Properties of two hypernovae entering the nebular phase: SN 1997ef and SN 1997dq

The two peculiar Type Ic supernovae (SNe) 1997ef and 1997dq are shown to have very similar photometric and spectral evolution in the epochs when both SNe are observed (i.e., beyond similar to80 days after the explosion). The early light curves and spectra of SN 1997ef suggested that this was a "hypernova,'' or "SN 1998bw-like Type Ic supernova.'' The fact that the two SNe are very similar allows us to extend the time coverage of this type of event, since SN 1997dq, unlike SN 1997ef, was observed well into the nebular phase. In contrast to SN 1998bw, the spectra of these two SNe did not become fully nebular until almost 1 yr after the explosion. During a long transition phase, lasting at least 6 months, the SNe developed nebular emission in lines of [O I] and [Ca II], but at the same time they retained an underlying, photospheric-type spectrum, originating at very low velocities. Spectral synthesis techniques are used to model the spectrum of SN 1997dq, suggesting that it produced similar to0.16 M. of (56)Ni, and that a significant fraction of this is located in a dense, low-velocity inner region.