Origin of the 6500 Å emission feature during the second peak

Determine the physical origin of the broad emission feature near rest wavelength 6500 Å observed in spectra at the second maximum of SN2020acct by distinguishing among Hα, C II λ6580 Å, [O I] λλ6300, 6364 Å, [N II], or blended combinations, and reconcile the implied line velocities with the measured bulk ejecta velocities.

Background

During the second peak, SN2020acct exhibits spectra characteristic of a stripped-envelope supernova but with relatively low expansion velocities and early-time forbidden emission. A prominent broad feature around 6500 Å is present, but its identification is ambiguous.

The authors discuss several possibilities for the 6500 Å feature, including Hα, C II λ6580 Å, [O I] λλ6300, 6364 Å, [N II], or a combination thereof, noting that attributing it to Hα would imply a velocity inconsistent with the bulk of the ejecta and that no corresponding Hβ emission is seen.

Resolving the line identification is important for constraining the physical conditions in the ejecta–CSM environment at peak and for distinguishing between hydrogen-bearing versus hydrogen-poor interaction and composition scenarios.