517 research outputs found
Excess low energy photon pairs from pion annihilation at the chiral phase transition
The photon pair production by pion annihilation in a hot and dense medium at
the chiral phase transition is investigated within a chiral quark model. As a
direct consequence of this transition the meson appears as a bound
state in the domain of temperatures and chemical potentials where the condition
is fulfilled. This effect results in a
strong enhancement of the cross section for the pion annihilation process compared with the vacuum case. The calculation of the photon
pair production rate as function of the invariant mass shows a strong
enhancement and narrowing of the meson resonance at threshold due to
chiral symmetry restoration.Comment: 15 pages, LaTeX, 6 figures, Phys. Lett.
Combustion of a neutron star into a strange quark star: The neutrino signal
There are strong indications that the process of conversion of a neutron star
into a strange quark star proceeds as a strong deflagration implying that in a
few milliseconds almost the whole star is converted. Starting from the
three-dimensional hydrodynamic simulations of the combustion process which
provide the temperature profiles inside the newly born strange star, we
calculate for the first time the neutrino signal that is to be expected if such
a conversion process takes place. The neutrino emission is characterized by a
luminosity and a duration that is typical for the signal expected from
protoneutron stars and represents therefore a powerful source of neutrinos
which could be possibly directly detected in case of events occurring close to
our Galaxy. We discuss moreover possible connections between the birth of
strange stars and explosive phenomena such as supernovae and gamma-ray-bursts.Comment: 9 pages, 5 figures, in print on Phys. Rev.
Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor
HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass
too low to ignite core helium burning and is therefore considered as a
pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M 0.3 Msun) are
He-core objects produced by the evolution of compact binary systems. We present
in this paper a detailed abundance analysis of HD 188112 based on
high-resolution Hubble Space Telescope (HST) near and far-ultraviolet
spectroscopy. We also constrain the mass of the star's companion. We use hybrid
non-LTE model atmospheres to fit the observed spectral lines and derive the
abundances of more than a dozen elements as well as the rotational broadening
of metallic lines. We confirm the previous binary system parameters by
combining radial velocities measured in our UV spectra with the already
published ones. The system has a period of 0.60658584 days and a WD companion
with M 0.70 Msun. By assuming a tidally locked rotation, combined with
the projected rotational velocity (v sin i = 7.9 0.3 km s) we
constrain the companion mass to be between 0.9 and 1.3 Msun. We further discuss
the future evolution of the system as a potential progenitor of a
(underluminous) type Ia supernova. We measure abundances for Mg, Al, Si, P, S,
Ca, Ti, Cr, Mn, Fe, Ni, and Zn, as well as for the trans-iron elements Ga, Sn,
and Pb. In addition, we derive upper limits for the C, N, O elements and find
HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE
effects on the line strength of some ionic species such as Si II and Ni II. The
metallic abundances indicate that the star is metal-poor, with an abundance
pattern most likely produced by diffusion effects.Comment: Accepted for publication in A&
The interaction of core-collapse supernova ejecta with a stellar companion
The progenitors of many core-collapse supernovae (CCSNe) are expected to be
in binary systems. By performing a series of three-dimensional hydrodynamical
simulations, we investigate how CCSN explosions affect their binary companion.
We find that the amount of removed stellar mass, the resulting impact velocity,
and the chemical contamination of the companion that results from the impact of
the SN ejecta, strongly increases with decreasing binary separation and
increasing explosion energy. Also, it is foud that the impact effects of CCSN
ejecta on the structure of main-sequence (MS) companions, and thus their long
term post-explosion evolution, is in general not be dramatic.Comment: 5 pages, 3 figures, poster contribution: IAU Symposium 346 "High Mass
X-ray Binaries: illuminating the passage from massive binaries to merging
compact objects", Vienna, Austria, 27-31 August 2018. arXiv admin note:
substantial text overlap with arXiv:1509.0363
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
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