Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-06-06
Astronomy and Astrophysics
Astrophysics
23 pages, 2 figures. Accepted for publication in ApJ
Scientific paper
10.1086/322420
The nebular spectra of the energetic Type Ic supernova SN1998bw (hypernova) are studied. The transition to the nebular phase occurred at an epoch of about 100 days after outburst, which is assumed to coincide with GRB980425. Early in the nebular epoch the spectra show the characteristics of a typical SN Ic spectrum, with strong lines of [OI], CaII and MgI], and lines of [FeII]. However, the [FeII] lines are unusually strong for a SN Ic. Also, lines of different elements have different widths, indicating different expansion velocities. In particular, iron appears to expand more rapidly than oxygen. Furthermore, the [OI] nebular lines decline more slowly than the [FeII] ones, signalling deposition of gamma-rays in a slowly-moving O-dominated region. These facts suggest that the explosion was aspherical. The absence of [FeIII] nebular lines can be understood if the ejecta are significantly clumped. A schematic picture of what this very unusual stellar explosion may have looked like is presented.
Maeda Kengo
Mazzali Paolo A.
Nomoto Ken
Patat Ferdinando
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