37,641 research outputs found
Effects of nuclear molecular configurations on the astrophysical S-factor for O + O
The impact of nuclear molecular configurations on the astrophysical S-factor
for O + O is investigated within the realistic two-center shell
model based on Woods-Saxon potentials. These molecular effects refer to the
formation of a neck between the interacting nuclei and the radial dependent
collective mass parameter. It is demonstrated that the former is crucial to
explain the current experimental data with high accuracy and without any free
parameter, whilst in addition the latter predicts a pronounced maximum in the
S-factor. In contrast to very recent results by Jiang et al., the S-factor does
not decline towards extremely low values as energy decreases.Comment: In press in Physics Letters
Conformal mapping of ultrasonic crystals: confining ultrasound and cochlear-like wave guiding
Conformal mapping of a slab of a two-dimensional ultrasonic crystal generate
a closed geometrical arrangement of ultrasonic scatterers with appealing
acoustic properties. This acoustic shell is able to confine ultrasonic modes.
Some of these internal resonances can be induced from an external wave source.
The mapping of a linear defect produces a wave-guide that exhibits a
spatial-frequency selection analogous to that characteristic of a synthetic
"cochlea". Both, experimental and theoretical results are reported here.Comment: 4 pages, 4 figure
How does breakup influence the total fusion of Li at the Coulomb barrier?
Total (complete + incomplete) fusion excitation functions of Li on
Co and Bi targets around the Coulomb barrier are obtained using
a new continuum discretized coupled channel (CDCC) method of calculating
fusion. The relative importance of breakup and bound-state structure effects on
total fusion is particularly investigated. The effect of breakup on fusion can
be observed in the total fusion excitation function. The breakup enhances the
total fusion at energies just around the barrier, whereas it hardly affects the
total fusion at energies well above the barrier. The difference between the
experimental total fusion cross sections for Li on Co is notably
caused by breakup, but this is not the case for the Bi target.Comment: 9 pages, 9 figures, Submitted to Phys. Rev.
A search for new hot subdwarf stars by means of Virtual Observatory tools
Hot subdwarf stars are faint, blue objects, and are the main contributors to
the far-UV excess observed in elliptical galaxies. They offer an excellent
laboratory to study close and wide binary systems, and to scrutinize their
interiors through asteroseismology, as some of them undergo stellar
oscillations. However, their origins are still uncertain, and increasing the
number of detections is crucial to undertake statistical studies. In this work,
we aim at defining a strategy to find new, uncatalogued hot subdwarfs. Making
use of Virtual Observatory tools we thoroughly search stellar catalogues to
retrieve multi-colour photometry and astrometric information of a known sample
of blue objects, including hot subdwarfs, white dwarfs, cataclysmic variables
and main sequence OB stars. We define a procedure to discriminate among these
spectral classes, particularly designed to obtain a hot subdwarf sample with a
low contamination factor. In order to check the validity of the method, this
procedure is then applied to two test sky regions: the Kepler FoV and to a test
region of around (RA:225, DEC:5) deg. As a result, we obtained 38 hot subdwarf
candidates, 23 of which had already a spectral classification. We have acquired
spectroscopy for three other targets, and four additional ones have an
available SDSS spectrum, which we used to determine their spectral type. A
temperature estimate is provided for the candidates based on their spectral
energy distribution, considering two-atmospheres fit for objects with clear
infrared excess. Eventually, out of 30 candidates with spectral classification,
26 objects were confirmed to be hot subdwarfs, yielding a contamination factor
of only 13%. The high rate of success demonstrates the validity of the proposed
strategy to find new uncatalogued hot subdwarfs. An application of this method
to the entire sky will be presented in a forthcoming work.Comment: 13 pages, 7 figure
Discrete-Time Fractional Variational Problems
We introduce a discrete-time fractional calculus of variations on the time
scale , . First and second order necessary optimality
conditions are established. Examples illustrating the use of the new
Euler-Lagrange and Legendre type conditions are given. They show that solutions
to the considered fractional problems become the classical discrete-time
solutions when the fractional order of the discrete-derivatives are integer
values, and that they converge to the fractional continuous-time solutions when
tends to zero. Our Legendre type condition is useful to eliminate false
candidates identified via the Euler-Lagrange fractional equation.Comment: Submitted 24/Nov/2009; Revised 16/Mar/2010; Accepted 3/May/2010; for
publication in Signal Processing
SACY - a Search for Associations Containing Young stars
The scientific goal of the SACY (Search for Associations Containing
Young-stars) was to identify possible associations of stars younger than the
Pleiades Association among optical counterparts of the ROSAT X-ray bright
sources. High-resolution spectra for possible optical counterparts later than
G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to
assess both the youth and the spatial motion of each target. More than 1000
ROSAT sources were observed, covering a large area in the Southern Hemisphere.
