Spectral Models of the Type Ic SN 1994I in M51

Abstract

We present detailed non-local thermodynamic equilibrium (NLTE) synthetic spectra for comparison with a time series of observed optical spectra of the Type Ic supernova 1994I which occurred in M51. With the exceptions of Si I and S I, we treat the important species in the formation of the spectrum in full NLTE. We present results for both a hydrodynamic model that has been fit to the light curve and for an illustrative custom crafted model that is more massive. Both models give reasonable fits to the overall observed spectra; however, neither is able to reproduce all the observed features. Some conspicuous observed features are absent and some predicted features are unobserved. No model that we have explored is able to satisfactorily reproduce the observed infrared feature near 1 micron on April 15, 1994 (+7d), which has been attributed to the triplet He I lambda 10830 transition. The low-mass hydrodynamic model produces an infrared feature with a blend of He I, C I, O I, and Si I--II lines, but it predicts a strong unobserved absorption feature near 6100 Angstroms due to Fe III, and the observed feature just blueward of 6000 Angstroms most likely due to Na D is not reproduced. The more massive model does a better job of reproducing the observed infrared lineshape, but also predicts the unobserved feature near 6100 Angstroms. The early-time spectrum of the low-mass model is far too blue; thus, a more massive model may be slightly favored. Since the predicted infrared feature is produced by a blend of so many elements and there is no overwhelming evidence for other helium features such as lambda 5876, it may be premature to conclude that SNe Ic unambiguously contain helium. Thus, we conclude that pure C+O cores are still viable progenitors for SNe Ic.