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33. HST Observations

Using the Wide Field and Planetary Camera 2 on HST, we have obtained a set of 12-epoch observations of NGC 1365. These observations were begun on August 6, and continued until September 24, 1995. The observing window of 44 days was selected to maximize target visibility, without necessitating any roll of the targeted field of view. Sampling within the window was prescribed by a power-law distribution, tailored to optimally cover the light and color curves of Cepheids with anticipated periods in the range 10 to 60 days (see Freedman et al. 1994a for additional details). Contiguous with 4 of the 12 V-band epochs, I-band exposures were also obtained so as to allow reddening corrections for the Cepheids to be determined. Each V-band epoch made use of the F555W filter and consisted of two exposures split between orbits (and allowing for cosmic ray rejection); a total of 5,100 sec of V-band data were obtained at each epoch in the course of the monitoring program. The I-band exposures (F814W) totaled 5,400 sec each, again cosmic-ray split and accumulated over two orbits.

All frames were pipeline pre-processed at the Space Telescope Science Institute in Baltimore and subsequently analyzed in Pasadena using ALLFRAME (a suite of special-purpose stellar photometry packages (Stetson 1994). A second independent reduction is being performed using the DoPhot photometry package. At this juncture, we are still allowing for the possibility of a 10% systematic offset in the true modulus derived from the two packages (Table 3); the results discussed here are based solely on the ALLFRAME analysis. Zero-point calibrations for the photometry were adopted (Holtzmann, J. et al. 1995; Hill et al. 1997), which agree to 0.05 mag on average. Details on the DoPHOT and ALLFRAME reduction and analysis of this data set are presented elsewhere (Silbermann, et al. 1997). We are also currently undertaking artificial star tests on these frames to quantify the uncertainty due to crowding.

Source of Uncertainty
on the Mean
Description of Uncertainty Percentage
[A] LMC True Modulus Independent Estimates = 18.50 ± 0.10 mag 5%
[B] V PL Zero Point LMC PL sigmaV = (0.27) / sqrt 31 = ± 0.05 mag 3%
[C] I PL Zero Point LMC PL sigmaI = (0.18) / sqrt 31 = ± 0.03 mag 2%
[SC] Systematic Uncertainty [A] + [B] + [C] combined in quadrature 6%
(D) HST V-Band Zero Point On-Orbit Calibration: ± 0.05 mag 3%
(E) HST I-Band Zero Point On-Orbit Calibration: ± 0.05 mag 3%
(M1) Cepheid True Modulus (D)(E) are uncorrelated, but coupled by reddening law: sigmaµ0 = ± 0.15 mag 7%
(F) Cepheid V Modulus NGC 1365 PL sigmaV = (0.32) / sqrt 37 = ± 0.05 mag 3%
(G) Cepheid I Modulus NGC 1365 PL sigmaI = (0.31) / sqrt 37 = ± 0.05 mag 3%
(M2) Cepheid True Modulus (F)(G) are partially correlated, giving sigmaµ0 = ± 0.06 mag 3%
(P1) V-Band Ap. Corr. Silberman et al. (1998) give ± 0.067 mag 3%
(P2) I-Band Ap. Corr. Silberman et al. (1998) give ± 0.061 mag 3%
(M3) Cepheid True Modulus (P1)(P2) are uncorrelated, but coupled by reddening law: sigmaµ0 = ± 0.15 mag 7%
[Z] Metallicity M31 metallicity gradient test gives sigmaµ0 = ± 0.08 mag 4%
M101 calibration gives +0.14 mag of Delta µ0 / Delta[Fe / H] = -0.25 mag/dex 7%
[H] Reduction Package Systematic differences in aperture corrections (achromatic) 4%
(J) Random Errors (M1) + (M2) + (M3) combined in quadrature 10%
[K] Systematic Errors [SC] + [Z] + [H] combined in quadrature 10%
D = 18.6 Mpc ± 1.9 (random) ± 1.9 [systematic]

Note: There are 32 Cepheids in the LMC with published VI photometry (Madore & Freedman 1991.) The measured dispersions in the period-luminosity relations at V and I are ± 0.27 and ± 0.18 mag, respectively.

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