Light curves and H$\alpha$ luminosities as indicators of $^{56}$Ni mass in type IIP supernovae

The possibility is investigated that the H$\alpha$ luminosity at the nebular epoch may be an additional indicator of $^{56}$Ni mass in type II supernovae with plateau (SNe IIP), on the basis of available photometry and spectra. We first derive the $^{56}$Ni mass from the $M_V$ magnitude on the radioactive tail using a standard approach. A confirmation of the correlation between $^{56}$Ni mass and plateau $M_V$ magnitude found recently by Hamuy (2003) is evident. There is strong evidence of a correlation between steepness of the $V$ light curve slope at the inflection time and the $^{56}$Ni mass. If confirmed, this relation may provide distance and extinction independent estimates of the amount of $^{56}$Ni in SNe IIP. We then apply upgraded radioactive models of H$\alpha$ luminosity at the nebular epoch and claim that it may be a good indicator of $^{56}$Ni, if mass, energy and mixing properties vary moderately (within factor $\sim 1.4$) among SNe IIP . This method of the $^{56}$Ni mass determination from H$\alpha$ luminosities yields results which are consistent with the photometric mass of $^{56}$Ni mass to within 20%. This result also implies that the parameters of SNe IIP events (mass, energy and mixing properties) are rather similar among the majority of SNe IIP, except for rare cases of SN II intermediate between IIP and IIL (linear), of which SN 1970G is an example.