Modelling the binary progenitor of Supernova 1993J

We have developed a detailed stellar evolution code capable of following the simultaneous evolution of both stars in a binary system, together with their orbital properties. To demonstrate the capabilities of the code, we investigate potential progenitors for the Type IIb Supernova 1993J, which is believed to have been an interacting binary system prior to its primary exploding. We use our detailed binary stellar evolution code to model this system to determine the possible range of primary and secondary masses that could have produced the observed characteristics of this system, with particular reference to the secondary. Using the luminosities and temperatures for both stars (as determined by Maund et al.) and the remaining mass of the hydrogen envelope of the primary at the time of explosion, we find that if mass transfer is 100 per cent efficient, the observations can be reproduced by a system consisting of a  15 M_⊙  primary and a  14 M_⊙  secondary in an orbit with an initial period of 2100 days. With a mass transfer efficiency of 50 per cent, a more massive system consisting of a  17 M_⊙  primary and a  16 M_⊙  secondary in an initial orbit of 2360 days is needed. We also investigate some of the uncertainties in the evolution, including the effects of tidal interaction, convective overshooting and thermohaline mixing.     

Spectral evolution of the peculiar Ic Supernova 1998bw

Supernova 1998bw holds the record for the most energetic Type Ic explosion, one of the brightest radio supernovae and probably the first supernova associated with a γ -ray burst. In this paper we present spectral observations of SN 1998bw observed in a cooperative monitoring campaign using the Anglo-Australian Telescope, the UK Schmidt Telescope and the Siding Springs Observatories 2.3-m telescope. We investigate the evolution of the spectrum between 7 and 94 d after V -band maximum in comparison with well-studied examples of Type Ic SNe in order to quantify the unusual properties of this supernova event. Though the early spectra differ greatly from observations of classical Ic supernovae (SNe), we find that the evolution from the photospheric to nebular phases is slow but otherwise typical. The spectra differ predominantly in the extensive line blending and blanketing which has been attributed to the high velocity of the ejecta. We find that by day 19, the absorption line minima blueshifts are 10–50 per cent higher than other SNe and on day 94 emission lines are 45 per cent broader, as expected if the progenitor had a massive envelope. However, it is difficult to explain the extent of line blanketing entirely by line broadening, and we argue that an additional contribution from other species is present, indicating unusual relative abundances or physical conditions in the envelope.     

Distribution of 56Ni Yields of Type Ia Supernovae and its Implication for Progenitors

The amount of 56Ni produced in Type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia lumi-nosities with their light curves. Based on an empirical relation between the 56Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56Ni masses for different SNe Ia could vary by a factor of ten (e.g., MNi = 0.1 - 1.3 M⊙),which cannot be explained in terms of the standard Chandraseldaar-mass model (with a 56Ni mass production of 0.4 - 0.8 M⊙). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with MNi 0.8 M⊙) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56Ni to power the light curve. This is also consistent with the find-ing that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △m15 < 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia (with MNi < 0.2 M⊙) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.     

Nucleosynthesis of 56Ni in wind-driven supernova explosions and constraints on the central engine of gamma-ray bursts

Theoretically expected natures of a supernova (SN) driven by a wind/jet are discussed. Approximate analytical formulations are derived to clarify basic physical processes involved in the wind/jet-driven explosions, and it is shown that the explosion properties are characterized by the energy injection rate     and the mass injection rate     . To explain observations of Supernova 1998bw associated with gamma-ray burst (GRB) 980425, the following conditions are required:     and     (if the wind Lorentz factor  Γ_w∼ 1  ) or     (if  Γ_w≫ 1  ). In Supernova 1998bw, ⁵⁶Ni  (∼0.4 M_⊙)  is probably produced in the shocked stellar mantle, not in the wind. The expected natures of SNe, e.g. ejected ⁵⁶Ni mass and ejecta mass, vary depending on     and     . The sequence of the SN properties from high     and     to low     and     is as follows: Supernova 1998bw-like – intermediate case – low mass ejecta  (≲1 M_⊙)  where ⁵⁶Ni is from the wind – whole collapse. This diversity may explain the diversity of SNe associated with GRBs. Our result can be used to constrain natures of the wind/jet, which are linked to the central engine of GRBs, by studying properties of the associated SNe.     

I_a型超新星爆发理论(Ⅱ):理论研究中的重要疑难问题

介绍与评述了I_a型超新星理论研究中最为突出的8个尚未解决的重要疑难问题,并简单介绍了国内围绕SNI_a开展的某些物理探讨研究．     

The 1.05-μm feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J -band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He I 10830 ... or Mg II 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia.
In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of ⁵⁶Ni and ⁵⁶Co.
The J -band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s⁻¹, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s⁻¹. Both models are in good agreement with the optical spectrum. In particular, a strong He I 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of He I differ by about an order of magnitude.
Unfortunately, neither model is able to reproduce the sudden disappearance of the J -band feature in the post-maximum spectra. Possible explanations are discussed.     

