AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 39. Results from the HST Key Project on the Extragalactic Distance Scale
Topical, Oral, Tuesday, June 1, 1999, 8:30-10:00am, 10:45am-12:30pm, 2:30-4:00pm, International Ballroom South

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[39.08] Type Ia Supernovae and the Value of the Hubble Constant

B.K. Gibson (CASA), P.B. Stetson (DAO), J.R. Mould (ANU), R.C. Kennicutt (Steward), W.L. Freedman (OCIW), J.P. Huchra (CfA), S. Sakai (KPNO), J.A. Graham, C.I. Fassett, D.D. Kelson (DTM), L. Ferrarese (Caltech), S.M.G. Hughes (IoA), G.D. Illingworth (UCSC), L.M. Macri (CfA), B.F. Madore (IPAC), K.M. Sebo (ANU), N.A. Silbermann (IPAC)

The primary observational task assigned the HST Key Project on the Extragalactic Distance Scale was the direct determination of distances to 18 nearby spirals via the search for, and discovery of, Cepheid variables. These galaxies provide the necessary calibration for secondary distance indicators such as the Tully-Fisher relation and Type Ia supernovae (SNe), thereby allowing a derivation of the Hubble Constant H\circ. Type Ia SNe are of particular importance as they (a) probe a regime in which complications due to the local flow field are minor, and (b) possess a Hubble Diagram intrinsic dispersion which is significantly smaller than other secondary indicators.

Unfortunately, our original sample was ill-suited for calibrating the Type Ia SNe Hubble Diagram due to its lack of Type Ia SNe-host galaxies. Because of this shortcoming, an important parallel program was undertaken by our team which led to the incorporation of complementary datasets from the Sandage/Saha Type Ia SNe and Tanvir et~al. Leo~I Projects, into our Key Project sample. These two datasets provide an additional 7 galaxies for which Cepheid-based distances can be derived and, more importantly, each plays host to a Type Ia SN event. Following the identical precepts and software ``pipeline'' adopted throughout the Key Project, a re-analysis of this archive data was completed, ensuring homogeneity with that of the Key Project's photometry and distances.

Our analysis of the local Type Ia SNe calibrators shows a systematic offset from the published Sandage/Saha and Tanvir et~al. distances, resulting in a 10% larger H\circ, bringing empirical Type Ia SNe-based H\circ determinations into excellent agreement with those inferred via other secondary indicators. I will describe the philosophy and mechanics of our re-analysis program, highlighting the divergence from the earlier published results, and stress the implications for the derived value of the Hubble Constant.

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