ALBERT

Notes: Abstract The initial stages of deceleration in the circumstellar medium of a stellar envelope, thrown off by a shock wave, are investigated. The equations of spherical-symmetric adiabatic hydrodynamics are shown to have a similarity solution in the case of the density of the expanding envelope being approximated by a reasonable power law. The overall flow pattern has such a form that the stellar material is decelerated in the internal shock wave while another shock propagates through the circumstellar matter. Between the shocks there is a contact discontinuity separating the circumstellar and stellar matter. The characteristics of the similarity solution are calculated for various exponents in the density laws of an expanding envelope and circumstellar matter and for two values of the adiabatic index (γ=5/3, 4/3). Some parts of the flow exhibit Rayleigh-Taylor instability. Special attention is paid to the validity of the hydrodynamics. In full agreement with D'yachenkoet al. (1969), we conclude that the kinetic and collisionless processes are of great importance if the initial stages of stellar envelope deceleration are to be properly monitored. The results obtained can also be employed to describe the interaction between the exploding core of a red giant star and its rarefied envelope. This is of interest for explosive nucleosynthesis. The similarity solution is applied to the envelopes expelled both by type-II supernovae and by rapid novae. In particular, the thermalization time-scale of circumstellar plasma is estimated. For SNii this time-scale proves to be of the order of 60 yr. This confirms with the observational data on the moment of the maximum radio-emission of young SNRs. In the case of rapid novae, this time is less by a factor of 10. Therefore, the peak radio and X-ray (∼2 keV) lumnosity may occur several years after the rapid nova outburst. The explosion of a degenerate carbon core is found to result in the heating of the hydrogen-helium envelope of a red giant star up to 3×106 K.

Notes: Abstract A statement of the problem of gravitational collapse and a computational method are described. The main feature of the collapse — its extremely high heterogeneity — is taken into account. The structure of a collapsing star is characterized by a dense and hot nucleon core which is opaque with respect to neutrino radiation and is embedded in to and extended envelope, almost transparent to neutrinos. The envelope is gradually being accreted onto the core. The enormous amount of energy, radiated in the form of neutrinos and antineutrinos, make us pay particular attention to relatively small absorption of neutrino radiation by extended envelope (so-called energy of deposition). The inclusion of the energy deposition in the calculations is of importance for the problem of transformation of an implosion into an explosion. The deposition is taken into consideration in the approximation of diluted neutrino radiation which escapes from neutrino photosphere and is partially absorbed in the envelope. Both the generation of energy due to deposition and the change of neutronto-proton ratio are taken into account. The increase of the mass of the core, which is opaque with respect to neutrino radiation, is fully taken into account in the calculations of the gravitational collapse.

Notes: Abstract The collapse of iron-oxygen stars with masses of 2M ⊙ has been calculated. The commencement of the collapse is due to dissociation of iron-group nuclei into free nucleons. After a while, the collapse proceeds in consequence of intensive energy losses due to neutrino volume radiation. At an intermediate stage of the collapse, the core — opaque with respect to neutrino radiation (neutrino core) — is formed inside the collapsing star. Both the gradual increase of the mass of the neutrino core and the partial absorption of neutrinos radiated from the surface of the neutrino core by the stellar envelope (deposition) were taken into account in our calculations. The kinetics of oxygen burning in the outer layers of the envelope was also allowed for. Neither the deposition, nor the oxygen burning, result in ejection of stellar envelopes.

Notes: Abstract A compact structure of a low-mass Type I presupernovae is assumed to be an essential feature of the hydrodynamical problem dealing with the supernova Type I (SNI) envelope outbursts. This structure is characterized by a degenerate carbon-oxygen core, which suffers a thermonuclear explosion of carbon fuel (M 0≃1.40M ⊙), and by a compact lowmass envelope (M e ≲0.1M ⊙) with external radiusR e≃109 cm. The parameters, of this hydrostatic envelope are specified and then, for a relatively small explosion energy, ofW 0≃(2–10)×1049 erg, hydrodynamic problem of the envelope ejection is solved numerically. This energy comes from neutrino-induced detonative carbon burning. The resulting structure of the SNI atmosphere expanding with the velocity gradient can be employed for an interpretation of the observed SNI spectra. In accordance with our previous papers, the SNI light curves are considered to occur due to an additional slow (with time-scale 106–107 s) release of the bulk of the SNI energy,W≃1051, erg. The slow energy release does not, however, affect the structure of the outermost expanding layers of the envelope which are responsible for the SNI spectra. A short (Δt≃10−2 s) burst of soft (2–10 keV) X-rays with total radiated energy of about 1040 erg is found to appear 10–20 days before the SNI optical maximum.

Notes: Abstract A series of hydrodynamical models of type-II supernova outbursts (SNII) has been calculated. Approximate relations connecting the total outburst energy ε, the mass of envelope ejectedM, the presupernova radiusR, and the amount of ionizing quanta radiated by the supernovaeN H with such values as the duration of the light curve plateau Δt, and absolute magnitude in the wavelength bandV and photospheric velocityU PH observed near the middle of the plateau have been established. Advantage has been taken of the relations to obtain a preliminary evaluation for the characteristics of the average SN II: ε=7×1050 erg,M=6M ⊙,R=500R ⊙,N H=2×1058. The SNIIs with plateau-like light curves seem to be accounted for by thermonuclear explosions of degenerate cores of red giant stars and result in a total disruption of the star without any stellar remnant. To the contrary, SNIIs with linear light curves have substantially different properties (in particular, they throw considerably less massive envelopes off). These SNII must signify the birth of collapsed objects—neutron stars (pulsars) or black holes.

