11,376 research outputs found
Nature of the low temperature ordering of Pr in PrBa_2Cu_3O_(6+x)
Theoretical model is presented to describe the anomalous ordered phase of Pr
ions in PrBa_2Cu_3O_(6+x) below T_Pr = 12-17 K. The model considers the Pr
multipole degrees of freedom and coupling between the Cu and Pr subsystems. We
identify the symmetry allowed coupling of Cu and Pr ions and conclude that only
an ab-plane Pr dipole ordering can explain the Cu spin rotation observed at
T_Pr by neutron diffraction by Boothroyd et al. [A. T. Boothroyd et al., Phys.
Rev. Lett. 78, 130 (1997)]. A substantial enhancement of the Pr ordering
temperature is shown to arise from the Cu-Pr coupling which is the key for the
anomalous magnetic behavior in PrBa_2Cu_3O_(6+x).Comment: 6 pages, 4 figure
Amplitude variations of modulated RV Tauri stars support the dust obscuration model of the RVb phenomenon
Context. RV Tauri-type variables are pulsating post-AGB stars that evolve
rapidly through the instability strip after leaving the Asymptotic Giant
Branch. Their light variability is dominated by radial pulsations. Members of
the RVb subclass show an additional variability in form of a long-term
modulation of the mean brightness, for which the most popular theories all
assume binarity and some kind of circumstellar dust. Here we address if the
amplitude modulations are consistent with the dust obscuration model. Aims. We
measure and interpret the overall changes of the mean amplitude of the
pulsations along the RVb variability. Methods. We compiled long-term
photometric data for RVb-type stars, including visual observations of the
American Association of Variable Star Observers, ground-based CCD photometry
from the OGLE and ASAS projects and ultra-precise space photometry of one star,
DF Cygni, from the Kepler space telescope. After converting all the
observations to flux units, we measure the cycle-to-cycle variations of the
pulsation amplitude and correlate them to the actual mean fluxes. Results. We
find a surprisingly uniform correlation between the pulsation amplitude and the
mean flux: they scale linearly with each other for a wide range of fluxes and
amplitudes. It means that the pulsation amplitude actually remains constant
when measured relative to the system flux level. The apparent amplitude
decrease in the faint states has long been noted in the literature but it was
always claimed to be difficult to explain with the actual models of the RVb
phenomenon. Here we show that when fluxes are used instead of magnitudes, the
amplitude attenuation is naturally explained by periodic obscuration from a
large opaque screen, one most likely corresponding to a circumbinary dusty disk
that surrounds the whole system.Comment: 8 pages, 6 figures, accepted for publication in A&
Beta-decay properties of neutron-rich Ca, Ti, and Cr isotopes
Beta-decay properties of neutron-rich Ca, Ti, and Cr isotopes are studied
within a deformed proton-neutron quasiparticle random-phase approximation. The
underlying mean field is described self-consistently from deformed Skyrme
Hartree-Fock calculations with pairing correlations. Residual spin-isospin
interactions in the particle-hole and particle-particle channels are also
included in the formalism. The energy distributions of the Gamow-Teller
strength, the beta-decay feedings, the beta-decay half-lives, and the
beta-delayed neutron emission probabilities are discussed and compared with
other theoretical results, as well as with the available experimental
information. The evolution of these nuclear beta-decay properties is
investigated in isotopic chains in a search for structural changes. A reliable
estimate of the beta-decay properties in this mass region is a valuable
information for evaluating decay rates in astrophysical scenarios.Comment: 11 pages, 12 figure
Attention modulates the processing of emotional expression triggered by foveal faces
To investigate whether the processing of emotional expression for faces presented within foveal vision is modulated by spatial attention, event-related potentials (ERPs) were recorded in response to stimulus arrays containing one fearful or neutral face at fixation, which was flanked by a pair of peripheral bilateral lines. When attention was focused on the central face, an enhanced positivity was elicited by fearful as compared to neutral faces. This effect started at 160 ms post-stimulus, and remained present for the remainder of the 700 ms analysis interval. When attention was directed away from the face towards the line pair, the initial phase of this emotional positivity remained present, but emotional expression effects beyond 220 ms post-stimulus were completely eliminated. These results demonstrate that when faces are presented foveally, the initial rapid stage of emotional expression processing is unaffected by attention. In contrast, attentional task instructions are effective in inhibiting later, more controlled stages of expression analysis
Evidence for fresh frost layer on the bare nucleus of comet Hale--Bopp at 32 AU distance
Here we report that the activity of comet Hale--Bopp ceased between late 2007
and March, 2009, at about 28 AU distance from the Sun. At that time the comet
resided at a distance from the Sun that exceeded the freeze-out distance of
regular comets by an order of magnitude. A Herschel Space Observatory PACS scan
was taken in mid-2010, in the already inactive state of the nucleus. The albedo
has been found to be surprisingly large (8.10.9%{}), which exceeds the
value known for any other comets. With re-reduction of archive HST images from
1995 and 1996, we confirm that the pre-perihelion albedo resembled that of an
ordinary comet, and was smaller by a factor of two than the post-activity
albedo. Our further observations with the Very Large Telescope (VLT) also
confirmed that the albedo increased significantly by the end of the activity.
We explain these observations by proposing gravitational redeposition of icy
grains towards the end of the activity. This is plausible for such a massive
body in a cold environment, where gas velocity is lowered to the range of the
escape velocity. These observations also show that giant comets are not just
the upscaled versions of the comets we know but can be affected by processes
that are yet to be fully identified.Comment: 7 pages, 6 figures, accepted for publication in Ap
Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278361) 2007 JJ43
We present the first photometric observations of trans-Neptunian objects
(TNOs) taken with the Kepler space telescope, obtained in the course of the K2
ecliptic survey. Two faint objects have been monitored in specifically designed
pixel masks that were centered on the stationary points of the objects, when
their daily motion was the slowest. In the design of the experiment, only the
apparent path of these objects were retrieved from the detectors, i.e. the
costs in terms of Kepler pixels were minimized. Because of the faintness of the
targets we employ specific reduction techniques and co-added images. We measure
rotational periods and amplitudes in the unfiltered Kepler band as follows: for
(278361) 2007 JJ43 and 2002 GV31 we get P_rot=12.097 h and P_rot=29.2 h while
0.10 and 0.35 mag for the total amplitudes, respectively. Future space
missions, like TESS and PLATO are not well suited to this kind of observations.
Therefore, we encourage to include the brightest TNOs around their stationary
points in each observing campaign to exploit this unique capability of the K2
Mission -- and therefore to provide unbiased rotational, shape and albedo
characteristics of many objects.Comment: Accepted for publication in ApJ Letters, 5.2 pages in emulateapj
style, misspelled 2007 JJ43 designation correcte
The 106Cd(α, α)106Cd elastic scattering in a wide energy range for γ process studies
Date of Acceptance: 15/04/2015Alpha elastic scattering angular distributions of the 106Cd(α, α)106Cd reaction were measured at three energies around the Coulomb barrier to provide a sensitive test for the α + nucleus optical potential parameter sets. Furthermore, the new high precision angular distributions, together with the data available from the literature were used to study the energy dependence of the locally optimized α + nucleus optical potential in a wide energy region ranging from ELab=27.0MeV down to 16.1 MeV.The potentials under study are a basic prerequisite for the prediction of α-induced reaction cross sections and thus, for the calculation of stellar reaction rates used for the astrophysical γ process. Therefore, statistical model predictions using as input the optical potentials discussed in the present work are compared to the available 106Cd + alpha cross section data.Peer reviewe
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