A study of the Type II-P supernova 2003gd in M74

We present photometric and spectroscopic data of the Type II-P supernova (SN II-P) 2003gd, which was discovered in M74 close to the end of its plateau phase. SN 2003gd is the first Type II supernova (SN) to have a directly confirmed red supergiant (RSG) progenitor. We compare SN 2003gd to SN 1999em, a similar SN II-P, and estimate an explosion date of 2003 March 18. We determine a reddening towards the SN of   E ( B − V ) = 0.14 ± 0.06  , using three different methods. We also calculate three new distances to M74 of  9.6 ± 2.8, 7.7 ± 1.7  and  9.6 ± 2.2 Mpc  . The former was estimated using the standard candle method (SCM), for Type II supernovae (SNe II), and the latter two using the brightest supergiants method (BSM). When combined with existing kinematic and BSM distance estimates, we derive a mean value of  9.3 ± 1.8 Mpc  . SN 2003gd was found to have a lower tail luminosity compared with other normal Type II-P supernovae (SNe II-P) bringing into question the nature of this SN. We present a discussion concluding that this is a normal SN II-P, which is consistent with the observed progenitor mass of  8⁺⁴₋₂ M_⊙  .     

An early-time infrared and optical study of the Type Ia Supernova 1998bu in M96

We present first-season infrared (IR) and optical photometry and spectroscopy of the Type Ia Supernova 1998bu in M96. We also report optical polarimetry of this event. SN 1998bu is one of the closest type Ia supernovae of modern times, and the distance of its host galaxy is well determined. We find that SN 1998bu is both photometrically and spectroscopically normal. However, the extinction to this event is unusually high, with     We find that SN 1998bu peaked at an intrinsic     Adopting a distance modulus of 30.25 (Tanvir et al.) and using Phillips et al.'s relations for the Hubble constant, we obtain     Combination of our IR photometry with those of Jha et al. provides one of the most complete early-phase IR light curves for a SN Ia published so far. In particular, SN 1998bu is the first normal SN Ia for which good pre- t _{B max} IR coverage has been obtained. It reveals that the J , H and K light curves peak about 5 days earlier than the flux in the B -band curve.     

The case and fate of HD 75767 – neutron star or supernova?

We report the discovery of the nearby  ( d = 24 pc)  HD 75767 as an eight billion year old quadruple system consisting of a distant M dwarf pair, HD 75767 C–D, in orbit around the known short-period   P = 10.25 d  single-lined binary HD 75767 A–B, the primary of which is a solar-like G star. On the reasonable assumption of synchronous orbital rotation as well as rotational and orbital coplanarity for the inner pair, we get   M _B= 0.96 M_⊙  for the unseen HD 75767 B, that is, the case of a massive white dwarf. Upon future evolution, mass transfer towards HD 75767 B will render the   M _A= 0.96 M_⊙  G-type primary, now a turnoff star, to become a helium white dwarf of   M _A∼ 0.33 M_⊙  . Depending on the mass accretion rate, accretion efficiency and composition of the massive white dwarf, this in turn may result in a collapse of HD 75767 B with the formation of a millisecond pulsar, i.e. the creation of a low-mass binary pulsar (LMBP), or, instead, a Type Ia supernova explosion and the complete disruption of HD 75767 B. Irrespective of which scenario applies, we point to the importance of the distant M dwarfs as the likely agents for the formation of the inner, short-period HD 75767 A–B pair, and hence a path that particularly avoids preceding phases of common envelope evolution.     

A single-degenerate model for the progenitor of the Type Ia supernova 2002ic

Supernova 2002ic was an atypical Type Ia supernova (SN Ia) with evidence for substantial amounts of hydrogen associated with the system. Contrary to previous claims, we show that its unusual properties can be understood within the framework of one of the most favoured progenitor models, the so-called supersoft channel. This requires that the donor star was initially relatively massive  (∼3 M_⊙)  and that the system experienced a delayed dynamical instability, leading to a large amount of mass-loss from the system in the last few 10⁴ yr before the explosion. This can produce the inferred hydrogen-rich circumstellar environment, most likely with a disc-like geometry. However, in order for this model to be feasible, it requires a larger accretion efficiency on to the white dwarf than is assumed in present parametrizations. If this is confirmed, it would most likely increase estimates for the frequency of the single-degenerate channel. Based on population synthesis simulations we estimate that not more than 1 in 100 SNe Ia should belong to this subgroup of SNe Ia.