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SN 2007bi and SN 1999as are among the first superluminous supernovaediscovered. SN 2007bi was suggested to be powered by the radioactive decay of alarge amount (5 - 10 Msun) of 56Ni. SN 1999as has a similar spectrum to SN2007bi. One suggested way to synthesize such a large amount of 56Ni is throughenergetic core-collapse supernovae from very massive progenitors. Although thesynthetic light curves of extremely energetic core-collapse supernovae havebeen shown to be consistent with SN 2007bi, no synthetic spectra have beenreported. Here, we present synthetic spectra of extremely energeticcore-collapse supernovae during the photospheric phases. We find that theejecta density structure above 13,000 - 16,000 km/s needs to be cut in order toexplain the co-existing broad and narrow line absorptions in SN 2007bi and SN1999as. The density cut is likely caused by the interaction between thesupernova ejecta and a dense circumstellar medium. Our results indicate thatabout 3 Msun of hydrogen-free dense circumstellar media might exist near theprogenitors of SN 2007bi and SN 1999as. These massive circumstellar media wouldsignificantly affect the light-curve and spectral properties of the supernovae.The precursors that are sometimes observed in superluminous supernovae might berelated to the collision of the ejecta with such dense circumstellar media. Wealso confirm results of previous studies that synthetic spectra frompair-instability supernova models do not match the early spectra of SN 2007biand SN 1999as.

Synthetic spectra of energetic core-collapse supernovae and the early spectra of SN 2007bi and SN 1999as