The Progenitor of SN 2004dj in a Star Cluster

The progenitor of Type II-P (P = plateau) supernova SN 2004dj is identified with a supergiant in a compact star cluster known as ``Sandage's star 96'' (S96) in the nearby spiral galaxy NGC 2403, which was fortuitously imaged as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2003 December prior to SN 2004dj. The superior photometry of BATC images for S96, taken with 14 intermediate-band filters covering 3000-10000 Å, unambiguously establishes the star cluster nature of S96, with an age of ~20 Myr, a reddening of E(B-V)~0.35 mag, and a total mass of ~96,000 M_solar. The compact star cluster nature of S96 is also consistent with the lack of light variations in the past decade. The SN progenitor is estimated to have a main-sequence mass of ~12 M_solar. The comparison of our intermediate-band data of S96 with the postoutburst photometry obtained as the SN significantly dims with time may hopefully conclusively establish the nature of the progenitor.