Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models

Details Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models

2005-08-17

arxiv.org/abs/astro-ph/0508373v2

Astrophys.J.640:854-877,2006

Astronomy and Astrophysics

Astrophysics

38 pages, 22 figures. ApJ, 640 (01 April 2006 issue), in press

Scientific paper

10.1086/500187

Refined one- and two-dimensional models for the nebular spectra of the hyper-energetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray burst GRB980425, from 125 to 376 days after B-band maximum are presented. One dimensional, spherically symmetric spectrum synthesis calculations show that reproducing features in the observed spectra, i.e., the sharply peaked [OI] 6300\AA doublet and MgI] 4570\AA emission, and the broad [FeII] blend around 5200\AA, requires the existence of a high-density O-rich core expanding at low velocities ($\lsim 8,000$ km s$^{-1}$) and of Fe-rich material moving faster than the O-rich material. Synthetic spectra at late phases from aspherical (bipolar) explosion models are also computed with a two-dimensional spectrum synthesis code. The above features are naturally explained by the aspherical model if the explosion is viewed from a direction close to the axis of symmetry ($\sim 30^{\rm o}$), since the aspherical model yields a high-density O-rich region confined along the equatorial axis. By examining a large parameter space (in energy and mass), our best model gives following physical quantities: the kinetic energy $E_{51} \equiv E_{\rm K}/10^{51}$ ergs $\gsim 8 - 12$ and the main-sequence mass of the progenitor star $M_{\rm ms} \gsim 30 - 35 \Msun$. The temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-, and C-rich regions, and highly clumpy structure.

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