AAS 205th Meeting, 9-13 January 2005
Session 69 Cosmology: Population III, Distant SNe, and Dark Energy
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

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[69.05] Cepheid Calibrations from the Hubble Space Telescope of the Luminosity of Two Modern Type Ia Supernovae and a Re-determination of the Hubble Constant

A. G. Riess (STScI), W. Li (UC Berkeley), P. B. Stetson (Herzberg Institute of Astrophysics, Victoria), A. V. Filippenko, S. Jha (UC Berkeley), R. P. Kirshner (Harvard), P. M. Garnavich (Notre Dame), R. Chornock (UC Berkeley), P. Challis (Harvard)

We report photometric observations of two nearby type Ia supernovae, SN 1994ae and SN 1998aq, and the farthest direct observations of Cepheids, using the Advanced Camera for Surveys on HST. Both of these SNe Ia are spectroscopically and photometrically normal with low-reddening, precise and complete light curves commencing before maximum and obtained with CCDs. These qualities make them rare, ``ideal'' calibrators of the zeropoint of the modern SN Ia Hubble diagram, free from significant systematic uncertainties which plague the majority of previous calibrators. Previous determinations of the Hubble constant from lower-quality calibrators yield differences of up to ~ 20%. Using more reliable calibrators appears to mitigate some sources of this discrepancy. We present a re-determination of the Hubble Constant. A detailed analysis of the sensitivity of our results to light curve fitting method, Cepheid selection, and period-luminosity relation is included.

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