Supernova 1987A: The envelope mass and the explosion energy

Astronomy Letters - Our study of the photometric and spectroscopic observations of SN 1987A based on the hydrodynamic modeling of its bolometric light curve and nonstationary hydrogen kinetics and...     

Effects of boundary conditions and viscous energy dissipation on carbon burning in thermonuclear supernova models

Based on a one-dimensional hydrodynamic model, we investigate carbon burning in a thermonuclear type-Ia supernova in the approximation of unsteady convection. The relatively broad range of convective parameters, 1×10^?3≤α_c≤2×10^?3, in which delayed detonation from the edge takes place was found to be preserved only for cases with a low boundary temperature at the presupernova stage, T _b ^(PS) = 6.4 × 10⁶ K, and with a high envelope mass, m_ex ? 2 × 10^?3M_⊙. In cases with a more realistic temperature, T _b ^(PS) = 2 × 10⁸ K, which corresponds to helium burning in the shell source, and with a lower mass m_ex, delayed detonation from the edge takes place only at α_c = 2 × 10^?3, while at α_c = 1 × 10^?3, numerous model pulsations occur during t?500 s. Artificial viscosity is shown to give a determining contribution to the increase in entropy in outer model shells, which is caused by the generation of weak shock waves during pulsations. We also show that the entropies calculated by two independent methods are equal.     

Dust formation and survival in supernova ejecta

The presence of dust at high redshift requires efficient condensation of grains in supernova (SN) ejecta, in accordance with current theoretical models. Yet observations of the few well-studied supernovae (SNe) and supernova remnants (SNRs) imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara for dust formation in the ejecta of core collapse SNe, and (ii) followed, for the first time, the evolution of newly condensed grains from the time of formation to their survival – through the passage of the reverse shock – in the SNR. We find that  0.1–0.6  M_⊙  of dust form in the ejecta of 12–40 M_⊙ stellar progenitors. Depending on the density of the surrounding interstellar medium, between 2 and 20 per cent of the initial dust mass survives the passage of the reverse shock, on time-scales of about  4–8 × 10⁴  yr  from the stellar explosion. Sputtering by the hot gas induces a shift of the dust size distribution towards smaller grains. The resulting dust extinction curve shows a good agreement with that derived by observations of a reddened QSO at   z = 6.2  . Stochastic heating of small grains leads to a wide distribution of dust temperatures. This supports the idea that large amounts (∼0.1 M_⊙) of cold dust  ( T ∼ 40   K)  can be present in SNRs, without being in conflict with the observed infrared emission.     

Dependence of the position of the knee in the Galactic cosmic ray spectrum on the explosion energy distribution of supernovae

The position of the knee in the Galactic cosmic ray (GCR) spectrum is shown to depend on the explosion energy distribution function of supernovae (SN). The position of the knee in the GCR spectrum can be quantitatively explained by the dominating contribution of hypernovae with explosion energies of (～30–50)×10⁵¹ erg, the fraction of which must be no less than 1% of all SN. The model reproduces the main features in the spectrum of all particles measured in extensive air shower (EAS) experiments: the knee in the spectrum of all particles at energy of about 3 PeV, the change in slope by δ_γ ～ 0.3–0.5 after the knee point, and the steepening of the spectrum near 10¹⁸ eV. The model predicts a smooth knee if the SN explosion energy distribution is universal and a sharp knee if the hypernovae represent a separate class of events. The suggested model of the GCR spectrum is essentially based on the assumption that a spread in explosion energies exists and that the assumptions of the standard model for the CR acceleration in supernova remnants are valid.     

A massive circumstellar envelope around the type-IIn supernova SN 1995G

We model the interaction of the supernova SN 1995G with a dense circumstellar (CS) gas in a thin-shell approximation. A model fit to the observed bolometric light curve combined with data on the supernova expansion velocity gives estimates for the density, mass (≈1 M_⊙), and age (≈8 yr) of the CS envelope. The determined CS-envelope density is shown to be virtually independent of the assumed mass of the supernova envelope because of the high CS-gas density at which the forward shock wave is essentially radiative. The derived CS-envelope density is consistent with the Hα luminosity and with the presence of distinct Thomson scattering in the red wing of this line. The mass of the CS envelope together with its expansion velocity and age indicate that the CS envelope was ejected by the presupernova eight years before the supernova explosion through violent energy release (～6×10⁴⁸ erg).     

