CONSTRAINTS ON COSMOLOGICAL MODELS FROM HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF HIGH-Z SUPERNOVAE

We have coordinated Hubble Space Telescope (HST) photometry with groun d-based discovery for three supernovae: Type Ia supernovae near z appr oximate to 0.5 (SN 1997ce, SN 1997cj) and a third event at z = 0.97 (S N 1997 ck). The superb spatial resolution of HST separates each supern ova from its host galaxy and leads to good precision in the light curv es. We use these light curves and relations between luminosity, light- curve shape, and color calibrated from low-z samples to derive relativ e luminosity distances that are accurate to 10% at z approximate to 0. 5 and 20% at z = 1. When the HST sample is combined with the distance to SN 1995K (z = 0.48), analyzed by the same precepts, we find that ma tter alone is insufficient to produce a flat universe. Specifically, f or Omega(m) + Omega(Lambda) = 1, Omega(m) is less than 1 with more tha n 95% confidence, and our best estimate of Omega(m) is -0.1 +/- 0.5 if Omega(Lambda) = 0. Although this result is based on a very small samp le whose systematics remain to be explored, it demonstrates the power of HST measurements for high-redshift supernovae.