We report on work to increase the number of well-measured Type Ia supernovae (SNe Ia) at high redshifts. Light curves, including high signal-to-noise HST data, and spectra of six SNe Ia that were discovered during 2001 are presented. Additionally, for the two SNe with z > 1, we present groundbased J-band photometry from Gemini and the VLT. These are among the most distant SNe Ia for which ground based near-IR observations have been obtained. We add these six SNe Ia together with other data sets that have recently become available in the literature to the Union compilation (Kowalski et al. 2008). We have made a number of refinements to the Union analysis chain, the most important ones being the refitting of all light curves with the SALT2 fitter and an improved handling of systematic errors. We call this new compilation, consisting of 557 supernovae, the Union2 compilation. The flat concordance ΛCDM model remains an excellent fit to the Union2 data with the best fit constant equation of state parameter w = −0.997 +0.050 −0.054 (stat)+0.077 −0.082 (stat + sys together) for a flat universe, or w = −1.035 +0.055 −0.059 (stat)+0.093 −0.097 (stat + sys together) with curvature. We also present improved constraints on w(z). While no significant change in w with redshift is detected, there is still considerable room for evolution in w. The strength of the constraints depend strongly on redshift. In particular, at z 1, the existence and nature of dark energy are only weakly constrained by the data.

We present a new compilation of Type Ia supernovae (SNe Ia), a new data set of low-redshift nearby-Hubble-flow SNe, and new analysis procedures to work with these heterogeneous compilations. This ''Union'' compilation of 414 SNe Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older data sets, as well as the recently extended data set of distant supernovae observed with the Hubble Space Telescope (HST ). A single, consistent, and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is à ¼ 0:713 þ0:027 À0:029 (stat) þ0:036 À0:039 (sys), for a flat, ÃCDM universe. Assuming a constant equation of state parameter, w, the combined constraints from SNe, BAO, and A CMB give w ¼ À0:969 þ0:059 À0:063 (stat) þ0:063 À0:066 (sys). While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a w that varies with redshift. In particular, the current SN data do not yet significantly constrain w at z > 1. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.

We report measurements of M , à , and w from 11 supernovae (SNe) at z ¼ 0:36 0:86 with high-quality light curves measured using WFPC2 on the Hubble Space Telescope (HST). This is an independent set of high-redshift SNe that confirms previous SN evidence for an accelerating universe. The high-quality light curves available from photometry on WFPC2 make it possible for these 11 SNe alone to provide measurements of the cosmological parameters comparable in statistical weight to the previous results. Combined with earlier Supernova Cosmology Project data, the new SNe yield a measurement of the mass density M ¼ 0:25 þ0:07 À0:06 ðstatisticalÞ AE 0:04 (identified systematics), or equivalently, a cosmological constant of à ¼ 0:75 þ0:06 À0:07 ðstatisticalÞ AE 0:04 (identified systematics), under the assumptions of a flat universe and that the dark energy equation-of-state parameter has a constant value w ¼ À1. When the SN results are combined with independent flat-universe measurements of M from cosmic microwave background and galaxy redshift distortion data, they provide a measurement of w ¼ À1:05 þ0:15 À0:20 ðstatisticalÞ AE 0:09 (identified systematic), if w is assumed to be constant in time. In addition to high-precision light-curve measurements, the new data offer greatly improved color measurements of the high-redshift SNe and hence improved host galaxy extinction estimates. These extinction measurements show no anomalous negative E(BÀV ) at high redshift. The precision of the measurements is such that it is possible to perform a host galaxy extinction correction directly for individual SNe without any assumptions or priors on the parent E(BÀV ) distribution. Our cosmological fits using full extinction corrections confirm that dark energy is required with Pð à > 0Þ > 0:99, a result consistent with previous and current SN analyses that rely on the identification of a low-extinction subset or prior assumptions concerning the intrinsic extinction distribution. , and the National Optical Astronomy Observatory. Based in part on observations made with the European Southern Observatory telescopes (ESO programs 60.A-0586 and 265.A-5721). Based in part on observations made with the Canada-France-Hawaii Telescope, operated by the National

Aims. We present a study of the optical and near-infrared (NIR) properties of the Type Ia Supernova (SN Ia) 2003du.Methods. An extensive set of optical and NIR photometry and low-resolution long-slit spectra was obtained using a number of facilities. The observations started 13 days before B-band maximum light and continued for 480 days with exceptionally good time sampling. The optical photometry was calibrated through the S-correction technique. Results. The UBVRIJHK light curves and the color indices of SN 2003du closely resemble those of normal SNe Ia. SN 2003du reached a B-band maximum of 13.49 ± 0.02 mag on JD2 452 766.38 ± 0.5. We derive a B-band stretch parameter of 0.988 ± 0.003, which corresponds to ∆m 15 = 1.02 ±0.05, indicative of a SN Ia of standard luminosity. The reddening in the host galaxy was estimated by three methods, and was consistently found to be negligible. Using an updated calibration of the V and JHK absolute magnitudes of SNe Ia, we find a distance modulus µ = 32.79 ± 0.15 mag to the host galaxy, UGC 9391. We measure a peak uvoir bolometric luminosity of 1.35(±0.20) × 10 43 erg s −1 and Arnett's rule implies that M56 Ni 0.68 ± 0.14 M of 56 Ni was synthesized during the explosion. Modeling of the uvoir bolometric light curve also indicates M56 Ni in the range 0.6−0.8 M . The spectral evolution of SN 2003du at both optical and NIR wavelengths also closely resembles normal SNe Ia. In particular, the Si ii ratio at maximum R(Si ii) = 0.22 ± 0.02 and the time evolution of the blueshift velocities of the absorption line minima are typical. The pre-maximum spectra of SN 2003du showed conspicuous high-velocity features in the Ca ii H&K doublet and infrared triplet, and possibly in Si ii λ6355, lines. We compare the time evolution of the profiles of these lines with other well-observed SNe Ia and we suggest that the peculiar pre-maximum evolution of Si ii λ6355 line in many SNe Ia is due to the presence of two blended absorption components.