1,019 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
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
Relativistic Winds from Compact Gamma-ray Sources: I. Radiative Acceleration in the Klein-Nishina Regime
We consider the radiative acceleration to relativistic bulk velocities of a
cold, optically thin plasma which is exposed to an external source of
gamma-rays. The flow is driven by radiative momentum input to the gas, the
accelerating force being due to Compton scattering in the relativistic
Klein-Nishina limit. The bulk Lorentz factor of the plasma, Gamma, derived as a
function of distance from the radiating source, is compared with the
corresponding result in the Thomson limit. Depending on the geometry and
spectrum of the radiation field, we find that particles are accelerated to the
asymptotic Lorentz factor at infinity much more rapidly in the relativistic
regime; and the radiation drag is reduced as blueshifted, aberrated photons
experience a decreased relativistic cross section and scatter preferentially in
the forward direction. The random energy imparted to the plasma by gamma-rays
can be converted into bulk motion if the hot particles execute many Larmor
orbits before cooling. This `Compton afterburn' may be a supplementary source
of momentum if energetic leptons are injected by pair creation, but can be
neglected in the case of pure Klein-Nishina scattering. Compton drag by
side-scattered radiation is shown to be more important in limiting the bulk
Lorentz factor than the finite inertia of the accelerating medium. The
processes discussed here may be relevant to a variety of astrophysical
situations where luminous compact sources of hard X- and gamma-ray photons are
observed, including active galactic nuclei, galactic black hole candidates, and
gamma-ray bursts.Comment: LateX, 20 pages, 5 figures, revised version accepted for publication
in the Ap
Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity
The precision of photometric and spectroscopic observations has been
systematically improved in the last decade, mostly thanks to space-borne
photometric missions and ground-based spectrographs dedicated to finding
exoplanets. The field of eclipsing binary stars strongly benefited from this
development. Eclipsing binaries serve as critical tools for determining
fundamental stellar properties (masses, radii, temperatures and luminosities),
yet the models are not capable of reproducing observed data well either because
of the missing physics or because of insufficient precision. This led to a
predicament where radiative and dynamical effects, insofar buried in noise,
started showing up routinely in the data, but were not accounted for in the
models. PHOEBE (PHysics Of Eclipsing BinariEs; http://phoebe-project.org) is an
open source modeling code for computing theoretical light and radial velocity
curves that addresses both problems by incorporating missing physics and by
increasing the computational fidelity. In particular, we discuss triangulation
as a superior surface discretization algorithm, meshing of rotating single
stars, light time travel effect, advanced phase computation, volume
conservation in eccentric orbits, and improved computation of local intensity
across the stellar surfaces that includes photon-weighted mode, enhanced limb
darkening treatment, better reflection treatment and Doppler boosting. Here we
present the concepts on which PHOEBE is built on and proofs of concept that
demonstrate the increased model fidelity.Comment: 60 pages, 15 figures, published in ApJS; accompanied by the release
of PHOEBE 2.0 on http://phoebe-project.or
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
Limits on the Boron Isotopic Ratio in HD 76932
Data in the 2090 A B region of HD 76932 have been obtained at high S/N using
the HST GHRS echelle at a resolution of 90,000. This wavelength region has been
previously identified as a likely candidate for observing the B11/B10 isotopic
splitting.
The observations do not match a calculated line profile extremely well at any
abundance for any isotopic ratio. If the B abundance previously determined from
observations at 2500 A is assumed, the calculated line profile is too weak,
indicating a possible blending line. Assuming that the absorption at 2090 A is
entirely due to boron, the best-fit total B abundance is higher than but
consistent with that obtained at 2500 A, and the best-fit isotopic ratio
(B11/B10) is in the range ~10:1 to ~4:1. If the absorption is not entirely due
to B and there is an unknown blend, the best-fit isotopic ratio may be closer
to 1:1. Future observations of a similar metal-poor star known to have
unusually low B should allow us to distinguish between these two possibilities.
