The surface abundances of red supergiants at core collapse

In the first weeks-to-months of a Type II-P supernova (SN), the spectrum formation region is within the hydrogen-rich envelope of the exploding star. Optical spectra taken within a few days of the SN explosion, when the photosphere is hot, show features of ionized carbon, nitrogen, and oxygen, as well as hydrogen and helium. Quantitative analysis of this very early phase may therefore constrain the chemical abundances of the stellar envelope at the point of core collapse. Using existing and new evolutionary calculations for red supergiants (RSGs), we show that the predictions for the terminal surface C/N ratio is correlated with the initial mass of the progenitor star. Specifically, a star with an initial mass above 20 M⊙ exploding in the RSG phase should have an unequivocal signal of a low C/N abundance. Furthermore, we show that the model predictions are relatively insensitive to uncertainties in the treatment of convective mixing. Although there is a dependence on initial rotation, this can be dealt with in a probabilistic sense by convolving the model predictions with the observed distribution of stellar rotation rates. Using numerical experiments, we present a strategy for using very early-time spectroscopy to determine the upper limit to the progenitor mass distribution for Type II-P SNe.