Formation and Diagnostic Use of Carbon Lines in Stripped-Envelope Supernovae

Carbon is one of the main end products of nucleosynthesis in massive stars. In this work, we study the emission signatures of carbon in spectra of stripped envelope supernovae (SESNe). A grid of model nebular spectra is created using the NLTE radiative transfer code SUMO, with stellar evolution- and explosion models as inputs. In the models, [C I] λ8727 and [C I] λλ9824, 9850 are identified as the only significant optical carbon lines, with contribution from both the O/C and He/C zones. To obtain estimates of L_[C I] λ8727, which is blended with the Ca II triplet, we introduce and apply the CaNARY code, a publicly available Monte Carlo scattering code. We study carbon lines in a sample of SESNe, and find that luminosities of [C I] λλ9824, 9850 relative to the optical spectrum increase with time, just as in our model grid. However, the relative luminosities of both [C I] λλ9824, 9850 and [C I] λ8727 are overproduced in our models. Multiple explanations for this discrepancy, such as too high carbon abundances in the stellar evolution models and underestimated cooling through molecule formation, are investigated. For those SNe where both lines are clearly observed, we use an analytical formalism to constrain their ejected carbon masses to the range ~0.2 - 2 Msun. However, several SNe yield upper limits of 0.05 Msun. We also show that [C I] λλ9824, 9850 is a useful line to diagnose both carbon mass and the extent of the He/C zone. We strongly encourage observers and instrumentalists to target [C I] λλ9824, 9850 in future SN observing campaigns.