2001ApJ...562..314S

Repeated imaging observations have been made of NGC 3982 with the Hubble Space Telescope between 2000 March and May, 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 (SN Ia) SN 1998aq. A total of 26 Cepheid candidates were identified, with periods ranging from 10 to 45 days, using photometry with the DoPHOT program. The dereddened distance to NGC 3982 is estimated from these data using various criteria to maximize signal-to-noise ratio 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 SN 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 and collaborators have been performed. The derived luminosities at hand are fully consistent with the mean of the eight normal SNe Ia previously calibrated with Cepheids. Together they yield H₀~60±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 H₀=58.7±6.3(internal) km.s^–1 Mpc^–1.