SN 2004A: Another Type II-P Supernova with a Red Supergiant Progenitor

We present a monitoring study of SN 2004A and probable discovery of a progenitor star in pre-explosion Hubble Space Telescope (HST) images. The photometric and spectroscopic monitoring of SN 2004A show that it was a normal Type II-P which was discovered in NGC 6207 about two weeks after explosion. We compare SN 2004A to the similar Type II-P SN 1999em and estimate an explosion epoch of 2004 January 6. We also calculate three new distances to NGC 6207 of 21.0 ± 4.3, 21.4 ± 3.5 and 25.1 ± 1.7 Mpc. The former was calculated using the Standard Candle Method (SCM) for SNe II-P, and the latter two from the brightest supergiants method (BSM). We combine these three distances with existing kinematic distances, to derive a mean value of 20.3 ± 3.4 Mpc. Using this distance, we estimate that the ejected nickel mass in the explosion is 0.046+0.031-0.017 Mo. The progenitor of SN 2004A is identified in pre-explosion WFPC2 F814W images with a magnitude of mF814W = 24.3 ± 0.3, but is below the detection limit of the F606W images. We show that this was likely a red supergiant (RSG) with a mass of 9+3-2 Mo. The object is detected at 4.7σ above the background noise. Even if this detection is spurious, the 5σ upper limit would give a robust upper mass limit of 12 Mo for a RSG progenitor. These initial masses are very similar to those of two previously identified RSG progenitors of the Type II-P SNe 2004gd (8+4-2 Mo) and 2005cs (9+3-2 Mo).