Spectrum Analysis of Type IIb Supernova 1996cb

Abstract

We analyze a time series of optical spectra of SN 1993J-like supernova 1996cb, from 14 days before maximum to 86 days after that, with a parameterized supernova synthetic-spectrum code SYNOW. Detailed line identification are made through fitting the synthetic spectra to observed ones. The derived photospheric velocity, decreasing from 11,000 km/s to 3,000 km/s, gives a rough estimate of the ratio of explosion kinetic energy to ejecta mass, i.e. E/M_ej \sim 0.2-0.5 \times 10^51 ergs / M_ej(M_\sun). We find that the minimum velocity of hydrogen is about 10,000 km/s, which suggests a small hydrogen envelope mass of \sim 0.02-0.1 M_ej, or 0.1-0.2 M_\sun if E is assumed 1 \times 10^51 ergs. A possible Ni II absorption feature near 4000 Åis identified throughout the epochs studied here and is most likely produced by primordial nickel. Unambiguous Co II features emerge from 16 days after maximum onward, which suggests that freshly synthesized radioactive material has been mixed outward to a velocity of at least 7,000 km/s as a result of hydrodynamical instabilities. Although our synthetic spectra show that the bulk of the blueshift of [O I] 5577 net emission, as large as \sim 70 Åat 9 days after maximum, is attributed to line blending, a still considerable residual \sim 20 Åremains till the late phase. It may be evidence of clumpy or large-scale asymmetric nature of oxygen emission region.