The type IIb SN 2008ax: spectral and light curve evolution

Pastorello, A.
Kasliwal, M. M.
Crockett, R. M.
Valenti, S.
Arbour, R.
Itagaki, K.
Kaspi, S.
Gal-Yam, A.
Smartt, S. J.
Griffith, R.
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We present spectroscopy and photometry of the He-rich supernova (SN) 2008ax. The early-time spectra show prominent P-Cygni H lines, which decrease with time and disappear completely about two months after the explosion. In the same period He I lines become the most prominent spectral features. SN 2008ax displays the ordinary spectral evolution of a type IIb supernova. A stringent pre-discovery limit constrains the time of the shock breakout of SN 2008ax to within only a few hours. Its light curve, which peaks in the B band about 20 days after the explosion, strongly resembles that of other He-rich core-collapse supernovae. The observed evolution of SN 2008ax is consistent with the explosion of a young Wolf-Rayet (of WNL type) star, which had retained a thin, low-mass shell of its original H envelope. The overall characteristics of SN 2008ax are reminiscent of those of SN 1993J, except for a likely smaller H mass. This may account for the findings that the progenitor of SN 2008ax was a WNL star and not a K supergiant as in the case of SN 1993J, that a prominent early-time peak is missing in the light curve of SN 2008ax, and that Halpha is observed at higher velocities in SN 2008ax than in SN 1993J.
Comment: 10 pages, including 8 figures and 4 tables. Accepted for publication in MNRAS