The study of a Type IIb supernova: SN 1996cb

We present spectroscopic and photometric results of SN 1996cb. The supernov a was independently discovered in NGC 3510 by M. Aoki, T. Cho, & K. Toyama of Japan and Qiao et al. of Beijing Astronomical Observatory on 1996 Decemb er 15 and 18, respectively. The results cover about 6 months following the discovery. The first few spectra showed strong Balmer lines with obvious P Cygni profiles, offering evidence of a Type II supernova. The emergence of He I lines could be inferred in these spectra. That the He I lines became q uite prominent in the spectra near optical maximum confirmed that SN 1996cb was definitely a Type IIb supernova, like SN 1987K and SN 1993J. The photometric results showed that the B-I color evolution was very simila r to that of SN 1993J. Comparing two color curves, we were able to estimate that the explosion of SN 1996cb occurred on UT 1996 December 12. Although the overall light curves resembled that of SN 1993J, they showed some diffe rences, especially for the B band. SN 1996cb had a broad peak in the light curves, and it declined somewhat slowly and, compared with SN 1993J, exhibi ted a plateau-like shape between 20 and 50 days after the maximum for the B and V bands. This indicates that there was relatively more hydrogen in the outer envelope of the progenitor of SN 1996cb. The spectral evolution of SN 1996cb displayed further differences from SN 1 993J. In the case of SN 1996cb, the Balmer lines showed strong P Cygni prof iles at a very early time, resembling the early spectra of SN 1987A, a supe rnova resulting from a compact blue supergiant star. The dramatic changes o f expansion velocities at early times indicated that the photosphere of SN 1996cb receded more quickly than it did in SN 1993J. The He I lines emerged much earlier and evolved more dramatically, causing SN 1996cb to display t he features of a Type Ib supernova before maximum. This might be the result of a dramatic recession of the photosphere at an early time; the He I line s and [O I] lines showed conspicuous blueshifts when they emerged, resembli ng the blueshifts of the [O I] lines that appeared in the late spectra of S N 1993J. This is probably observational evidence of Rayleigh-Taylor instabi lities occurring at the interfaces between the H and He and the He and O+C layers, respectively; H alpha emission and absorption components, especiall y the latter, were conspicuous 100 days after the explosion. We also conclu de that the outer envelope of SN 1996cb had relatively more hydrogen than w as the case for SN 1993J, even though the amount remained much less than is typical of other Type II supernovae. This finding is consistent with the r esults of the photometry. The [O I] and O lines emerged very late and exhib ited weak emission, indicating that the He-rich layer was relatively thick. Combining the analyses of photometric and spectroscopic evolution, we conc lude that the progenitor of SN 1996cb, like that of SN 1993J, was a strippe d massive star exhibiting some special features: it was probably a more com pact star with a thick helium layer and a relatively more massive hydrogen envelope.