The newly identified young stars present a patchy distribution in UVW and XYZ,
revealing the existence of huge nearby young associations. Here we present the
associations identified in this survey.Comment: 8 pages, 2 figures, to appear in the Proceedings of Open Issues in
Local Formation and Early Stellar Evolution, Ouro Preto, Brazi
The light curve of the companion to PSR B1957+20
We present a new analysis of the light curve for the secondary star in the
eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous
data and new data points at minimum from the Hubble Space Telescope, we have
100% coverage in the R-band. We also have a number of new K_s-band data points,
which we use to constrain the infrared magnitude of the system. We model this
with the Eclipsing Light Curve code (ELC). From the modelling with the ELC code
we obtain colour information about the secondary at minimum light in BVRI and
K. For our best fit model we are able to constrain the system inclination to 65
+/- 2 degrees for pulsar masses ranging from 1.3 -- 1.9 M_sun. The pulsar mass
is unconstrained. We also find that the secondary star is not filling its Roche
lobe. The temperature of the un-irradiated side of the companion is in
agreement with previous estimates and we find that the observed temperature
gradient across the secondary star is physically sustainable.Comment: 6 pages, 4 figures & 3tables. Accepted for publication in MNRA
Time-resolved spectroscopy of the pulsating CV GW Lib
We present time-resolved optical spectroscopy of the dwarf nova GW Librae during its rare 2007 April superoutburst and compare these with quiescent epochs. The data provide the first opportunity to track the evolution of the principal spectral features. In the early stages of the outburst, the optically thick disc dominates the optical and the line components show clear orbital radial velocity excursions. In the course of several weeks, optically thin regions become more prominent as strong emission lines replace the broad disc absorption.
Post-outburst spectroscopy covering the I band illustrates the advantages of Ca II relative to the commonly used Balmer lines when attempting to constrain binary parameters. Due to the lower ionization energy combined with smaller thermal and shear broadening of these lines, a sharp emission component is seen to be moving in between the accretion disc peaks in the Ca II line. No such component is visible in the Balmer lines. We interpret this as an emission component originating on the hitherto unseen mass donor star. This emission component has a mean velocity of similar to -15 +/- 5 km s(-1) which is associated with the systemic velocity., and a velocity semi-amplitude of K-em = 82.2 +/- 4.9 km s(-1). Doppler tomography reveals an asymmetric accretion disc, with the S-wave mapping to a sharp spot in the tomogram with a velocity consistent to what is obtained with line profile fitting. A centre of symmetry analysis of the disc component suggests a very small value for the WD orbital velocity K-1 as is also inferred from double Gaussian fits to the spectral lines.
While our conservative dynamical limits place a hard upper limit on the binary mass ratio of q < 0.23, we favour a significantly lower value near q similar to 0.06. Pulsation modelling suggests a white dwarf mass similar to 1 M-circle dot. This, paired with a low-mass donor, near the empirical sequence of an evolved cataclysmic variable close to the period bounce, appears to be consistent with all the observational constraints to date
- …