Circumstellar Na I and Ca II absorption lines of type Ia supernovae in the symbiotic scenario

The formation of circumstellar Na I and Ca II resonance absorption lines in a type Ia supernova is studied in the case where the supernova explodes in a binary system with a red giant. The model suggests a spherically symmetric wind and takes into account the nonstationary ionization and heating of the wind by X rays from the shock wave and by gamma rays from radioactive ⁵⁶Ni decay. For wind densities typical of a red giant, the expected optical depth of the wind in Na I lines is shown to be too small (τ < 10^?3) for their detection. Under the same conditions, the optical depth of the predicted Ca II 3934 Å absorption line is sufficient for its detection (τ > 0.1). It is concluded that the Na I and Ca II absorption lines detected in SN 2006X could not be formed in the red giant wind and are most likely related to clouds at distances exceeding the dust evaporation radius (r > 10¹⁷ cm). An upper limit for the rate of mass loss through a stationary wind with velocity u has been obtained from the absence of Ca II absorption lines in SN 2006X unrelated to the similar Na I components: ? < 10^?8 (u/10 km s^?1) M _⊙ yr^?1.     

Monte Carlo simulations of supernova light curves and the hypernova SN 1998bw

We propose a method for computing the bolometric light curves of type Ia and Ib/c supernovae based on Monte Carlo simulations of unsteady-state radiative transfer. The method is used to analyze the bolometric light curve of the unusual type Ib/c supernova SN 1998bw associated with GRB 980 425. We show that the previously noted inconsistency in the behavior of simulated light curves at early and late stages, which is attributable to asymmetry effects, can be overcome in a spherically symmetric model. Agreement with observations requires complete ⁵⁶Ni mixing and a higher matter density in the central part of the envelope in the velocity range v<5000 km s^?1 compared to standard models.     

Distribution of type Ib/c supernovae relative to galactic spiral arms

Type Ib/c supernovae are shown to be concentrated to the inner edges of the spiral arms, with the distributions for type Ib and Ic supernovae being identical. We have found differences between the distributions of type II and Ib/c supernovae relative to spiral arms, suggesting that the type Ib/c presupernovae are, on average, younger.     

Type-Ia supernovae in dense circumstellar gas

We propose a model for the bolometric light curve of a type-Ia supernova (SN Ia) that explodes in a dense circumstellar (CS) envelope. Our modeling of the light curves for SN 2002ic and SN 1997cy shows that the densities of the CS envelopes around both supernovae at a radius of ～7×10¹⁵ cm are similar, while the characteristic ejection time for this envelope around SN 1997cy does not exceed 600 yr. We analyze two possible evolutionary scenarios that could lead to the explosion of a SN Ia inside a dense C S hydrogen envelope: accretion onto a CO white dwarf in a symbiotic binary and the evolution of a single star with an initial mass of about 8M_⊙. If the hypothesis of a SN Ia explosion in a dense CS envelope is correct for SN 2002ic and SN 1997cy, then we must assume that the the rapid loss of the red-supergiant envelope in several hundred years and the subsequent explosion of the CO white dwarf are synchronized by some physical mechanism. This mechanism may be related to the contraction of the white dwarf as it approaches the Chandrasekhar limit. We show that the formation of a (super-)Chandrasekhar mass due to the merger of a CO white dwarf and the CO core of a red supergiant followed by a supernovae explosion is unlikely, since this mechanism does not provide the required synchronization of the rapid mass loss and the explosion.     

Observations of supernovae in the period 1997–1999

Astronomy Letters - We present our photometric observations of the 15 supernovae (SN) discovered in the period 1997–1999; of these, six are type Ia SN, two are peculiar type Ia SN, three are...     

A rotating collapsar and possible interpretation of the LSD neutrino signal from SN 1987A

We consider an improved rotational mechanism of the explosion of a collapsing supernova. We show that this mechanism leads to two-stage collapse with a phase difference of ～5 h. Based on this model, we attempt a new interpretation of the events in underground neutrino detectors on February 23, 1987, related to the supernova SN 1987A.     

Radial distributions of gamma-ray bursts and type Ib/c supernovae in galaxies

Data on the positions of gamma-ray bursts (GRBs) in galaxies are used to construct the radial distributions of their surface density. The gradient in GRB surface density is shown to decrease sharply at a galactocentric distance equal to the effective galactic radius. In central galactic regions, the GRB density distribution agrees with the galactic surface-brightness distribution; in outer regions, the GRB density decreases more slowly than does the surface brightness. Based on improved statistics, we analyze the radial distribution of type Ib/c supernovae. We show that it differs insignificantly from the distributions of other types of supernova and exhibits a much closer similarity to the distribution of star-forming regions than do GRBs. Although the statistics for GRBs is poor, the deviation of their distribution from the distribution of active star-forming regions in nearby galaxies seems to have been firmly established. A correlation of GRBs with the distribution of dark matter in outer galactic regions is not ruled out.     

Observational effects in supernova remnants associated with H II regions and dense gaseous clouds

The dependences of various parameters for S-and C-type supernova remnants (SNRs) on their diameters are investigated. Only SNRs with D≤40 pc that expand initially within H II regions and, subsequently, in dense media are considered. The expansion velocities and thermal electron densities of these SNRs were found to decrease with increasing diameter, on average, as D ^?1 and D ^?0.5, respectively. H II regions hamper the detection of SNRs; this effect is particularly pronounced in regions with 270°≤1≤300° and 330°≤1≤360°. The X-ray luminosities of SNRs born in dense media increase by an order of magnitude when their diameters reach ～30 pc. After the SNR diameters reach ～40 pc, their radio and X-ray luminosities decrease sharply.     

