AstroNote 2021-76

Motivated by recent discovery and classification reports of transients with spectra showing strong narrow (<~1000 km/s) emission lines (including potential narrow P-Cygni profiles) of carbon and/or oxygen, but lacking lines of hydrogen and helium, we propose to use a new spectroscopic SN type - SN Icn - to refer to such objects, following the current SN classification as implemented in types IIn and Ibn (events with narrow lines of hydrogen and helium, respectively).   

Fiducially we propose that events showing spectra with strong emission lines of CNO elements but lacking prominent lines of H and He (in particular the He II 4686) would be classified as SNe Icn. Several relevant example are listed in the table below, including recent reports by Perley et al. (AstroNote 2021-62; 2021TNSAN..62....1P), Pastorello et al. (AstroNote 2021-71; 2021TNSAN..71....1P) and the classification of the earlier SN 2019hgp (Class. report 9002; 2021TNSCR.547....1G). A potentially related object could be SN 2010mb where Ben-Ami et al. 2014 (2014ApJ...785...37B) reported transient narrow emission lines of oxygen, though that object appears to differ photometrically and spectroscopically from the recently reported events.

We propose that should transitional events between types Ibn (prominent He lines, no C/O lines) and Icn (prominent C/O, no/weak He) appear, i.e., events whose spectra show narrow C/N/O and He lines with comparable strengths, those events should be classified as Ibn/Icn.