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New methodology to measure the dynamics of ocular wave front aberrations during small amplitude changes of accommodation
We present a methodology to measure the systematic changes of aberrations induced by small changes in amplitude of accommodation. We use a method similar the one used in electrophysiology, where a periodic stimulus is presented to the eye and many periods (epochs) of the stimulus are averaged. Using this technique we have measured changes in higher order aberrations from 0.006μm to 0.02μm and correlated them with amplitude changes of accommodation as small as 0.14D. These small changes would have been undetectable without epoch averaging. The correlation coefficients of Zernike terms with defocus were calculated, demonstrating higher values of correlation for epoch averaging. The accurate monitoring of defocus at the start of the accommodation response has shown some interesting trends that may be related with the mechanisms behind accommodation
Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K
are covered by ice mantles. A nonthermal desorption mechanism is invoked to
explain the presence of gas-phase molecules in these environments, such as the
photodesorption induced by irradiation of ice due to secondary ultraviolet
photons. To quantify the effects of ice photoprocessing, an estimate of the
photon absorption in ice mantles is required. In a recent work, we reported the
vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the
solid phase. The aim was to estimate the VUV-absorption cross sections of
nonpolar molecular ice components, including CH4, CO2, N2, and O2. The column
densities of the ice samples deposited at 8 K were measured in situ by infrared
spectroscopy in transmittance. VUV spectra of the ice samples were collected in
the 120-160 nm (10.33-7.74 eV) range using a commercial microwave-discharged
hydrogen flow lamp. We found that, as expected, solid N2 has the lowest
VUV-absorption cross section, which about three orders of magnitude lower than
that of other species such as O2, which is also homonuclear. Methane (CH4) ice
presents a high absorption near Ly-alpha (121.6 nm) and does not absorb below
148 nm. Estimating the ice absorption cross sections is essential for models of
ice photoprocessing and allows estimating the ice photodesorption rates as the
number of photodesorbed molecules per absorbed photon in the ice.Comment: 9 pages, 6 figures, 7 table
Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4
In both spinel compounds GeCoO and GeNiO which order
antiferromagnetically (at and , ) with different Curie Weiss temperatures (=80.5 K and -15 K),
the usual magnetic frustration criterion is not fulfilled.
Using neutron powder diffraction and magnetization measurements up to 55 T,
both compounds are found with a close magnetic ground state at low temperature
and a similar magnetic behavior (but with a different energy scale), even
though spin anisotropy and first neighbor exchange interactions are quite
different. This magnetic behavior can be understood when considering the main
four magnetic exchange interactions. Frustration mechanisms are then
enlightened.Comment: submitted to Phys.Rev.B (2006
XMM-Newton EPIC and OM observation of Nova Centauri 1986 (V842 Cen)
We report the results from the temporal and spectral analysis of an
XMM-Newton observation of Nova Centauri 1986 (V842 Cen). We detect a period at
3.510.4 h in the EPIC data and at 4.00.8 h in the OM data. The X-ray
spectrum is consistent with the emission from an absorbed thin thermal plasma
with a temperature distribution given by an isobaric cooling flow. The maximum
temperature of the cooling flow model is keV. Such a
high temperature can be reached in a shocked region and, given the periodicity
detected, most likely arises in a magnetically-channelled accretion flow
characteristic of intermediate polars. The pulsed fraction of the 3.51 h
modulation decreases with energy as observed in the X-ray light curves of
magnetic CVs, possibly due either to occultation of the accretion column by the
white dwarf body or phase-dependent to absorption. We do not find the 57 s
white dwarf spin period, with a pulse amplitude of 4 mmag, reported by Woudt et
al. (2009) either in the Optical Monitor (OM) data, which are sensitive to
pulse amplitudes 0.03 magnitudes, or the EPIC data, sensitive to
pulse fractions 14 2%.Comment: 5 pages, 3 figures; MNRAS, accepte
No temperature fluctuations in the giant HII region H 1013
While collisionally excited lines in HII regions allow one to easily probe
