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تسجيل مستخدم جديدSupernova explosions and the birth of neutron stars
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We report here on recent progress in understanding the birth conditions of neutron stars and the way how supernovae explode. More sophisticated numerical models have led to the discovery of new phenomena in the supernova core, for example a generic hydrodynamic instability of the stagnant supernova shock against low-mode nonradial deformation and the excitation of gravity-wave activity in the surface and core of the nascent neutron star. Both can have supportive or decisive influence on the inauguration of the explosion, the former by improving the conditions for energy deposition by neutrino heating in the postshock gas, the latter by supplying the developing blast with a flux of acoustic power that adds to the energy transfer by neutrinos. While recent two-dimensional models suggest that the neutrino-driven mechanism may be viable for stars from about 8 solar masses to at least 15 solar masses, acoustic energy input has been advocated as an alternative if neutrino heating fails. Magnetohydrodynamic effects constitute another way to trigger explosions in connection with the collapse of sufficiently rapidly rotating stellar cores, perhaps linked to the birth of magnetars. The global explosion asymmetries seen in the recent simulations offer an explanation of even the highest measured kick velocities of young neutron stars.
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We present results from an extensive set of one- and two-dimensional radiation-hydrodynamic simulations of the supernova core collapse, bounce, and postbounce phases, and focus on the protoneutron star (PNS) spin periods and rotational profiles as a
function of initial iron core angular velocity, degree of differential rotation, and progenitor mass. For the models considered, we find a roughly linear mapping between initial iron core rotation rate and PNS spin. The results indicate that the magnitude of the precollapse iron core angular velocities is the single most important factor in determining the PNS spin. Differences in progenitor mass and degree of differential rotation lead only to small variations in the PNS rotational period and profile. Based on our calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming angular momentum conservation, we estimate final neutron star rotation periods. We find periods of one millisecond and shorter for initial central iron core periods of below ~ 10 s. This is appreciably shorter than what previous studies have predicted and is in disagreement with current observational data from pulsar astronomy. After considering possible spindown mechanisms that could lead to longer periods we conclude that there is no mechanism that can robustly spin down a neutron star from ~ 1 ms periods to the injection periods of tens to hundreds of milliseconds observed for young pulsars. Our results indicate that, given current knowledge of the limitations of neutron star spindown mechanisms, precollapse iron cores must rotate with periods around 50-100 seconds to form neutron stars with periods generically near those inferred for the radio pulsar population.
We have studied the X-ray properties of ageing historical core-collapse supernovae in nearby galaxies, using archival data from Chandra, XMM-Newton and Swift. We found possible evidence of a young X-ray pulsar in SN 1968D and in few other sources, bu
t none more luminous than ~ a few 10^{37} erg/s. We compared the observational limits to the X-ray pulsar luminosity distribution with the results of Monte Carlo simulations for a range of birth parameters. We conclude that a pulsar population dominated by periods <~ 40 ms at birth is ruled out by the data.
The mass function for black holes and neutron stars at birth is explored for mass-losing helium stars. These should resemble, more closely than similar studies of single hydrogen-rich stars, the results of evolution in close binary systems. The effec
ts of varying the mass-loss rate and metallicity are calculated using a simple semi-analytic approach to stellar evolution that is tuned to reproduce detailed numerical calculations. Though the total fraction of black holes made in stellar collapse events varies considerably with metallicity, mass-loss rate, and mass cutoff, from 5$%$ to 30$%$, the shapes of their birth functions are very similar for all reasonable variations in these quantities. Median neutron star masses are in the range 1.32 - 1.37 $M_odot$ regardless of metallicity. The median black hole mass for solar metallicity is typically 8 to 9 $M_odot$ if only initial helium cores below 40 $M_odot$ (ZAMS mass less than 80 $M_odot$) are counted, and 9 - 13 $M_odot$, in most cases, if helium cores with initial masses up to 150 $M_odot$ (ZAMS mass less than 300 $M_odot$) contribute. As long as the mass-loss rate as a function of mass exhibits no strong non-linearities, the black hole birth function from 15 to 35 $M_odot$ has a slope that depends mostly on the initial mass function for main sequence stars. These findings imply the possibility of constraining the initial mass function and the properties of mass loss in close binaries using ongoing measurements of gravitational wave radiation. The expected rotation rates of the black holes are briefly discussed.
The properties of compact stars and their formation processes depend on many physical ingredients. The composition and the thermodynamics of the involved matter is one of them. We will investigate here uniform strongly interacting matter at densities
and temperatures, where potentially other components than free nucleons appear such as hyperons, mesons or even quarks. In this paper we will put the emphasis on two aspects of stellar matter with non-nucleonic degrees of freedom. First, we will study the phase diagram of baryonic matter with strangeness, showing that the onset of hyperons, as that of quark matter, could be related to a very rich phase structure with a large density domain covered by phase coexistence. Second, we will investigate thermal effects on the equation of state (EoS), showing that they favor the appearance of non-nucleonic particles. We will finish by reviewing some recent results on the impact of non-nucleonic degrees freedom in compact star mergers and core-collapse events, where thermal effects cannot be neglected.
One prediction of particle acceleration in the supernova remnants in the magnetic wind of exploding Wolf Rayet and Red Super Giant stars is that the final spectrum is a composition of a spectrum $E^{-7/3}$ and a polar cap component of $E^{-2}$ at the
source. This polar cap component contributes to the total energy content with only a few percent, but dominates the spectrum at higher energy. The sum of both components gives spectra which curve upwards. The upturn was predicted to occur always at the same rigidity. An additional component of cosmic rays from acceleration by supernovae exploding into the Inter-Stellar Medium (ISM) adds another component for Hydrogen and for Helium. After transport the predicted spectra $J(E)$ for the wind-SN cosmic rays are $E^{-8/3}$ and $E^{-7/3}$; the sum leads to an upturn from the steeper spectrum. An upturn has now been seen. Here, we test the observations against the predictions, and show that the observed properties are consistent with the predictions. Hydrogen can be shown to also have a noticeable wind-SN-component. The observation of the upturn in the heavy element spectra being compatible with the same rigidity for all heavy elements supports the magneto-rotational mechanism for these supernovae. This interpretation predicts the observed upturn to continue to curve upwards and approach the $E^{-7/3}$ spectrum. If confirmed, this would strengthen the case that supernovae of very massive stars with magnetic winds are important sources of Galactic cosmic rays.
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