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THE ASTRONOMICAL JOURNAL, 116 : 1009 õ1038, 1998 September
( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

OBSERVATIONAL EVIDENCE FROM SUPERNOVAE FOR AN ACCELERATING UNIVERSE AND A COSMOLOGICAL CONSTANT ADAM G. RIESS,1 ALEXEI PETER M. GARNAVICH,2 B. LEIBUNDGUT,6 M. R. V. FILIPPENKO,1 PETER CHALLIS,2 ALEJANDRO CLOCCHIATTI,3 ALAN DIERCKS,4 RON L. GILLILAND,5 CRAIG J. HOGAN,4 SAURABH JHA,2 ROBERT P. KIRSHNER,2 M. PHILLIPS,7 DAVID REISS,4 BRIAN P. SCHMIDT,8,9 ROBERT A. SCHOMMER,7 CHRIS SMITH,7,10 J. SPYROMILIO,6 CHRISTOPHER STUBBS,4 NICHOLAS B. SUNTZEFF,7 AND JOHN TONRY11
Received 1998 March 13 ; revised 1998 May 6

ABSTRACT We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 ¹ z ¹ 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters : the Hubble constant (H ), the mass density () ), the cosmological constant (i.e., the 0 M vacuum energy density, ) ), the deceleration parameter (q ), and the dynamical age of the universe (t ). " 0 õ15% farther than expected in a low mass 0 The distances of the high-redshift SNe Ia are, on average, 10% density () \ 0.2) universe without a cosmological constant. Dierent light curve ïtting methods, SN Ia M subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., ) [ 0) and a current acceleration of the expansion (i.e., q \ 0). With no prior " 0 constraint on mass density other than ) º 0, the spectroscopically conïrmed SNe Ia are statistically M p conïdence levels, and with ) [ 0 at the 3.0 p and 4.0 p consistent with q \ 0 at the 2.8 p and 3.9 " conïdence levels, 0for two dierent ïtting methods, respectively. Fixing a "" minimal îî mass density, ) \ M 0.2, results in the weakest detection, ) [ 0 at the 3.0 p conïdence level from one of the two methods. " the spectroscopically conïrmed SNe Ia require ) [ 0 at 7 p For a ÿat universe prior () ] ) \ 1), M " and 9 p formal statistical signiïcance for the two dierent ïtting methods. A universe closed"by ordinary matter (i.e., ) \ 1) is formally ruled out at the 7 p to 8 p conïdence level for the two dierent ïtting M methods. We estimate the dynamical age of the universe to be 14.2 ^ 1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely eect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these eects appear to reconcile the data with ) \ 0 and q º 0. " 0 Key words : cosmology : observations õ supernovae : general

õõõõõõõõõõõõõõõ 1 Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720-3411. 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. 3 Departamento de Astronom·a y Astrof ·sica, Pontiïcia Universidad Catolica, Casilla 104, Santiago 22, Chile. 4 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195. 5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218. 6 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei Munchen, Germany. 7 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, Casilla 603, La Serena, Chile. NOAO is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 8 Mount Stromlo and Siding Spring Observatories, Private Bag, Weston Creek, ACT 2611, Australia. 9 Visiting Astronomer, Cerro Tololo Inter-American Observatory. 10 Department of Astronomy, University of Michigan, 834 Dennison Building, Ann Arbor, MI 48109. 11 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822.

1

. INTRODUCTION

This paper reports observations of 10 new high-redshift Type Ia supernovae (SNe Ia) and the values of the cosmological parameters derived from them. Together with the four high-redshift supernovae previously reported by our High-z Supernova Search Team (Schmidt et al. 1998 ; Garnavich et al. 1998a) and two others (Riess et al. 1998b), the sample of 16 is now large enough to yield interesting cosmological results of high statistical signiïcance. Conïdence in these results depends not on increasing the sample size but on improving our understanding of systematic uncertainties. The time evolution of the cosmic scale factor depends on the composition of mass-energy in the universe. While the universe is known to contain a signiïcant amount of ordinary matter, ) , which decelerates the expansion, its M dynamics may also be signiïcantly aected by more exotic forms of energy. Preeminent among these is a possible energy of the vacuum () ), Einsteinîs "" cosmological con" 1009