the chemical composition of the interstellar medium in galaxies, the possible
presence of important temperature fluctuations casts some doubt on the derived
abundances. To provide new insights into this question, we have carried out a
detailed study of a giant HII region, H 1013, located in the galaxy M101, for
which many observational data exist and which has been claimed to harbour
temperature fluctuations at a level of t^2 = 0.03-0.06. We have first
complemented the already available optical observational datasets with a
mid-infrared spectrum obtained with the Spitzer Space Telescope. Combined with
optical data, this spectrum provides unprecedented information on the
temperature structure of this giant HII region. A preliminary analysis based on
empirical temperature diagnostics suggests that temperature fluctuations should
be quite weak. We have then performed a detailed modelling using the pyCloudy
package based on the photoionization code Cloudy. We have been able to produce
photoionization models constrained by the observed Hb surface brightness
distribution and by the known properties of the ionizing stellar population
than can account for most of the line ratios within their uncertainties. Since
the observational constraints are both strong and numerous, this argues against
the presence of significant temperature fluctuations in H 1013. The oxygen
abundance of our best model is 12 + log O/H = 8.57, as opposed to the values of
8.73 and 8.93 advocated by Esteban et al. (2009) and Bresolin (2007),
respectively, based on the significant temperature fluctuations they derived.
However, our model is not able to reproduce the intensities of the oxygen
recombination lines . This cannot be attributed to observational uncertainties
and requires an explanation other than temperature fluctuations.Comment: accepted in Astronomy & Astrophysic
A Gravitational Wave Background from Reheating after Hybrid Inflation
The reheating of the universe after hybrid inflation proceeds through the
nucleation and subsequent collision of large concentrations of energy density
in the form of bubble-like structures moving at relativistic speeds. This
generates a significant fraction of energy in the form of a stochastic
background of gravitational waves, whose time evolution is determined by the
successive stages of reheating: First, tachyonic preheating makes the amplitude
of gravity waves grow exponentially fast. Second, bubble collisions add a new
burst of gravitational radiation. Third, turbulent motions finally sets the end
of gravitational waves production. From then on, these waves propagate
unimpeded to us. We find that the fraction of energy density today in these
primordial gravitational waves could be significant for GUT-scale models of
inflation, although well beyond the frequency range sensitivity of
gravitational wave observatories like LIGO, LISA or BBO. However, low-scale
models could still produce a detectable signal at frequencies accessible to BBO
or DECIGO. For comparison, we have also computed the analogous gravitational
wave background from some chaotic inflation models and obtained results similar
to those found by other groups. The discovery of such a background would open a
new observational window into the very early universe, where the details of the
process of reheating, i.e. the Big Bang, could be explored. Moreover, it could
also serve in the future as a new experimental tool for testing the
Inflationary Paradigm.Comment: 22 pages, 18 figures, uses revtex
The accretion disk in the post period-minimum cataclysmic variable SDSS J080434.20+510349.2
This study of SDSS0804 is primarily concerned with the double-hump shape in
the light curve and its connection with the accretion disk in this bounce-back
system. Time-resolved photometric and spectroscopic observations were obtained
to analyze the behavior of the system between superoutbursts. A geometric model
of a binary system containing a disk with two outer annuli spiral density waves
was applied to explain the light curve and the Doppler tomography. Observations
were carried out during 2008-2009, after the object's magnitude decreased to
V~17.7(0.1) from the March 2006 eruption. The light curve clearly shows a
sinusoid-like variability with a 0.07 mag amplitude and a 42.48 min
periodicity, which is half of the orbital period of the system. In Sept. 2010,
the system underwent yet another superoutburst and returned to its quiescent
level by the beginning of 2012. This light curve once again showed a
double-humps, but with a significantly smaller ~0.01mag amplitude. Other types
of variability like a "mini-outburst" or SDSS1238-like features were not
