Cepheid Calibration of the Peak Brightness of SNe Ia. XI. SN 1998aq in NGC 3982

A. Saha, Allan Sandage, G. A. Tammann, A. E. Dolphin, J. Christensen, N. Panagia, F. D. Macchetto

Published 2001-07-19Version 1

Repeated imaging observations have been made of NGC 3982 with the Hubble Space Telescope between March and May 2000, over an interval of 53 days. Images were obtained on 12 epochs in the F555W band and on five epochs in the F814W band. The galaxy hosted the type Ia supernova SN1998aq. A total of 26 Cepheid candidates were identified, with periods ranging from 10 to 45 days, using photometry with the DoPHOT program. The de-reddened distance to NGC 3982 is estimated from these data using various criteria to maximize signal to noise and reliability: the values lie between 31.71 and 31.82, with uncertainties in the mean of typically +/- 0.14 mag for each case. A parallel analysis using photometry with HSTphot discovered 13 variables, yielding a distance modulus of 31.85 +/- 0.16. The final adopted modulus is (M-m)_0 = 31.72 +/- 0.14 (22 +/- 1.5 Mpc). Photometry of 1998aq that is available in the literature is used in combination with the derived distance to NGC 3982 to obtain the peak absolute magnitude of this supernova. The lower limit (no extinction within the host galaxy) for M_V is -19.47 +/- 0.15 mag. Corrections for decline rate and intrinsic color to carry these to the reduced system of Parodi et al. (2000) have been performed. The derived luminosities at hand are fully consistent with the mean of the 8 normal SNe Ia previously calibrated with Cepheids. Together they yield H_0 ~ 60 +/- 2 (internal) km s-1 Mpc-1 based on an assumed LMC distance modulus of 18.50. We point out that correcting some of the systematic errors and including uncertainty estimates due to them leads to H_0 = 58.7 +/- 6.3 (internal) km s-1 Mpc-1.