Universal mechanism of thermonuclear explosion for single (type I + 1/2) and binary (type Ia) presupernovae: The history and prospects

We discuss the history and prospects for the one-dimensional models of thermonuclear explosions in carbon-oxygen stellar cores. In connection with the recently studied SN 2002ic, which combines the properties of classical type Ia and IIn supernovae, we hope that our delayed detonation mechanism is applicable not only to binary, but also to single presupernovae. Since a large amount of ⁵⁶Ni is synthesized, it can also describe adequately the light curves of supernovae similar to SN 2002ic.     

Parameters of the classical type-IIP supernova SN 1999em

Based on observations of SN 1999em, we determined the physical parameters of this supernova using hydrodynamic calculations including nonequilibrium radiative transfer. Taking the distance to SN 1999em estimated by the expanding photosphere method (EPM) to be D = 7.5 Mpc, we found the parameters of the presupernova: radius R = 450R_⊙, mass M = 15M_⊙, and explosion energy E = 7 × 10⁵⁰ erg. For the distance D = 12 Mpc determined from Cepheids, R, M, and E must be increased to the following values: R = 1000R_⊙, M = 18M_⊙, and E = 10⁵¹ erg. We show that one cannot restrict oneself to using the simple analytical formulas relating the supernova and presupernova parameters to obtain reliable parameters for type-IIP presupernovae.     

Dust formation in primordial Type II supernovae

We have investigated the formation of dust in the ejecta of Type II supernovae (SNe), mostly of primordial composition, to answer the question of where the first solid particles are formed in the Universe. However, we have also considered non-zero progenitor metallicity values up to Z = Z_⊙ . The calculations are based on standard nucleation theory, and the scheme has been tested for the first time on the well-studied case of SN1987A, yielding results that are in agreement with the available data. We find that: (i) the first dust grains are predominantly made of silicates, amorphous carbon (AC), magnetite and corundum; and (ii) the largest grains are the AC ones, with sizes around 300 Å, whereas the other grain types have smaller radii, around 10–20 Å . The grain size distribution depends somewhat on the thermodynamics of the ejecta expansion, and variations in the results by a factor ≈2 might occur within reasonable estimates of the relevant parameters. Also, and for the same reason, the grain size distribution is essentially unaffected by metallicity changes. The predictions on the amount of dust formed are very robust: for Z =0 , we find that SNe with masses in the range (12–35) M_⊙ produce about 0.08 M_⊙≲ M _d≲0.3 M_⊙ of dust per supernova. The above range increases by roughly three times as the metallicity is increased to solar values. We discuss the implications and the cosmological consequences of the results.     

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...     

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.     

SN 1988Z: spectro-photometric catalogue and energy estimates★

We present a spectro-photometric catalogue of the evolution of supernova 1988Z, which combines new and published observations in the radio, optical and X-ray bands, with the aim of offering a comprehensive view of the evolution of this object and deriving the total energy radiated since its discovery. The major contribution to the total radiated energy comes at optical to X-ray frequencies, with a total emission of at least 2×10⁵¹ erg (for H₀=50 km s⁻¹ Mpc⁻¹) in 8.5 yr. A model-dependent extrapolation of this value indicates that the total radiated energy may be as high as 10⁵² erg. The high value of the radiated energy supports a scenario in which most of the kinetic energy of the ejecta is thermalized and radiated in a short interaction with a dense circumstellar medium of nearly constant density. In this sense, 1988Z is not a supernova but a young and compact supernova remnant.     

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.     

H I distribution in the region of the supernova remnant G78.2+2.1

Based on RATAN-600 21-cm line observations with an angular resolution of 2.4′ over a wide range of radial velocities, we analyze the neutral-hydrogen distribution in the region of the SNR G78.2+2.1. In addition to an H I shell at low radial velocities immediately surrounding the radio remnant, we detected an extended expanding H I shell, ≈3° in diameter, at a radial velocity of ?25 km s^?1, which closely coincides in coordinates and angular sizes with the outer X-ray shell discovered by Lozinskaya et al. (2000). The Hα emission studied by these authors in the SNR region also has a secondary peak at radial velocities from ?45 to ?20 km s^?1. Since the radial velocities of these two objects differ significantly, their distances can be assumed to differ as well; i.e., a chance projection of two distinct objects is observed.     

An iterative method for simultaneously computing the synthesis of light and heavy elements

We propose an iterative algorithm for computing the synthesis of heavy elements through the rapid capture of neutrons (r-process) and, at sufficiently high temperatures, protons by simultaneously using two distinct computer codes. One of the codes describes the kinetics of nuclear reactions between light and intermediate chemical elements, which are the source of free neutrons and protons used by the second code to synthesize heavy elements from seed nuclides (isotopes near the iron peak of the cosmic abundance curve). The two codes interact through the neutron and proton reaction channels. We demonstrate the efficiency of our method with an example of the nucleosynthesis in a supernova’s helium shell triggered by the evaporation of neutrons and protons from α particles exposed to the neutrino flux from a collapsing stellar core. In this case, three or four iterations are enough to obtain an almost exact self-consistent solution.     

What path the <Emphasis Type="Italic">r</Emphasis>-process takes: Extreme cases and comparison with observations

The concept of the r-process path is considered from the standpoint of a dynamic model. Rapid nucleosynthesis is shown to proceed not along certain preferential lines called the r-process paths but in the region of nuclei bounded on the one side by the existing nuclei and on the other side by nuclei upon reaching which the r-process enters a cooling phase. This view is shown to account for the main heavy-element abundance patterns.