Double detonations at the core–envelope boundary in Type Ia supernovae

An off-centre detonation propagating near the interface between a C–O core and a He envelope in a Type Ia supernova explosion is modelled as a steady two-dimensional similarity solution at a plane interface. We assume that in both regions the energy release occurs in an infinitely thin detonation, which produces material in nuclear statistical equilibrium (NSE) in He and in nuclear statistical quasi-equilibrium in C–O. An α-network is then used to determine the effect of the associated rarefaction wave in the C–O on the final abundance of intermediate elements. We find that, although there is a significant effect, the rarefaction is not strong enough to quench the reactions and prevent the C–O from burning to NSE.     

A hydrodynamic model for asymmetric weak explosion of collapsing supernovae with rapid initial rotation

We further investigate the two-dimensional hydrodynamic explosion model for rapidly rotating and collapsing supernovae (Aksenov et al. 1997), in which the initial energy release inside a fragmenting low-mass neutron star of critical mass ≈0.1M _⊙ moving in a circular orbit at a velocity of ≈18000 km s^?1 is reduced considerably. This velocity closely corresponds to a pulsar escape velocity of ≈1000 km s^?1 (at a total mass of ≈1.9M _⊙ for the binary of neutron stars). Compared to our previous study (Zabrodina and Imshennik 1999), this energy release was reduced by more than a half. Otherwise, the model in question does not differ from the explosion model with a self-consistent chemical composition of nuclides investigated in the above paper. In particular, the initial energy release was carefully reconciled with a chemical composition. Our numerical solution shows that the reduction in energy release due to the time scales of β processes and neutrino energy losses being finite does not alter the qualitative results of our previous studies (Aksenov et al. 1997; Imshennik and Zabrodina 1999). An intense undamped diverging shock wave (with a total post-shock energy ? 10⁵¹ erg at a front radius of ≈10 000 km) is formed; a large asymmetry of explosion with a narrow cone (with a solid angle of ≈π/4) around the leading direction, which coincides with the velocity direction of the low-mass neutron star at the instant of its explosive fragmentation in the two-dimensional model, emerges. A jet of synthesized radioactive nickel, whose mass is estimated by using simple threshold criteria to be M _Ni≈(0.02?0.03)M _⊙ is concentrated inside this cone. This appears to be the integrated parameter that is most sensitive to the specified reduction in initial energy release; it is also reduced by almost a half compared to our previous estimate (Imshennik and Zabrodina 1999). The time of propagation of the shock wave inferred in our model to the presupernova surface was estimated for SN 1987A to be 0.5–1.0 h, in agreement with observations.     

Type Ia supernovae: An explosion in the regime of a convergent delayed detonation wave

The model of a presupernova’s carbon-oxygen (C-O) core with an initial mass of 1.33 M _⊙, an initial carbon abundance X _C ⁽⁰⁾ =0.27, and a mean rate of increase in mass of 5 × 10^?7 M _⊙ yr^?1 through accretion in a binary system evolved from the central density and temperature ρ_c=10⁹ g cm^?3 and T _c=2.05 × 10⁸K, respectively, by forming a convective core and its subsequent expansion to an explosive fuel ignition at the center. The evolution and explosion equations included only the carbon burning reaction ¹²C+¹²C with energy release corresponding to the complete conversion of carbon and oxygen (at the same rate as that of carbon) into ⁵⁶Ni. The ratio of mixing length to convection-zone size α_c was chosen as the parameter. Although the model assumptions were crude, we obtained an acceptable (for the theory of supernovae) pattern of explosion with a strong dependence of its duration on α_c. In our calculations with sufficiently large values of this parameter, α_c=4.0 × 10^?3 and 3.0×10^?3, fuel burned in the regime of prompt detonation. In the range 2.0×10^?3≥α_c≥3.0×10^?4, there was initially a deflagration with the generation of model pulsations whose amplitude gradually increased. Eventually, the detonation regime of burning arose, which was triggered from the model surface layers (with m ? 1.33 M _⊙) and propagated deep into the model up to the deflagration front. The generation of model pulsations and the formation of a detonation front are described in detail for α_c=1.0 × 10^?3.     

The effect of massive binaries on stellar populations and supernova progenitors

We compare our latest single and binary stellar model results from the Cambridge stars code to several sets of observations. We examine four stellar population ratios: the number of blue to red supergiants, the number of Wolf–Rayet stars to O supergiants, the number of red supergiants to Wolf–Rayet stars and the relative number of Wolf–Rayet subtypes, WC to WN stars. These four ratios provide a quantitative measure of nuclear burning lifetimes and the importance of mass loss during various stages of the stars' lifetimes. In addition, we compare our models to the relative rate of Type Ib/c to Type II supernovae to measure the amount of mass lost over the entire lives of all stars. We find reasonable agreement between the observationally inferred values and our predicted values by mixing single and binary star populations. However, there is evidence that extra mass loss is required to improve the agreement further, to reduce the number of red supergiants and increase the number of Wolf–Rayet stars.     

