Abstract
ABSTRACTSpectroscopic analyses of Type Ia supernovae have shown that there exist four spectroscopic groups—cool, broad line, shallow silicon, and core normal—defined by the widths of the Si II features λ5972 and λ6355. The 1991bg-like SNe Ia are classified as “cool.” Cool SNe Ia are dim, undergo a rapid decline in luminosity, and produce significantly less 56Ni than normal Type Ia supernovae. They also have an unusually deep and wide trough in their spectra at around 4200 Å and a relatively strong Si II absorption attributed to λ5972. We examine the spectra of supernova (SN) 1991bg and the cool SN 1997cn, SN 1999by, and SN 2005bl using the highly parameterized synthetic spectrum code SYNOW, and we find general agreement with similar spectroscopic studies. Our analysis reveals that this group of supernovae is fairly homogeneous, with many of the blue spectral features well fit by Fe II. The nature of the spectroscopic commonalities and the variations in the class are discussed. Finally, we examine intermediates such as SN 2004eo and discuss the spectroscopic subgroup distribution of Type Ia supernovae.
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