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Vol. 116 1.2. A Brief History of Supernova Cosmology

stant,îî whose negative pressure would do work to accelerate the expansion (Carroll, Press, & Turner 1992 ; Schmidt et al. 1998). Measurements of the redshift and apparent brightness of SNe Ia of known intrinsic brightness can constrain these cosmological parameters. 1.1. T he High-z Program Measurement of the elusive cosmic parameters ) and ) through the redshift-distance relation depends onMcom" paring the apparent magnitudes of low-redshift SNe Ia with those of their high-redshift cousins. This requires great care to assure uniform treatment of both the nearby and distant samples. The High-z Supernova Search Team has embarked on a program to measure supernovae at high redshift and to develop the comprehensive understanding of their properties required for their reliable use in cosmological work. Our team pioneered the use of supernova light curve shapes to reduce the scatter about the Hubble line from pB 0.4 mag to pB 0.15 mag (Hamuy et al. 1996a, 1996c, 1995 ; Riess, Press, & Kirshner 1995, 1996a). This dramatic improvement in the precision of SNe Ia as distance indicators increases the power of statistical inference for each object by an order of magnitude and sharply reduces their susceptibility to selection bias. Our team has also pioneered methods for using multicolor observations to estimate the reddening to each individual supernova, near and far, with the aim of minimizing the confusion between eects of cosmology and dust (Riess et al. 1996a ; Phillips et al. 1998). Because the remaining scatter about the Hubble line is so small, the discussion of the Hubble constant from lowredshift SNe Ia has already passed into a discussion of the best use of Cepheid distances to galaxies that have hosted SNe Ia (Saha et al. 1997 ; Kochanek 1997 ; Madore & Freedman 1998 ; Riess et al. 1996a ; Hamuy et al. 1996c ; Branch 1998). As the use of SNe Ia for measuring ) and ) proM " gresses from its infancy into childhood, we can expect a similar shift in the discussion from results limited principally by statistical errors to those limited by our depth of understanding of SNe Ia. Published high-redshift SN Ia data are a small fraction of the data in hand both for our team and for the Supernova Cosmology Project (Perlmutter et al. 1995, 1997, 1998). Now is an opportune time to spell out details of the analysis, since further increasing the sample size without scrupulous attention to photometric calibration, uniform treatment of nearby and distant samples, and an eective way to deal with reddening will not be proïtable. Besides presenting results for four high-z supernovae, we have published details of our photometric system (Schmidt et al. 1998) and stated precisely how we used ground-based photometry to calibrate our Hubble Space T elescope (HST ) light curves (Garnavich et al. 1998b). In this paper, we spell out details of newly observed light curves for 10 objects, explain the recalibration of the relation of light curve shape and luminosity for a large low-redshift sample, and combine all the data from our teamîs work to constrain cosmological parameters. We also evaluate how systematic eects could alter the conclusions. While some comparison with the stated results of the Supernova Cosmology Project (Perlmutter et al. 1995, 1997, 1998) is possible, an informed combination of the data will have to await a similarly detailed description of their measurements.

While this paper emphasizes new data and constraints for cosmology, a brief summary of the subject may help readers connect work on supernovae with other approaches to measuring cosmological parameters. Empirical evidence for SNe I presented by Kowal (1968) showed that these events had a well-deïned Hubble diagram whose intercept could provide a good measurement of the Hubble constant. Subsequent evidence showed that the original spectroscopic class of Type I should be split (Doggett & Branch 1985 ; Uomoto & Kirshner 1985 ; Wheeler & Levreault 1985 ; Wheeler & Harkness 1990 ; Porter & Filippenko 1987). The remainder of the original group, now called Type Ia, had peak brightness dispersions of 0.4 mag to 0.6 mag (Tammann & Leibundgut 1990 ; Branch & Miller 1993 ; Miller & Branch 1990 ; Della Valle & Panagia 1992 ; Rood 1994 ; Sandage & Tammann 1993 ; Sandage et al. 1994). Theoretical models suggested that these "" standard candles îî arise from the thermonuclear explosion of a carbon-oxygen white dwarf that has grown to the Chandrasekhar mass (Hoyle & Fowler 1960 ; Arnett 1969 ; Colgate & McKee 1969). Because SNe Ia are so luminous (M B [19.5 mag), Colgate (1979) suggested that B observations of SNe Ia at z B 1 with the forthcoming Space Telescope could measure the deceleration parameter, q . From a methodical CCD-based supernova search0 that spaced observations across a lunation and employed prescient use of image-subtraction techniques to reveal new objects, Hansen, Jòrgensen, & Nòrgaard-Nielsen (1987) detected SN 1988U, a SN Ia at z \ 0.31 (Nòrgaard-Nielsen et al. 1989). At this redshift and distance precision (pB 0.4 to 0.6 mag), D100 SNe Ia would have been needed to distinguish between an open and a closed universe. Since the Danish group had already spent 2 years to ïnd one object, it was clear that larger detectors and faster telescopes needed to be applied to this problem. Evidence of systematic problems also lurked in supernova photometry, so that merely increasing the sample would not be adequate. Attempts to correct supernova magnitudes for reddening by dust (Branch & Tammann 1992) based on the plausible (but incorrect) assumption that all SNe Ia have the same intrinsic color had the unfortunate eect of increasing the scatter about the Hubble line or alternately attributing bizarre properties to the dust absorbing SN Ia light in other galaxies. In addition, wellobserved supernovae such as SN 1986G (Phillips et al. 1987 ; Cristiani et al. 1992), SN 1991T (Filippenko et al. 1992a ; Phillips et al. 1992 ; Ruiz-Lapuente et al. 1992), and SN 1991bg (Filippenko et al. 1992b ; Leibundgut et al. 1993 ; Turatto et al. 1996) indicated that large and real inhomogeneity was buried in the scatter about the Hubble line. Deeper understanding of low-redshift supernovae greatly improved their cosmological utility. Phillips (1993) reported that the observed peak luminosity of SNe Ia varied by a factor of 3. But he also showed that the decrease in B brightness in the 15 days after peak [*m (B)] was a good predictor of the SN Ia luminosity,15with slowly declining supernovae more luminous than those which fade rapidly. A more extensive database of carefully and uniformly observed SNe Ia was needed to reïne the understanding of SN Ia light curves. The Calan/Tololo survey (Hamuy et al. 1993a) made a systematic photographic search for supernovae between cycles of the full Moon. This search was


