Takayoshi Nakamura

The Metamorphosis of SN 1998bw

We present and discuss the photometric and spectroscopic evolution of the peculiar SN 1998bw, associated with GRB 980425, through an analysis of optical and near-IR data collected at ESOLa Silla. The spectroscopic data, spanning the period from day ( 9t o day)376 (relative to B maximum), have shown that this supernova (SN) was unprecedented, although somewhat similar to SN 1997ef. Maximum expansion velocities as high as 3 ) 104 km s~1 to some extent mask its resemblance to other Type Ic SNe. At intermediate phases, between photospheric and fully nebular, the expansion velocities (D104 km s~1) remained exceptionally high compared to those of other recorded core-collapse SNe at a similar phase. The mild linear polarization detected at early epochs suggests the presence of asymmetry in the emitting material. The degree of asymmetry, however, cannot be decoded from these measurements alone. The He I 1.083 and 2.058 km lines are identi—ed, and He is suggested to lie in an outer region of the envelope. The temporal behavior of the —uxes and pro—les of emission lines of Mg I) j4571, (O I) jj6300, 6364, and a feature ascribed to Fe are traced to stimulate future modeling work. The uniqueness of SN 1998bw became less obvious once it entered the fully nebular phase (after 1 yr), when it was very similar to other Type Ib/cIIb objects, such as the Type Ib SN 1996N and the Type IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN 1993J and 3 mag brighter than SN 1996N at a com- parable phase. The late-phase optical photometry, which extends up to 403 days after B maximum, shows that the SN luminosity declined exponentially but substantially faster than the decay rate of 56Co. The ultraviolet-optical-infrared bolometric light curve, constructed using all available optical data and the early JHK photometry presented in this work, shows a slight —attening starting on about day )300. Since no clear evidence of ejecta-wind interaction was found in the late-time spectroscopy (see also the work of Sollerman and coworkers), this may be due to the contribution of the positrons since most c-rays escape thermalization at this phase. A contribution from the superposed H II region cannot, however, be excluded. Subject headings: gamma rays: burstssupernovae: generalsupernovae: individual (SN 1998bw)

Explosive Nucleosynthesis in Aspherical Hypernova Explosions and Late-Time Spectra of SN 1998bw

Aspherical explosion models for the hypernova (hyperenergetic supernova) SN 1998bw are presented. Nucleosynthesis in aspherical explosions is examined with a two-dimensional hydrodynamical code and a detailed nuclear reaction network. Aspherical explosions lead to a strong α-rich freezeout, thus enhancing the abundance ratios [44Ca, 48Ti, 64Zn/Fe] in the ejecta. The nebular line profiles of the Fe-dominated blend near 5200 A and of [O I] 6300, 6363 A are calculated and compared with the observed late-time spectra of SN 1998bw. Compared with the spherical model, the unusual features of the observed nebular spectra can be better explained if SN 1998bw is a strongly aspherical explosion with a kinetic energy of ~1052 ergs viewed from near the jet direction.

THE PECULIAR TYPE Ic SUPERNOVA 1997ef: ANOTHER HYPERNOVA

SN 1997ef has been recognized as a peculiar supernova from its light curve and spectral properties. The object was classified as a Type Ic supernova (SN Ic) because its spectra were dominated by broad absorption lines of oxygen and iron, lacking any clear signs of hydrogen or helium line features. The light curve is very different from that of previously known SNe Ic, showing a very broad peak and a slow tail. The strikingly broad line features in the spectra of SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest the interesting possibility that SN 1997ef may also be a hypernova. The light curve and spectra of SN 1997ef were modeled first with a standard SN Ic model assuming an ordinary kinetic energy of explosion EK = 1051 ergs. The explosion of a CO star of mass MCO ≈ 6 M☉ gives a reasonably good fit to the light curve but clearly fails to reproduce the broad spectral features. Then, models with larger masses and energies were explored. Both the light curve and the spectra of SN 1997ef are much better reproduced by a C+O star model with EK = 8 × 1051 ergs and MCO = 10 M☉. Therefore, we conclude that SN 1997ef is very likely a hypernova on the basis of its kinetic energy of explosion. Finally, implications for the deviation from spherical symmetry are discussed in an effort to improve the fits to the observations.SN 1997ef has been recognized as a peculiar supernova from its light curve and spectral properties. The object was classified as a Type Ic supernova (SN Ic) because its spectra are dominated by broad absorption lines of oxygen and iron, lacking any clear signs of hydrogen or helium line features. The light curve is very different from that of previously known SNe Ic, showing a very broad peak and a slow tail. The strikingly broad line features in the spectra of SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest the interesting possibility that SN 1997ef may also be a hypernova. The light curve and spectra of SN 1997ef were modeled first with a standard SN~Ic model assuming an ordinary kinetic energy of explosion

Light Curve and Spectral Models for the Hypernova SN 1998bw Associated with GRB 980425

E_{\rm K} = 10^{51}

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

erg. The explosion of a CO star of mass

Can Differences in the Nickel Abundance in Chandrasekhar-Mass Models Explain the Relation between the Brightness and Decline Rate of Normal Type Ia Supernovae?

M_{\rm CO} \approx 6 M_\odot

Formation of the Black Hole in Nova Scorpii

gives a reasonably good fit to the light curve but clearly fails to reproduce the broad spectral features. Then, models with larger masses and energies were explored. Both the light curve and the spectra of SN 1997ef are much better reproduced by a C+O star model with

Evolution and Nucleosynthesis of Metal-Free Massive Stars

E_{\rm K} =

Gamma-ray signatures of supernovae and hypernovae

8 \e{51} erg and

SN1998bw and Hypernovae

M_{\rm CO} = 10 M_\odot