Improved Distances to Type Ia Supernovae with Multicolor Light Curve Shapes: MLCS2k2

We present an updated version of the Multicolor Light Curve Shape method to measure distances to type Ia supernovae (SN Ia), incorporating new procedures for Kcorrection and extinction corrections. We also develop a simple model to disentangle intrinsic color variations and reddening by dust, and expand the method to incorporate U-band light curves and to more easily accommodate prior constraints on any of the model parameters. We apply this method to 133 nearby SN Ia, including 95 objects in the Hubble flow (cz ≥ 2500 km s−1), which give an intrinsic dispersion of less than 7% in distance. The Hubble flow sample, which is of critical importance to all cosmological uses of SN Ia, is the largest ever presented with homogeneous distances. We find the Hubble flow supernovae with H0dSN ≥ 7400 km s−1 yield an expansion rate that is 6.5 ± 1.8% lower than the rate determined from supernovae within that distance, and this can have a large effect on measurements of the dark energy equation of state with SN Ia. Peculiar velocities of SN Ia host galaxies in the rest frame of the Local Group are consistent with the dipole measured in the Cosmic Microwave Background. Direct fits of SN Ia that are significantly reddened by dust in their host galaxies suggest their mean extinction law may be described by RV ≃ 2.7, but optical colors alone provide weak constraints on RV . Subject headings: cosmology: observations — distance scale — galaxies: distances and redshifts — supernovae: general 1Miller Institute and Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720 2present address: Kavli Institute for Particle Astrophysics and Cosmology, Stanford Linear Accelerator Center, 2575 Sand Hill Road MS 29, Menlo Park, CA 94025 3Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 4Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138