1,452 research outputs found
A New Limit on the Antiproton Lifetime
Measurements of the cosmic ray pbar/p ratio are compared to predictions from
an inhomogeneous disk-diffusion model of pbar production and propagation within
the Galaxy, combined with a calculation of the modulation of the interstellar
cosmic ray spectra as the particles propagate through the heliosphere to the
Earth. The predictions agree with the observed pbar/p spectrum. Adding a finite
pbar lifetime to the model, we obtain the limit tau_pbar > 0.8 Myr (90 % C.L.).Comment: 13 pages, 3 encapsulated Postscript figures, uses AASTeX; accepted by
Astrophysical Journal; minor change
Physics of Eclipsing Binaries: Modelling in the new era of ultra-high precision photometry
Recent ultra-high precision observations of eclipsing binaries, especially data acquired by the Kepler satellite, have made accurate light curve modelling increasingly challenging but also more rewarding. In this contribution, we discuss low-amplitude signals in light curves that can now be used to derive physical information about eclipsing binaries but that were unaccessible before the Kepler era. A notable example is the detection of Doppler beaming, which leads to an increase in flux when a star moves towards the satellite and a decrease in flux when it moves away. Similarly, Rømer delays, or light travel time effects, also have to taken into account when modelling the supreme quality data that is now available. The detection of offsets between primary and secondary eclipse phases in binaries with extreme mass ratios, and the observation of Rømer delays in the signals of pulsators in binary stars, have allowed us to determine the orbits of several binaries without the need for spectroscopy. A third example of a small-scale effect that has to be taken into account when modelling specific binary systems, are lensing effects. A new binary light curve modelling code, PHOEBE 2.0, that takes all these effect into account is currently being developed
The final COS-B database: In-flight calibration of instrumental parameters
A method for the determination of temporal variation of sensitivity is designed to find a set of parameters which lead to maximum consistency between the intensities derived from different observation periods. This method is briefly described and the resulting sensitivity and background variations presented
Diffractive Interaction and Scaling Violation in pp->pi^0 Interaction and GeV Excess in Galactic Diffuse Gamma-Ray Spectrum of EGRET
We present here a new calculation of the gamma-ray spectrum from pp->pi^0 in
the Galactic ridge environment. The calculation includes the diffractive pp
interaction and incorporates the Feynman scaling violation for the first time.
Galactic diffuse gamma-rays come, predominantly, from pi^0->gamma gamma in the
sub-GeV to multi-GeV range. Hunter et al. found, however, an excess in the GeV
range ("GeV Excess") in the EGRET Galactic diffuse spectrum above the
prediction based on experimental pp->pi^0 cross-sections and the Feynman
scaling hypothesis. We show, in this work, that the diffractive process makes
the gamma-ray spectrum harder than the incident proton spectrum by ~0.05 in
power-law index, and, that the scaling violation produces 30-80% more pi^0 than
the scaling model for incident proton energies above 100GeV. Combination of the
two can explain about a half of the "GeV Excess" with the local cosmic proton
(power-law index ~2.7). The excess can be fully explained if the proton
spectral index in the Galactic ridge is a little harder (~0.2 in power-law
index) than the local spectrum. Given also in the paper is that the diffractive
process enhances e^+ over e^- and the scaling violation gives 50-100% higher
p-bar yield than without the violation, both in the multi-GeV range.Comment: 35 pages, 11 figures, to appear in Astrophysical Journa
Investigating magnetic activity of F stars with the it Kepler mission
The dynamo process is believed to drive the magnetic activity of stars like
the Sun that have an outer convection zone. Large spectroscopic surveys showed
that there is a relation between the rotation periods and the cycle periods:
the longer the rotation period is, the longer the magnetic activity cycle
period will be. We present the analysis of F stars observed by Kepler for which
individual p modes have been measure and with surface rotation periods shorter
than 12 days. We defined magnetic indicators and proxies based on photometric
observations to help characterise the activity levels of the stars. With the
Kepler data, we investigate the existence of stars with cycles (regular or
not), stars with a modulation that could be related to magnetic activity, and
stars that seem to show a flat behaviour.Comment: 2 pages, 1 figure, proceedings of IAU Symposium 302 'Magnetic fields
through stellar evolution', 25-30 August 2013, Biarritz, Franc
The blue-edge problem of the V1093 Her instability strip revisited using evolutionary models with atomic diffusion
We have computed a new grid of evolutionary subdwarf B star (sdB) models from
the start of central He burning, taking into account atomic diffusion due to
radiative levitation, gravitational settling, concentration diffusion, and
thermal diffusion. We have computed the non-adiabatic pulsation properties of
the models and present the predicted p-mode and g-mode instability strips. In
previous studies of the sdB instability strips, artificial abundance
enhancements of Fe and Ni were introduced in the pulsation driving layers. In
our models, the abundance enhancements of Fe and Ni occur naturally,
eradicating the need to use artificial enhancements. We find that the abundance
increases of Fe and Ni were previously underestimated and show that the
instability strip predicted by our simulations solves the so-called blue edge
problem of the subdwarf B star g-mode instability strip. The hottest known
g-mode pulsator, KIC 10139564, now resides well within the instability strip
{even when only modes with low spherical degrees (l<=2) are considered.Comment: 7 pages, 7 figures. Accepted for publication in Astronomy &
Astrophysic
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