The Extragalactic Distance Scale Key Project. V. Photometry of the Brightest Stars in M100 and the Calibration of WFPC2

We present multiepoch VI observations of the brightest stars in the Virgo spiral galaxy M100 obtained with the WFPC2 aboard the Hubble Space Telescope. Particular attention has been given to the WFPC2 calibration process, and a new calibration is presented here. We find that a calibration based on short exposure (t ≤ 60 sec) observations of the galactic globular clusters ω Cen, Pal 4, and NGC 2419 yields results that are in excellent agreement with the calibration of Holtzmann et al. However, a calibration based on longer exposures (t ≥ 1300 s) yields magnitudes that are systematically brighter by 0.05 mag, on average, in both V and I.

We have independently employed both the DAOPHOT/ALLFRAME and DoPHOT photometry routines in reducing the M100 data and have found that the two packages yield magnitudes that agree to better than 0.05 mag except for the WFPC2 chip 3 and WFPC2 chip 4 I data. Severe crowding appears to be the problem in the case of the WFPC2 chip 3 data, but we have been unable to determine the source of the discrepancy for the WFPC2 chip 4 I data. We consider the better than 0.05 mag agreement for the other chip/filter combinations to be quite good under the circumstances prevailing in this data set: an undersampled point-spread function (PSF), faint stars, a rapidly varying background, and moderate to severe crowding. A color-magnitude diagram for the brightest stars in M100 is also presented, which shows a plume of bright blue supergiants extending to at least V ~ 23 mag, or M_V ~ -8 mag.