessary to succeed. Alternative models
of gender allow a greater range and fluidity of gender identities, and
highlight deficiencies in data that exclude women's experiences. We suggest new
investigations that diverge from this expanded gender framework in PER.Comment: 27 pages, accepted to Phys. Rev. Special Topics - PE
Multi-filter transit observations of WASP-39b and WASP-43b with three San Pedro M\'artir telescopes
Three optical telescopes located at the San Pedro M\'artir National
Observatory were used for the first time to obtain multi-filter defocused
photometry of the transiting extrasolar planets WASP-39b and WASP-43b. We
observed WASP-39b with the 2.12m telescope in the U filter for the first time,
and additional observations were carried out in the R and I filters using the
0.84m telescope. WASP-43b was observed in VRI with the same instrument, and in
the i filter with the robotic 1.50m telescope. We reduced the data using
different pipelines and performed aperture photometry with the help of custom
routines, in order to obtain the light curves. The fit of the light curves
(1.5--2.5mmag rms), and of the period analysis, allowed a revision of the
orbital and physical parameters, revealing for WASP-39b a period ( days) which is seconds larger than
previously reported. Moreover, we find for WASP-43b a planet/star radius
() which is larger in the i filter
with respect to previous works, and that should be confirmed with additional
observations. Finally, we confirm no evidence of constant period variations in
WASP-43b.Comment: 13 pages, 7 figures, accepted in PASP, scheduled for the February 1,
2015 issu
Josephson current through a Kondo molecule
We investigate transport of Cooper pairs through a double quantum dot (DQD)
in the Kondo regime and coupled to superconducting leads. Within the
non-perturbative slave boson mean-field theory we evaluate the Josephson
current for two different configurations, the DQD coupled in parallel and in
series to the leads. We find striking differences between these configurations
in the supercurrent as a function of the ratio t/\Gamma, where t is the
interdot coupling and \Gamma is the coupling to the leads: the critical current
I_c decreases monotonously with t/\Gamma for the parallel configuration whereas
I_c exhibits a maximum at t/\Gamma=1 in the serial case. These results
demonstrate that a variation of the ratio t/\Gamma enables to control the flow
of supercurrent through the Kondo resonance of the DQD.Comment: 5 pages, 4 figure
Model of Low-pass Filtering of Local Field Potentials in Brain Tissue
Local field potentials (LFPs) are routinely measured experimentally in brain
tissue, and exhibit strong low-pass frequency filtering properties, with high
frequencies (such as action potentials) being visible only at very short
distances (10~) from the recording electrode. Understanding
this filtering is crucial to relate LFP signals with neuronal activity, but not
much is known about the exact mechanisms underlying this low-pass filtering. In
this paper, we investigate a possible biophysical mechanism for the low-pass
filtering properties of LFPs. We investigate the propagation of electric fields
and its frequency dependence close to the current source, i.e. at length scales
in the order of average interneuronal distance. We take into account the
presence of a high density of cellular membranes around current sources, such
as glial cells. By considering them as passive cells, we show that under the
influence of the electric source field, they respond by polarisation, i.e.,
creation of an induced field. Because of the finite velocity of ionic charge
movement, this polarization will not be instantaneous. Consequently, the
induced electric field will be frequency-dependent, and much reduced for high
frequencies. Our model establishes that with respect to frequency attenuation
properties, this situation is analogous to an equivalent RC-circuit, or better
a system of coupled RC-circuits. We present a number of numerical simulations
of induced electric field for biologically realistic values of parameters, and
show this frequency filtering effect as well as the attenuation of
extracellular potentials with distance. We suggest that induced electric fields
in passive cells surrounding neurons is the physical origin of frequency
filtering properties of LFPs.Comment: 10 figs, revised tex file and revised fig
Comprehensive transient-state study for CARMENES-NIR high thermal stability
CARMENES has been proposed as a next-generation instrument for the 3.5m Calar
Alto Telescope. Its objective is finding habitable exoplanets around M dwarfs
through radial velocity measurements (m/s level) in the near-infrared.
Consequently, the NIR spectrograph is highly constraint regarding
thermal/mechanical requirements. As a first approach, the thermal stability has
been limited to \pm 0.01K (within year period) over a working temperature of
243K. This can be achieved by means of several temperature-controlled rooms.
The options considered to minimise the complexity of the thermal design are
here presented, as well as the transient-state thermal analyses realised to
make the best choice
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