2,880 research outputs found
Molecular opacities for low-mass metal-poor AGB stars undergoing the Third Dredge Up
The concomitant overabundances of C, N and s-process elements are commonly
ascribed to the complex interplay of nucleosynthesis, mixing and mass loss
taking place in Asymptotic Giant Branch stars. At low metallicity, the
enhancement of C and/or N may be up to 1000 times larger than the original iron
content and significantly affects the stellar structure and its evolution. For
this reason, the interpretation of the already available and still growing
amount of data concerning C-rich metal-poor stars belonging to our Galaxy as
well as to dwarf spheroidal galaxies would require reliable AGB stellar models
for low and very low metallicities. In this paper we address the question of
calculation and use of appropriate opacity coefficients, which take into
account the C enhancement caused by the third dredge up. A possible N
enhancement, caused by the cool bottom process or by the engulfment of protons
into the convective zone generated by a thermal pulse and the subsequent huge
third dredge up, is also considered. Basing on up-to-date stellar models, we
illustrate the changes induced by the use of these opacity on the physical and
chemical properties expected for these stars.Comment: 23 pages, 8 figures, accepted for publication in Ap
Magneto-Roton Modes of the Ultra Quantum Crystal: Numerical Study
The Field Induced Spin Density Wave phases observed in quasi-one-dimensional
conductors of the Bechgaard salts family under magnetic field exhibit both Spin
Density Wave order and a Quantized Hall Effect, which may exhibit sign
reversals. The original nature of the condensed phases is evidenced by the
collective mode spectrum. Besides the Goldstone modes, a quasi periodic
structure of Magneto-Roton modes, predicted to exist for a monotonic sequence
of Hall Quantum numbers, is confirmed, and a second mode is shown to exist
within the single particle gap. We present numerical estimates of the
Magneto-Roton mode energies in a generic case of the monotonic sequence. The
mass anisotropy of the collective mode is calculated. We show how differently
the MR spectrum evolves with magnetic field at low and high fields. The
collective mode spectrum should have specific features, in the sign reversed
"Ribault Phase", as compared to modes of the majority sign phases. We
investigate numerically the collective mode in the Ribault Phase.Comment: this paper incorporates material contained in a previous cond-mat
preprint cond-mat/9709210, but cannot be described as a replaced version,
because it contains a significant amount of new material dealing with the
instability line and with the topic of Ribault Phases. It contains 13 figures
(.ps files
NASAs Orbital Debris JAO/ES-MCAT Optical Telescope Facility on Ascension Island
The NASA Orbital Debris Program Office has a long-standing optical program begun over three and a half decades ago in 1984, designed to observe the Earth-orbiting environment with optical telescopes. Photometrically calibrated optical data provides a statistical sample for input to NASA models of the debris population for understanding the current and future debris environment around the Earth. Tracked objects and orbits allow for analysis of break-up events. Both known (correlated target in the SSN catalogue, or CT) and unknown (uncorrelated target, or UCT) objects are of interest to better understand how to protect current spacecraft and design more robust future operational satellites, and advise on how policies and practices can lead to protecting the environment itself for future generations. In 2015, a joint NASA JSC Air Force Research Labs (AFRL) project culminated in the installation of the 1.3-meter Eugene Stansbery Meter Class Autonomous Telescope, ES-MCAT (a.k.a. MCAT) on Ascension Island. This DFM Engineering designed telescope provides nearly five-times greater light-collecting power than its predecessor, the 0.6-m MODEST telescope, and faster tracking capabilities by both the telescope and the 7-m ObservaDome. This allows for all orbital regimes to be easily within reach, ranging from low Earth to geosynchronous orbits. Extensive testing and commissioning activities of this custom system led to successfully reaching Initial Operational Capability in 2018, and the facility is currently on track to reach Full Operational Capability. The John Africano Observatory (JAO) comprises the primary 1.3-m ES-MCAT facility, the adjacent tower platform with a 0.4-m telescope, a sophisticated suite of weather instruments, and custom software by Euclid Research for autonomously running the entire system, including monitoring the weather and hardware, tasking all components, and collecting, processing, and analyzing the data. The mission of JAO and MCAT will be discussed, including survey and tracking tasking, a full discussion of data calibration, and both optics and weather-dependent performance
Stellar (n,γ) cross sections of ²³Na
