The nature of the progenitor of the Type II - P supernova 1999

The masses and the evolutionary states of the progenitors of core-collapse supernovae are not well constrained by direct observations. Stellar evolution theory generally predicts that massive stars with initial masses less than about 30M should undergo core-collapse when they are cool M-type supergiants. However the only two detections of a SN progenitor before explosion are SN1987A and SN1993J, and neither of these was an M-type supergiant. Recently we have set an upper mass-limit to the progenitor of Type II-P SN1999gi of 9 −2M (Smartt et al. 2001a) from pre-explosion Hubble Space Telescope images. In this paper we present high quality ground-based V RI images of the site of the Type II-P SN1999em (in NGC1637) taken before explosion, which were extracted from the CFHT archive. We determine a precise position of the SN on these images to an accuracy of 0.17′′. The host galaxy is close enough (7.5 ± 0.5 Mpc) that the bright supergiants are resolved as individual objects, however we show that there is no detection of an object at the SN position before explosion that could be interpreted as the progenitor star. By determining the sensitivity limits of the V RI data, and assuming the reddening toward the progenitor is similar to that toward the SN itself, we derive bolometric luminosity limits for the progenitor. Comparing these to standard stellar evolutionary tracks which trace evolution up to the point of core carbon ignition, we initially derive an upper mass limit of approximately 12M . However we present evolutionary calculations that follow 7-12M stars throughout their C-burning lifetime and show that we can restrict the mass of the progenitor even further. Our calculations indicate that progenitors initially of 8-10M , undergoing expected mass loss, can also be excluded because a second dredge up sends them to somewhat higher luminosities than a star of initially 12 M . These results limit the progenitor’s initial main-sequence