The Spectral Characteristics of Stripped‐Envelope Supernovae

Abstract

Supernovae (SNe) of Type Ib and Ic are distinguished by what they lack. They do not have the hydrogen lines indicative of Type II SNe, nor do they show the silicon absorption feature near 6150 A that is the hallmark of Type Ia SNe. (Type Ib SNe show helium absorption lines, while Type Ic SNe do not.) Type Ib and Ic SNe 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. We present various studies of optical spectra of SNe Ib/c and the metamorphosing SNe that link them to SNe II as core-collapse events. The spectroscopic characteristics of SNe Ib and Ic may help resolve questions about the nature of their progenitors. SN 1993J appeared originally to be a Type II SN, but it underwent a dramatic transformation to one resembling a Type Ib, although the hydrogen lines never completely faded away. This metamorphosis is seen in the 42 low-dispersion spectra shown by T. Matheson et al. (2000, AJ, 120, 1487). Detailed analysis of the SN 1993J spectra is presented by T. Matheson et al. (2000, AJ, 120, 1499). The spectra show that the ejecta are clumped, especially the synthesized oxygen. The calcium emission is smooth, implying that only preexisting calcium contributes to the emission. The late-time spectra exhibit signs of circumstellar interaction with a roughly spherical shell. Some emission lines develop a two-horned profile, indicating the formation of a flattened, disklike emission source. SN 1999cq exhibits the first unambiguous detection of helium emission lines in SNe Ic (T. Matheson et al. 2000, AJ, 119, 2303). 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. We also present more than 80 spectra of various SNe Ib and Ic (T. Matheson et al. 2001, AJ, 121, 1648). SNe Ib are found to exhibit fairly homogeneous spectra and helium line strength. The relative depths of the helium absorption lines appear to provide a measurement of the temporal evolution of SNe Ib. 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, indicating 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. This lends strength to the argument that the difference between SNe Ib and SNe Ic arises from the constituents of the overlying envelope of the star at the time of core collapse, rather than the extent of mixing of radioactive nickel into the helium layers of the progenitor. SNe Ic themselves are heterogeneous, with diverse spectroscopic characteristics. No strong evidence for helium is found in SNe Ic, although some objects may have a low level of helium. These may be SNe that bridge the gap between SNe Ib and Ic.