The constraints that can be placed on the isotopic ratio based on comparisons
with similar observations of HD 102870 and HD 61421 (Procyon) are also
discussed.Comment: Accepted for Nov 1998 Ap
Surrogate and reduced-order modeling: a comparison of approaches for large-scale statistical inverse problems [Chapter 7]
Solution of statistical inverse problems via the frequentist or Bayesian approaches described in earlier chapters can be a computationally intensive endeavor, particularly when faced with large-scale forward models characteristic of many engineering and science applications. High computational cost arises in several ways. First, thousands or millions of forward simulations may be required to evaluate estimators of interest or to characterize a posterior distribution. In the large-scale setting, performing so many forward simulations is often computationally intractable. Second, sampling may be complicated by the large dimensionality of the input space--as when the inputs are fields represented with spatial discretizations of high dimension--and by nonlinear forward dynamics that lead to multimodal, skewed, and/or strongly correlated posteriors. In this chapter, we present an overview of surrogate and reduced order modeling methods that address these computational challenges. For illustration, we consider a Bayesian formulation of the inverse problem. Though some of the methods we review exploit prior information, they largely focus on simplifying or accelerating evaluations of a stochastic model for the data, and thus are also applicable in a frequentist context.Sandia National Laboratories (Laboratory Directed Research and Development (LDRD) program)United States. Dept. of Energy (Contract DE-AC04-94AL85000)Singapore-MIT Alliance Computational Engineering ProgrammeUnited States. Dept. of Energy (Award Number DE-FG02-08ER25858 )United States. Dept. of Energy (Award Number DESC00025217
Kepler Eclipsing Binary Stars. V. Identification of 31 Eclipsing Binaries in the K2 Engineering Data-set
Over 2500 eclipsing binaries were identified and characterized from the
ultra-precise photometric data provided by the Kepler space telescope. Kepler
is now beginning its second mission, K2, which is proving to again provide
ultra-precise photometry for a large sample of eclipsing binary stars. In the
1951 light curves covering 12 days in the K2 engineering data-set, we have
identified and determined the ephemerides for 31 eclipsing binaries that
demonstrate the capabilities for eclipsing binary science in the upcoming
campaigns in K2. Of those, 20 are new discoveries. We describe both manual and
automated approaches to harvesting the complete set of eclipsing binaries in
the K2 data, provide identifications and details for the full set of eclipsing
binaries present in the engineering data-set, and discuss the prospects for
application of eclipsing binary searches in the K2 mission.Comment: 12 pages, 2 figures, submitted to PAS
The Diffuse Gamma-Ray Background from Supernovae
The Cosmic Gamma-ray Background (CGB) in the MeV region is believed to be due
to photons from radioactivity produced in SNe throughout the history of
galaxies in the universe. In particular, gamma-ray line emission from the decay
chain 56Ni-> 56Co->56Fe provides the dominant photon source. Although iron
synthesis occurs in all types of SNe, the contribution to the CGB is dominated
by SNIa events due to their higher photon escape probabilities. Estimates of
the star formation history in the universe suggest a rapid increase by a factor
\~ 10 from the present to a redshift z_p ~ 1.5, beyond which it either remains
constant or decreases slowly. We integrate the observed star formation history
to determine the CGB from the corresponding SN rate history. In addition to
gamma-rays from short-lived radioactivity in SNIa and SNII/Ibc we also
calculate the minor contributions from long-lived radioactivities (26Al, 44Ti,
60Co, and electron-positron pair annihilation). Although progenitor evolution
for SNIa is not yet fully understood, various arguments suggest delays of order
1-2 Gy between star formation and the production of SNIa's. The effect of this
delay on the CGB is discussed. We emphasize the value of gamma-ray observations
of the CGB in the MeV range as an independent tool for studies of the cosmic
star formation history. If the delay between star formation and SNIa activity
exceeds 1 Gy substantially, and/or the peak of the cosmic star formation rate
occurs at a redshift much larger than unity, the gamma-ray production of SNIa
would be insufficient to explain the observed CGB. Alternatively, the cosmic
star formation rate would have to be higher (by a factor 2-3) than commonly
assumed, which is in accord with several upward revisions reported in the
recent literature.Comment: Minor changes, 26 pages, 9 figures, Accepted by Ap
Cyg X-3: Not seen in high-energy gamma rays by COS-B
COS-B had Cyg X-3 within its field of view during 7 observation periods between 1975 and 1982 for in total approximately 300 days. In the skymaps (70 meV E 5000 meV) of the Cyg-X region produced for each of these observations and in the summed map, a broad complex structure is visible in the region 72 deg approximately less than 1 approximately less than 85 deg, approximately less than 5 deg. No resolved source structure is visible at the position of Cyg X-3, but a weak signal from Cyg X-3 could be hidden in the structured gamma-ray background. Therefore, the data has been searched for a 4.8 h timing signature, as well as for a source signal in the sky map in addition to the diffuse background structure as estimated from tracers of atomic and molecular gas
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