Evidence for Core Collapse in the Type Ib/c SN 1999ex

Abstract.

We present optical and infrared spectra of SN 1999ex, which are characterized by the lack of strong hydrogen lines, weak optical He I lines, and strong He I $\lambda$10830,20581. SN 1999ex provides a clear example of an intermediate case between pure Ib and Ic supernovae, which suggests a continuous spectroscopic sequence between SNe Ic to SNe Ib. Our UBVRIz photometric observations of SN 1999ex started only one day after explosion, which permitted us to witness an elusive transient cooling phase that lasted 4 days. The initial cooling and subsequent heating due to ⁵⁶Ni$\rightarrow$ ⁵⁶Co$\rightarrow$ ⁵⁶Fe produced a dip in the light curve which is consistent with explosion models involving core collapse of evolved massive helium stars, and not consistent with light curves resulting from the thermonuclear runaway of compact white dwarfs.