A spectroscopic analysis of the energetic Type Ic hypernova SN 1997ef

The properties of the bright and energetic Type Ic SN 1997ef are investigated using a Monte Carlo spectrum synthesis code. Analysis of the earliest spectra is used to determine the time of outburst. The changing features of the spectrum and the light curve are used to probe the ejecta and to determine their composition, verifying the results of explosion calculations. Since synthetic spectra computed using our best explosion model, CO100, are only moderately good reproductions of the observations, the inverse approach is adopted, and a density structure is derived by demanding that it gives the best possible fit to the observed spectrum at every epoch analyzed. It is found that the density structure of model CO100 is adequate at intermediate velocities (5000-25,000 km s(-1)), but that a slower density decline (rho proportional to r(-4)) is required to obtain the extensive line blending at high velocities (25,000-50,000 km s(-1)) that is the characterizing feature of this and other energetic Type Ic Supernovae. Also, the inner hole in the density predicted by the model is found not to be compatible with the observed evolution of the spectrum, which reaches very low photospheric velocities at epochs of about 2 months. The best-fit in the density distribution results in somewhat different parameters for the SN, namely an ejecta mass of 9.6 M-. (vs. 7.6 M-. in CO100) and an explosion kinetic energy of 1.75 x 10(52) ergs (vs. 8 x 10(51) ergs in CO100). This revised value of the kinetic energy brings SN 1997ef closer to the value for the prototypical Type Ic hypernova SN 1998bw. The abundance distribution of model CO100 is found to hold well. The modified density structure is used to compute a synthetic light curve, which is found to agree very well with the observed bolometric light curve around maximum. The amount of radioactive Ni-56 produced by the SN is confirmed at 0.13 M-.. In the context of an axisymmetric explosion, a somewhat smaller kinetic energy than that of SN 1998bw may have resulted from the nonalignment of the symmetry axis of the SN and the line of sight. This might also explain the lack of evidence for a gamma-ray burst correlated with SN 1997ef.