SN 2017dio shows both spectral characteristics of a type-Ic supernova ( SN ) and signs of a hydrogen-rich circumstellar medium ( CSM ) .
Prominent , narrow emission lines of H and He are superposed on the continuum .
Subsequent evolution revealed that the SN ejecta are interacting with the CSM .
The initial SN Ic identification was confirmed by removing the CSM interaction component from the spectrum and comparing with known SNe Ic , and reversely , adding a CSM interaction component to the spectra of known SNe Ic and comparing them to SN 2017dio .
Excellent agreement was obtained with both procedures , reinforcing the SN Ic classification .
The light curve constrains the pre-interaction SN Ic peak absolute magnitude to be around M _ { g } = -17.6 mag .
No evidence of significant extinction is found , ruling out a brighter luminosity required by a SN Ia classification .
These pieces of evidence support the view that SN 2017dio is a SN Ic , and therefore the first firm case of a SN Ic with signatures of hydrogen-rich CSM in the early spectrum .
The CSM is unlikely to have been shaped by steady-state stellar winds .
The mass loss of the progenitor star must have been intense , \dot { M } \sim 0.02 ( \epsilon _ { H \alpha } / 0.01 ) ^ { -1 } ( v _ { \textrm { wind } } / 500 km s ^ { -1 } ) ( v _ { \textrm { shock } } / 10000 km s ^ { -1 } ) ^ { -3 } M _ { \odot } yr ^ { -1 } , peaking at a few decades before the SN .
Such a high mass loss rate might have been experienced by the progenitor through eruptions or binary stripping .