Quantitative Classification of Type I Supernovae Using Spectroscopic Features at Maximum Brightness

We present a set of new quantitative classification criteria for major subclasses of Type I Supernovae (SNe). We analyze peak spectra of 146 SNe Ia from the Berkeley Supernova Ia Program (BSNIP), 12 SNe Ib, 19 SNe Ic (including 5 SNe Ic-BL) and 4 SNe Ib/c. All the spectra are within 5 days of maximum light. We study their absorption line depths relative to the pseudo-continuum at around observed wavelength 6150{\AA}, attributed to Si II 6355{\AA} for SNe Ia and Ic and to hydrogen for SNe Ib, and O I $\lambda 7774$\AA\ observed around $\lambda 7500$\AA. We found that Type I SNe could be quantitatively classified using the line depths at these two absorption regions. Type Ia SNe, including normal SNe Ia, Ia-1991bg and Ia-1999aa, show strong signatures of Si II near peak with relative line depth values of $a{\rm (\lambda6150\AA)>0.35}$. Only SNe Ia-2002cx (Iax) do not satisfy this criterion, while the $a{\rm (O\ I\ \lambda 7774\AA)}$ index can separate SNe Ia-1991bg and Ia-1999aa cleanly. Type Ib SNe satisfy that $a{\rm (\lambda6150\AA)<0.35}$ and $a{\rm (\lambda6150\AA)} / a{\rm (O\ I\ \lambda 7774\AA)>1}$, while regular Type Ic SNe show $a{\rm (\lambda6150\AA)<0.35}$ but $a{\rm (\lambda6150\AA)} / a{\rm (O\ I\ \lambda 7774\AA)<1}$. These quantitive classification criteria work well for the majority of normal Type I SN subclasses. Peculiar SNe Ia-2002cx and SNe Ic-BL are the exception. We apply these criteria to provide clear identifications to intermediate or unclear SNe Ib/c. The results show good consistency with classification using specific spectral line features. Though there are still some puzzles about the physical mechanisms driving the diverse line ratios, our method is an accurate empirical method to identify the majority of Type I SNe.