The cross section of the ²³Na(n,γ)²⁴Na reaction has been measured via the activation method at the Karlsruhe 3.7 MV Van de Graaff accelerator. NaCl samples were exposed to quasistellar neutron spectra at kT = 5.1 and 25 keV produced via the ¹⁸O(p,n)¹⁸F and ⁷Li(p,n)⁷Be reactions, respectively. The derived capture cross sections (σ)kT=5keV = 9.1 ± 0.3mb and (σ)kT=25keV = 2.03 ± 0.05 mb are significantly lower than reported in literature. These results were used to substantially revise the radiative width of the first ²³Na resonance and to establish an improved set of Maxwellian average cross sections. The implications of the lower capture cross section for current models of s-process nucleosynthesis are discussed
Two-dimensional solitons with hidden and explicit vorticity in bimodal cubic-quintic media
We demonstrate that two-dimensional two-component bright solitons of an
annular shape, carrying vorticities in the components, may be
stable in media with the cubic-quintic nonlinearity, including the
\textit{hidden-vorticity} (HV) solitons of the type , whose net
vorticity is zero. Stability regions for the vortices of both types
are identified for , 2, and 3, by dint of the calculation of stability
eigenvalues, and in direct simulations. A novel feature found in the study of
the HV solitons is that their stability intervals never reach the (cutoff)
point at which the bright vortex carries over into a dark one, hence dark HV
solitons can never be stable, contrarily to the bright ones. In addition to the
well-known symmetry-breaking (\textit{external}) instability, which splits the
ring soliton into a set of fragments flying away in tangential directions, we
report two new scenarios of the development of weak instabilities specific to
the HV solitons. One features \textit{charge flipping}, with the two components
exchanging the angular momentum and periodically reversing the sign of their
spins. The composite soliton does not split in this case, therefore we identify
such instability as an \textit{intrinsic} one. Eventually, the soliton splits,
as weak radiation loss drives it across the border of the ordinary strong
(external) instability. Another scenario proceeds through separation of the
vortex cores in the two components, each individual core moving toward the
outer edge of the annular soliton. After expulsion of the cores, there remains
a zero-vorticity breather with persistent internal vibrations.Comment: 10 pages, 11 figure
Scattering cancellation of the magnetic dipole field from macroscopic spheres
Based on the scattering cancellation technique we suggest a cloak that allows to conceal macroscopic objects, i.e. objects with an optical size comparable to wavelengths in the visible and whose scattering response is dominated by a magnetic dipole contribution. The key idea in our approach is to use a shell of polaritonic spheres around the object to be cloaked. These spheres exhibit an artificial magnetism. In a systematic investigation, where we progressively increase the complexity of the considered structure, we devise the requirements imposed on the shell and outline how it can be implemented with natural available materials
An alternative search for the electron capture of Te-123
A search for the electron capture of Te-123 has been performed using CdZnTe
detectors. After a measuring time of 195 h no signal could be found resulting
in a lower half-life limt of yrs (95 % CL) for
this process. This clearly discriminates between existing experimental results
which differ by six orders of magnitude and our data are in strong favour of
the result with longer half-lifes.Comment: 2 pages, 2 eps-figures, reanalysis of data set
Stellar neutron capture cross sections of ²⁰ ²¹ ²²Ne
The stellar (n,γ) cross sections of the Ne isotopes are important for a number of astrophysical quests, i.e., for the interpretation of abundance patterns in presolar material or with respect to the s-process neutron balance in red giant stars. This paper presents resonance studies of experimental data in the keV range, which had not been fully analyzed before. The analyses were carried out with the R-matrix code sammy. With these results for the resonant part and by adding the components due to direct radiative capture, improved Maxwellian-averaged cross sections (MACS) could be determined. At kT=30keV thermal energy we obtain MACS values of 240±29,1263±160, and 53.2±2.7 μbarn for ²⁰Ne,²¹Ne, and ²²Ne, respectively. In earlier work the stellar rates of ²⁰Ne and ²¹Ne had been grossly overestimated. ²²Ne and ²⁰Ne are significant neutron poisons for the s process in stars because their very small MACS values are compensated by their large abundances
Emission of gamma rays shifted from resonant absorption by electron-nuclear double transitions in ^{151}Eu^{2+}:CaF_2
We show that the emission of a gamma-ray photon by a nucleus can be
influenced by a microwave magnetic field acting on the atomic electrons. We
study theoretically these electron-nuclear double transitions (ENDTs) for
^{151}Eu nuclei in a CaF_2 lattice at low temperature, in the presence of a
static magnetic field and of a microwave magnetic field. The ENDTs acquire a
significant intensity for certain resonance frequencies. The ENDTs are of
interest for the identification of the position of the lines in complex
M\"{o}ssbauer spectra.Comment: 8 pages; 3 Postscript figures: Fig. 1, Fig. 2(a), Fig. 2(b
Exploring SR Calcium and Cytosolic Calcium Wave Dynamics using a 3D Stochastic Myocyte Model
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