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We present a set of new quantitative classification criteria for majorsubclasses of Type I Supernovae (SNe). We analyze peak spectra of 146 SNe Iafrom the Berkeley Supernova Ia Program (BSNIP), 12 SNe Ib, 19 SNe Ic (including5 SNe Ic-BL) and 4 SNe Ib/c. All the spectra are within 5 days of maximumlight. We study their absorption line depths relative to the pseudo-continuumat around observed wavelength 6150{\AA}, attributed to Si II 6355{\AA} for SNeIa and Ic and to hydrogen for SNe Ib, and O I $\lambda 7774$\AA\ observedaround $\lambda 7500$\AA. We found that Type I SNe could be quantitativelyclassified 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 SiII 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. TypeIb 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 $a{\rm (\lambda6150\AA)} / a{\rm (O\ I\ \lambda7774\AA)<1}$. These quantitive classification criteria work well for themajority of normal Type I SN subclasses. Peculiar SNe Ia-2002cx and SNe Ic-BLare the exception. We apply these criteria to provide clear identifications tointermediate or unclear SNe Ib/c. The results show good consistency withclassification using specific spectral line features. Though there are stillsome puzzles about the physical mechanisms driving the diverse line ratios, ourmethod is an accurate empirical method to identify the majority of Type I SNe.

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