detected. Doppler tomograms, obtained from spectroscopic data during the same
period of time, show a large accretion disk with uneven brightness, implying
the presence of spiral waves. We constructed a geometric model of a bounce-back
system containing two spiral density waves in the outer annuli of the disk to
reproduce the observed light curves. The Doppler tomograms and the
double-hump-shape light curves in quiescence can be explained by a model system
containing a massive >0.7Msun white dwarf with a surface temperature of
~12000K, a late-type brown dwarf, and an accretion disk with two outer annuli
spirals. According to this model, the accretion disk should be large, extending
to the 2:1 resonance radius, and cool (~2500K). The inner parts of the disk
should be optically thin in the continuum or totally void.Comment: 12 pages, 15 figures, accepted for publication in A&
Evolution of Phase-Space Density in Dark Matter Halos
The evolution of the phase-space density profile in dark matter (DM) halos is
investigated by means of constrained simulations, designed to control the
merging history of a given DM halo. Halos evolve through a series of quiescent
phases of a slow accretion intermitted by violent events of major mergers. In
the quiescent phases the density of the halo closely follows the NFW profile
and the phase-space density profile, Q(r), is given by the Taylor & Navarro
power law, r^{-beta}, where beta ~ 1.9 and stays remarkably stable over the
Hubble time. Expressing the phase-space density by the NFW parameters, Q(r)=Qs
(r/Rs)^{-beta}, the evolution of Q is determined by Qs. We have found that the
effective mass surface density within Rs, Sigma_s = rhos Rs, remains constant
throughout the evolution of a given DM halo along the main branch of its
merging tree. This invariance entails that Qs ~ Rs^{-5/2} and Q(r) ~
Sigma_s^{-1/2} Rs^{-5/2} (r/ Rs)^{-beta}. It follows that the phase-space
density remains constant, in the sense of Qs=const., in the quiescent phases
and it decreases as Rs^{-5/2} in the violent ones. The physical origin of the
NFW density profile and the phase-space density power law is still unknown.
Yet, the numerical experiments show that halos recover these relations after
the violent phases. The major mergers drive Rs to increase and Qs to decrease
discontinuously while keeping Qs Rs^{5/2} = const. The virial equilibrium in
the quiescent phases implies that a DM halos evolves along a sequence of NFW
profiles with constant energy per unit volume (i.e., pressure) within Rs.Comment: 7 pages, 5 figures, accepted by the Astrophysical Journal. Revised, 2
figures adde
Dipolar glass polymers containing polarizable groups as dielectric materials for energy storage applications. A minireview
Materials that have high dielectric constants, high energy densities and minimum dielectric losses are highly desirable for use in capacitor devices. In this sense, polymers and polymer blends have several advantages over inorganic and composite materials, such as their flexibilities, high breakdown strengths, and low dielectric losses. Moreover, the dielectric performance of a polymer depends strongly on its electronic, atomic, dipolar, ionic, and interfacial polarizations. For these reasons, chemical modification and the introduction of specific functional groups (e.g., F, CN and R−S(=O)2−R´) would improve the dielectric properties, e.g., by varying the dipolar polarization. These functional groups have been demonstrated to have large dipole moments. In this way, a high orientational polarization in the polymer can be achieved. However, the decrease in the polarization due to dielectric dissipation and the frequency dependency of the polarization are challenging tasks to date. Polymers with high glass transition temperatures (Tg) that contain permanent dipoles can help to reduce dielectric losses due to conduction phenomena related to ionic mechanisms. Additionally, sub-Tg transitions (e.g., γ and β relaxations) attributed to the free rotational motions of the dipolar entities would increase the polarization of the material, resulting in polymers with high dielectric constants and, hopefully, dielectric losses that are as low as possible. Thus, polymer materials with high glass transition temperatures and considerable contributions from the dipolar polarization mechanisms of sub-Tg transitions are known as “dipolar glass polymers”. Considering this, the main aspects of this combined strategy and the future prospects of these types of material were discussed
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