On the progenitor of the type II-plateau supernova 2003gd in M74

Hubble Space Telescope (HST) WFPC2 archival F606W and F300W images obtained within 1 year prior to the explosion of the nearby Type II supernova (SN) 2003gd in M74 (=NGC 628) have been analyzed to isolate the progenitor star. The SN site was located using precise astrometry applied to the HST images. Two plausible candidates are identified within 0.6 of the SN position in the F606W image. Neither candidate was detected in the F300W image. SN 2003gd appears to be of Type II-plateau (II-P), with age approximate to87 days on June 17 UT and with low reddening [E(B-V) = 0.13 mag]. The most likely of the two progenitor candidates has M-V0 approximate to -3.5 mag (for an extinction-corrected distance modulus of 29.3 mag), and on the basis of additional color information derived from a high-quality, archival ground-based I-band image, we estimate that this star was a red supergiant with initial (zero-age main sequence) mass M-ZAMS approximate to 8-9 M.. This mass estimate is somewhat lower than, but relatively consistent with, recent limits placed on the progenitor masses of other SNe II-P, using HST data. Future HST imaging with the HST Advanced Camera for Surveys, when the SN has faded considerably, will be very useful in pinpointing the exact SN location and securing identification of the progenitor. If our proposed candidate is confirmed, it will be only the sixth SN progenitor ever directly identified.