Gravitational radiation from approaching neutron stars and the rotational explosion mechanism for collapsing supernovae

We consider the evolution of a neutron star binary system under the effect of two factors: gravitational radiation and mass transfer between the components. Gravitational radiation is specified under the justified assumption of a circular orbit and point masses and in the approximation of a weak gravitational field at nonrelativistic velocities of the binary components. During the first evolutionary phase determined only by gravitational radiation, the neutron stars approach each other according to a simple analytical solution. The second evolutionary phase begins at the time of Roche-lobe filling by the low-mass component, when the second factor, mass transfer as a result of mass loss by the latter, also begins to affect the evolution. Under the simplest assumptions of conservative mass transfer and exact equality between the Roche-lobe radius and the radius of the low-mass neutron star, it is still possible to extend the analytical solution of the problem of evolution to its second phase. We present this complete solution at both phases and, in particular, give theoretical light curves of gravitational radiation that depend only on two dimensionless parameters (m _t and δ ₀). Based on the solution found, we analyze the theoretical gravitational signals from SN 1987A; this analysis includes the hypothesis about the rotational explosion mechanism for collapsing supernovae.     

Consequences for some dark energy candidates from the type Ia supernova SN 1997ff

We examine the status of various dark energy models in light of the recently observed SN 1997ff at   z ≈1.7  . The modified data still fit a pure cosmological constant Λ or a quintessence with an equation of state similar to that of Λ. The kinematical Λ models,  Λ∼ S ^-2  and  Λ∼ H ²  , also fit the data reasonably well and require less dark energy density (hence more matter energy density) than is required by the constant Λ model. However, the model  Λ∼ S ^-2  with low energy density becomes unphysical as it cannot accommodate higher redshift objects.
We also examine an alternative explanation of the data, namely the absorption by the intervening whisker-like dust, and find that the quasi-steady state (QSS) model and the Friedmann–Robertson–Walker (FRW) model  Ω_m0=0.33  without any dark energy also fit the data reasonably well.
We notice that the addition of SN 1997ff to the old data has worsened the fit to most of the models, except a closed FRW model with a constant Λ and a closed quintessence model with   ω _φ =-0.82  , and the models have started departing from each other as we go above   z =1  . However, to make a clear discrimination possible, a few more supernovae with   z >1  are required.
We have also calculated the age of the Universe in these models and find that, in the models with a constant Λ, the expansion age is uncomfortably close to the age of the globular clusters. Quintessence models show even lower age. The kinematical Λ models are, however, interesting in this connection (especially the model  Λ∼ H ²)  , as they give a remarkably large age of the Universe.     

A model of particle acceleration to high energies by multiple supernova explosions in OB associations

The possibility of cosmic-ray (CR) acceleration to energies above 10⁹ GeV per nucleus in extended Galactic OB associations is analyzed. A two-stage acceleration mechanism is justified: at the first stage, the acceleration by separate shock fronts from spatially and temporally correlated supernovae explosions takes place, and, at the second stage, the Fermi acceleration by supersonic turbulence in an extended, strongly perturbed region near the OB association takes place. We calculate the CR energy spectrum, the change in CR chemical composition with energy, and the energy dependence of the mean logarithm of atomic mass, ?lnA?, for the accelerated particles. The calculated values are compared with those observed near the break in the energy spectrum. We estimate the turbulence parameters, which allow the observed features of the energy spectrum and the CR enrichment with heavy elements to be explained.     

Influence of a strong magnetic field on the neutrino heating of a supernova shock

Based on the magnetorotational model of a supernova explosion with core collapse, we investigate the significant processes of neutrino heating of the supernova shock. These processes should be taken into account in self-consistent modeling, since the neutrino heating mechanism is capable of increasing the explosion efficiency. We show that, even in the presence of a strong magnetic field (B ～ 10¹⁵ G) in the shock formation region, the heating rate is determined with good accuracy by the absorption and emission of neutrinos in direct URCA processes. Moreover, the influence on them of a magnetic field is reduced to insignificant corrections.     

On the possible observational manifestation of the impact of a supernova shock on the neutron star magnetosphere

The impact of a supernova explosion on the magnetosphere of a neutron star in a massive binary system is considered. The supernova shock impact on a plasma-filled neutron star magnetosphere can give rise to a long magnetospheric tail with a considerable store of magnetic energy. Magnetic reconnection in the formed current sheet can transform the magnetic energy stored in the tail into the kinetic energy of charged particles. The plasma instabilities excited by beams of accelerated relativistic particles can lead to the formation of a short pulse of coherent radio emission with parameters similar to those measured for the bright extragalactic millisecond radio burst detected in 2007.