Schedule Feb 08, 2006
The Unique Type Ib Supernova 2005bf and Aspherical Explosions
Ken'ichi Nomoto (Tokyo)

Authors: K. Nomoto, N. Tominaga, P.A. Mazzali, J. Deng, K. Maeda, M. Tanaka(Tokyo), and M. Modjaz, R.P. Kirshner (CfA)
Observations and modeling for the light curve (LC) and spectra ofsupernova (SN) 2005bf are reported. This SN showed unique features: theLC had two maxima, and declined rapidly after the second maximum, whilethe spectra showed strengthening He lines whose velocityincreased with time. The double-peaked LC can be reproduced by adouble-peaked 56Ni distribution, with most 56Ni at low velocity and asmall amount at high velocity. The rapid post-maximum declinerequires a large fraction of the gamma-rays to escape from the56Ni-dominated region, possibly because of low-density ``holes''. Thepresence of Balmer lines in the spectrum suggests that the He layer of theprogenitor was substantially intact. Increasing gamma-raydeposition in the He layer due to enhanced gamma-ray escape from the56Ni-dominated region may explain both the delayed strengthening and theincreasing velocity of the He lines. The SN has massive ejecta (6-7Msun), normal kinetic energy (1.0-1.5 E51 ergs), high peakbolometric luminosity (5 E42 erg/s) for an epoch as late as 40 days, and alarge 56Ni mass (0.3 Msun). These properties, and the presence of a smallamount of H suggest that the progenitor was initiallymassive (25-30 Msun) and had lost most of its H envelope, possibly a WNstar. The double-peaked 56Ni distribution suggests that theexplosion may have formed jets that did not reach the He layer.The properties of SN 2005bf resemble those of the explosion ofCassiopeia A.

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