The Progenitor of the Type II-P SN 2004dj in NGC 2403

The Type II-P supernova 2004dj in the nearby galaxy NGC 2403 occurred at a position coincident with object 96 in the list of luminous stars and clusters in this galaxy published by Sandage in 1984. We compare archival images of NGC 2403 with recent images showing the SN to establish a definitive association between Sandage 96 and the SN. The archival images show that Sandage 96 is slightly resolved from the ground. Pre-outburst spectrograms in the blue region obtained by Humphreys and Aaronson reveal that Sandage 96 has a composite spectrum, dominated in the blue region by A- and B-type stars, while infrared photometry shows that Sandage 96 also contains red supergiants. These results show that Sandage 96 is a young compact cluster. We have studied the stellar population of Sandage 96, using published photometric measurements combined with a chi-square-fitting code. We derive a cluster age of 14 Myr, a reddening of E(4405-5495)=0.135, and a total stellar mass of 24 000 M_Sun. For this cluster age, the SN progenitor had a main-sequence mass of 15 M_Sun. Post-outburst photometry of Sandage 96 should establish whether the progenitor was a red or blue supergiant.