Notes: Abstract The cooling wave apparently arises at some phases of the supernova expansion. A number of basic properties of the cooling wave are considered. The dependence of light curve and colour temperature on chemical composition and density distribution law has been investigated in detail. An estimation of total mass ejected by supernovae type II leads to the value of ≲20M ⊙. The necessity of further careful spectroscopic and photometric observations at prolonged postmaximum phases, especially for supernovae type II, is emphasized.

Notes: Abstract Three mechanisms generating pulsed γ-ray emission at late stages of stellar evolution are investigated: (1) γ-ray burst produced by the absorption of neutrino emission of a collapsing star in its envelope; (2) γ-ray burst of thermal emission when the outer layers of a compact star (R=0.01–0.1R ⊙) are heated up by a powerful shock wave; and (3) γ-ray emission due to ejection of matter from neutron stars at an active stage of their existence. In case (1) the expected flux of γ-quanta with the energy 0.1 MeV amounts to about 10−4 cm−2 which is considerably less than the observed flux. However, observation of such γ-ray pulses would be an important supplement to the direct observations of neutrino emission of collapsing stars. In case (2) the outer layers of the star are heated up to the temperature of about 108 K. This results in a short burst with the emission energy ∼ 1042–43 erg whose main part is concentrated in the X-ray range (∼ 25 keV). In this case approximately 10% of the energy is emitted in the γ-ray range (≳ 0.1 MeV). Generally speaking, this mechanism is sufficient as to the energy for accounting for the observed bursts; however, probably under the condition that supernovae are exploding in our Galaxy. This involves difficulties concerning the frequency of bursts and the spectrum of emission. In case (3) ejection of chemically non-equilibrium matter from the neutron star leads to an intensive emission which is produced due to fission of superheavy nuclei, β-decay of radioactive elements and radiative capture of free neutrons. Ejection of matter from the neutron stars may be related to the observed jumps of periods of pulsars. From the observed gain of kinetic energy of the filaments of the Crab Nebula (∼ 2×1041 erg) the mass of the ejected material may be estimated (∼ 1021 g). This leads to energies of the γ-ray bursts of the order of 1038–1039 erg, which agrees fully with observations at the mean distance up to the sources 0.25 kpc. As distinct from the outbursts of supernovae it would seem that no difficulties concerning the burst frequency and the spectrum of emission are encountered. From the mechanisms of γ-ray emission examined here the mechanism connected with the ejection of matter from neutron stars seems to be the most promising as far as the interpretation of observations is concerned.

Notes: Abstract The cross-sections of interaction of neutrinos emitted by collapsing stellar cores with matter of an expelled envelope are estimated (Table I). The neutrinos are shown to be able to produce isotopes of light elements (lithium, beryllium and boron) effectively in the stellar envelope and also, possibly, to create some deuterium. The neutrino-induced production of light elements accounts quite naturally for the observed enhanced abundances of odd isotopes of lithium and boron, as compared with even ones. It is emphasized that the reliable quantitative evaluation of the light element yields has to be based on the thoroughly elaborated hydrodynamical models of the envelope ejection. A slow expulsion of the envelope due to non-thermal forces seems to be the most favourable case for the neutrino-induced production of light elements. The proposed mechanism for the production of light elements can be verified observationally. The enhanced abundances of lithium and boron with respect to their cosmic values are expected to be characteristic of Crab nebula filaments. Also, it is of interest to search for the lines of light elements in the spectra of supernovae.

Notes: Abstract The equations of neutrino hydrodynamics are derived in two different approximations taking into consideration the neutrino scattering from stellar material. In a thermal-conductivity approximation which holds good when neutrino optical depth with respect to absorption exceeds 1, the neutrino scattering is taken into account, analogously with photon radiative conductivity, by introducing the transport cross-section in the neutrino mean free path. In a practically important case when the neutrino optical thickness with respect to scattering is high enough, whereas that concerning absorption is sufficiently low, another approximation of ‘Comptonized’ neutrinos is valid. In this case, the neutrino and antineutrino chemical potentials are independent of each other. They have to be calculated from equations of continuity established for neutrino and antineutrino alongside with the diffusion equation expressing the law of lepton-charge conservation. The equations of neutrino hydrodynamics are written out both with and without inclusion of muon neutrinos and antineutrinos. The equations obtained are established to deal properly with neutrino diffusion inside collapsing stars.

Notes: Abstract Neutrinos emitted by a collapsing stellar core are shown to interact effectively with the carbon layer of the stellar envelope, thereby producing a sufficiently large amount of isotope11B to account for the cosmic abundance observed. Since the mechanism of stellar envelope expulsion which accompanies the gravitational collapse of the core has not yet been developed in detail, the temperature conditions of the expelled envelope are not quite clear. Hence, the net yield of isotope11B, relative to carbon—as shown in the present work—ranges from 5×10−6 to 5×10−4 (the observed cosmic value of this ratio is (2.5–5)×10−7). The upper limit of the above range of calculated values refers to the case when the neutrino-induced isotope11B burns via thermonuclear reactions due to the carbon layer heated by the neutrino emission itself. The lower limit, however, was obtained within the model assuming a considerably higher temperature of matter which results from the condition of hydrostatic equilibrium.