Browsing by Author "Riess, AG"

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    Hubble Space Telescope observations of nine high-redshift essence supernovae
    (2005) Krisciunas, K; Garnavich, PM; Challis, P; Prieto, JL; Riess, AG; Barris, B; Aguilera, C; Becker, AC; Blondin, S; Chornock, R; Clocchiatti, A; Covarrubias, R; Filippenko, AV; Foley, RJ; Hicken, M; Jha, S; Kirshner, RP; Leibundgut, B; Li, WD; Matheson, T; Miceli, A; Miknaitis, G; Rest, A; Salvo, ME; Schmidt, BP; Smith, RC; Sollerman, J; Spyromilio, J; Stubbs, CW; Suntzeff, NB; Tonry, JL; Wood-Vasey, WM
    We present broadband light curves of nine supernovae ranging in redshift from 0.5 to 0.8. The supernovae were discovered as part of the ESSENCE project, and the light curves are a combination of Cerro Tololo 4 m and Hubble Space Telescope (HST) photometry. On the basis of spectra and/or light-curve fitting, eight of these objects are definitely Type Ia supernovae, while the classification of one is problematic. The ESSENCE project is a 5 yr endeavor to discover about 200 high-redshift Type Ia supernovae, with the goal of tightly constraining the time average of the equation-of-state parameter [w = p/(rho c(2))] of the "dark energy." To help minimize our systematic errors, all of our ground-based photometry is obtained with the same telescope and instrument. In 2003 the highest redshift subset of ESSENCE supernovae was selected for detailed study with HST. Here we present the first photometric results of the survey. We find that all but one of the ESSENCE supernovae have slowly declining light curves and that the sample is not representative of the low-redshift set of ESSENCE Type Ia supernovae. This is unlikely to be a sign of evolution in the population. We attribute the decline-rate distribution of HST events to a selection bias at the high-redshift edge of our sample and find that such a bias will infect other magnitude-limited Type Ia supernova searches unless appropriate precautions are taken.
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    The luminous Type Ic supernova 1992ar at z=0.145
    (2000) Clocchiatti, A; Phillips, MM; Suntzeff, NB; DellaValle, M; Cappellaro, E; Turatto, M; Hamuy, M; Avilés, R; Navarrete, M; Smith, C; Rubenstein, EP; Covarrubias, R; Stetson, PB; Maza, J; Riess, AG; Zanin, C
    We present spectroscopic and photometric observations of SN 1992ar, the most distant supernova (SN) in the Calan/Tololo Survey. We compare its spectrum with those of nearby Type Ia and Ic SNe and conclude that the latter type is a better match to SN 1992ar. Using K-corrections based on the spectra of well-observed Type Ic and Ia SNe, we compute different possible rest-frame light curves of SN 1992ar and compare them with those of representative SNe of each type observed in the nearby universe. From the photometry and the spectra, we are able to conclude that SN 1992ar cannot be matched by any known example of a Type Ia SN. Even though the data set collected is fairly complete (one spectrum and 10 photometric points), it is not possible to decide whether SN 1992ar was a fast Type Ic SN, like SN 1994I, or a slow one, like SN 1983V. The absolute V magnitudes at maximum implied by each of these possibilities are -19.2 and -20.2, respectively. The latter would make SN 1992ar one of the brightest SNe on record. SN 1992ar, hence, illustrates the problem of contamination faced by the high-z Type Ia SNe samples whose luminosity distances are used to determine the cosmological parameters of the universe. We, present observational criteria to distinguish the two SN types when the Si II 6355 Angstrom line is redshifted out of the sensitivity range of typical CCD detectors and discuss the effect that these luminous Type Ic SNe would have on the measured cosmological parameters, if not removed from the high-z Type Ia SN samples.