SN Ib 1990I: Clumping and dust in the ejecta?

Photometry and spectra of the type Ib SN 19901 are presented and analysed, covering about 400 days of evolution. The good quality of the data allow one to set reliably the supernova age (i.e. both date of maximum and explosion time estimates are given). The presence of optical helium lines is shown. SN 19901 seems to show high velocities compared to a sample of type Ib events. The nebular emission lines display a high degree of asymmetry and the presence of fine structures, suggestive of non-spherical clumping in the ejecta of SN 19901. Using the [O I] 6300,64 A flux, we estimate a lower limit on the oxygen mass to fall in the range (0.7-1.35) M ○. . The oxygen mass requires a filling factor as small as ∼10 -2 on day 254, indicating a highly clumpy distribution of the oxygen material. The amplitude and evolution of the [Ca II]/[O I] flux ratio is similar to that observed in a sample of SNe Ib/c events and thus suggests similar progenitor masses. A blueshift of order 600 km s -1 is reported in the [O I] 6300,64 A after day 254. The [Ca II] 7307.5 A emission profile appears blueshifted as well at late epochs. We recover the quasi-bolometric BVRI light curve of SN 19901. The constructed bolometric light curve shows a change of slope at late phases, with an e-folding time of 60 ± 2 d in the [50:200] d time interval, considerably faster than the one of 56 Co decay (i.e. 111.3 d), suggesting that the γ-rays escape with lower deposition, owing to the low mass nature of the ejecta. After day 200, an e-folding time ≃47 ± 2.8 d is measured. While the light curves of SNe 19901 and 1993J are similar in the [30:100] d time range, they tend to behave differently after day 200. A simplified γ-ray deposition model is applied after adding a contribution of about 35% to the computed pseudo-bolometric light curves to account for near-IR luminosities to estimate the ejecta and 56 Ni masses (M( 56 Ni) = 0.11 M ○. and M ej = 3.7 M ○. ). The deficit in luminosity is estimated to be about 50% around day 308. The observed spectral blueshift combined with the dramatic and sudden drop in the pseudo-bolometric light curve and (B - V) colour is interpreted to be a consequence of dust condensation in the ejecta of SN 19901 around day 250.