Cepheid calibration of the peak brightness of Type Ia supernovae. XI. SN 1998aq in NGC 3982

Repeated imaging observations have been made of NGC 3982 with the Hubble Sp ace Telescope between 2000 March and May, over an interval of 53 days. Imag es were obtained on 12 epochs in the F555W band and on five epochs in the F 814W band. The galaxy hosted the Type Ia supernova (SN Ia) SN 1998aq. A tot al of 26 Cepheid candidates were identified, with periods ranging from 10 t o 45 days, using photometry with the DoPHOT program. The dereddened distanc e to NGC 3982 is estimated from these data using various criteria to maximi ze 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 ca se. A parallel analysis using photometry with HSTphot discovered 13 variabl es, yielding a distance modulus of 31.85 +/- 0.16. The final adopted modulu s is (M-m)(0) = 31.72 +/- 0.14 (22 +/- 1.5 Mpc). Photometry of SN 1998aq th at is available in the literature is used in combination with the derived d istance 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 the se to the reduced system of Parodi and collaborators have been performed. T he derived luminosities at hand are fully consistent with the mean of the e ight normal SNe Ia previously calibrated with Cepheids. Together they yield H-0 approximate to 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 sy stematic errors and including uncertainty estimates due to them leads to H0 = 58.7 +/- 6.3(internal) km s(-1) Mpc(-1).