OPTICAL-SPECTRA OF SUPERNOVAE

The temporal evolution of the optical spectra of various types of supe rnovae (SNe) is illustrated, in part to aid observers classifying supe rnova candidates. Type II SNe are defined by the presence of hydrogen, and they exhibit a very wide variety of photometric and spectroscopic properties. Among hydrogen-deficient SNe (Type I), three subclasses a re now known: those whose early-time spectra show strong Si II (Ia), p rominent He I (Ib), or neither Si II nor He I (Ic). The late-time spec tra of SNe Ia consist of a multitude of blended emission lines of iron -group elements; in sharp contrast, those of SNe Ib and SNe Ic (which are similar to each other) are dominated by several relatively unblend ed lines of intermediate-mass elements. Although SNe Ia, which result from the thermonuclear runaway of white dwarfs, constitute a rather ho mogeneous subclass, important variations in their photometric and spec troscopic properties are undeniably present. SNe Ib/Ic probably result from core collapse in massive stars largely stripped of their hydroge n (Ib) and helium (Ic) envelopes, and hence they are physically relate d to SNe II. Indeed, the progenitors of some SNe II seem to have only a low-mass skin of hydrogen; their spectra gradually evolve to resembl e those of SNe Ib. In addition to the two well-known photometric subcl asses (linear and plateau) of SNe II, which may exhibit minor spectros copic differences, there is a new subclass (SNe IIn) distinguished by relatively narrow emission lines with little or no P Cygni absorption component and slowly declining light curves. These objects probably ha ve unusually dense circumstellar gas with which the ejecta interact.