The spectral characteristics of stripped -envelope supernovae

Supernovae (SNe) of Type Ib and Ic are rare and remain fairly mysterious objects. As they are thought to result from progenitors that have lost their massive envelopes, they are referred to as "stripped-envelope" SNe. This dissertation presents various studies of optical spectra of SNe Ib/c and the metamorphosing SNe that link them to SNe II as core-collapse events. The characteristics of SNe Ib and Ic may help resolve questions about the differences of their progenitors.;SN 1993J underwent a dramatic transformation from Type II to one resembling Type Ib. This metamorphosis is seen in the 42 low-dispersion spectra shown. Detailed analysis of the SN 1993J spectra shows that the ejecta are clumped, especially the synthesized oxygen. The calcium emission is smooth, implying that only pre-existing calcium contributes to the emission. The late-time spectra that exhibit signs of circumstellar interaction are studied in relation to theoretical models. Some emission lines develop a two-horned profile, indicating the formation of a flattened emission source.;SN 1999cq exhibits the first unambiguous detection of helium emission lines in SNe Ic. They are of intermediate velocity width and appear to show the presence of dense circumstellar material composed of almost pure helium. This material may be stellar matter lost through a dense wind or mass transfer to a companion.;More than 80 spectra of various SNe Ib and Ic are also presented. SNe Ib are found to exhibit fairly homogeneous spectra and helium strength. SNe Ib and Ic are shown to have distinguishing characteristics other than the presence or absence of helium lines. SNe Ic have clearly broader nebular-phase emission lines, implying that SNe Ic have a higher explosion energy and/or lower envelope mass than SNe Ib. SNe Ic also have stronger permitted oxygen lines at early times, implying that they are less diluted by a massive helium envelope. SNe Ic themselves are heterogeneous, with diverse spectroscopic characteristics. No strong evidence for helium is found, although some objects may have a low level of helium. These may be SNe that bridge the gap between SNe Ib and Ic.