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EVIDENCE FOR AN ACCELERATING UNIVERSE

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extensive enough to guarantee the need for scheduled follow-up observations, which were supplemented by the cooperation of visiting observers, to collect well-sampled light curves. Analysis of the Calan/Tololo results generated a broad understanding of SNe Ia and demonstrated their remarkable distance precision (after template ïtting) of pB 0.15 mag (Hamuy et al. 1995, 1996a, 1996b, 1996c, 1996d ; Tripp 1997, 1998). A parallel eort employed data from the Calan/Tololo survey and from the Harvard Smithsonian Center for Astrophysics (CfA) to develop detailed empirical models of SN Ia light curves (Riess et al. 1995 ; Riess 1996). This work was extended into the multicolor light curve shape (MLCS) method, which employs up to four colors of SN Ia photometry to yield excellent distance precision (B0.15 mag) and a statistically valid estimate of the uncertainty for each object with a measurement of the reddening by dust for each event (Riess et al. 1996a ; see Appendix of this paper). This work has also placed useful constraints on the nature of dust in other galaxies (Riess et al. 1996b ; but see Tripp 1998). The complete sample of nearby SNe Ia light curves from the Calan/Tololo and CfA samples provides a solid founda tion from which to extend the redshift-distance relation to explore cosmological parameters. The low-redshift sample used here has 34 SNe Ia with z \ 0.15. Since the high-redshift observations reported here consumed large amounts of observing time at the worldîs ïnest telescopes, we have a strong incentive to ïnd efficient ways to use the minimum set of observations to derive the distance to each supernova. A recent exploration of this by Riess et al. (1998b) is the "" snapshot îî method, which uses only a single spectrum and a single set of photometric measurements to infer the luminosity distance to a SN Ia with D10% precision. In this paper, we employ the snapshot method for six SNe Ia with sparse data, but a shrewdly designed program that was intended to use the snapshot approach could be even more eective in extracting useful results from slim slices of observing time. Application of large-format CCDs and sophisticated image analysis techniques by the Supernova Cosmology Project (Perlmutter et al. 1995) led to the discovery of SN 1992bi (z \ 0.46), followed by six more SNe Ia at z B 0.4 (Perlmutter et al. 1997). Employing a correction for the luminosity/light curve shape relation (but none for host galaxy extinction), comparison of these SNe Ia to the Calan/Tololo sample gave an initial indication of "" low îî ) " and "" high îî ) : ) \ 0.06`0.28 for a ÿat universe and Mfor a universe without a cosmological con" ~0.34 ) \ 0.88`0.69 M ~0.60 stant () 4 0). The addition of one very high redshift " SN Ia observed with HST had a signiïcant eect (z \ 0.83) on the results : ) \ 0.4 ^ 0.2 for a ÿat universe, and ) \ M 0.2 ^ 0.4 for a " universe with ) 4 0. (Perlmutter et al. " volatile the subject is 1998). This illustrates how young and at present. 1.3. T his Paper Our own High-z Supernova Search Team has been assiduously discovering high-redshift supernovae, obtaining their spectra, and measuring their light curves since 1995 (Schmidt et al. 1998). The goal is to provide an independent set of measurements that uses our own techniques and compares our data at high and low redshifts to constrain the cosmological parameters. Early results from four SNe Ia (three observed with HST ) hinted at a non-negligible cosmological constant and "" low îî ) but were limited by M

statistical errors : ) \ 0.65 ^ 0.3 for a ÿat universe, ) \ " M [0.1 ^ 0.5 when ) 4 0 (Garnavich et al. 1998a). Our aim " in this paper is to move the discussion forward by increasing the data set from four high-redshift SNe to 16, to spell out exactly how we have made the measurement, and to consider various possible systematic eects. In ° 2 we describe the observations of the SNe Ia including their discovery, spectral identiïcation, photometric calibration, and light curves. We determine the luminosity distances (including K-corrections) via two methods, MLCS and a template-ïtting method [*m (B)], as 15 explained in ° 3. Statistical inference of the cosmological parameters including H , ) , ) , q , t , and the fate of the 0 M "00 universe is contained in ° 4. Section 5 presents a quantitative discussion of systematic uncertainties that could aect our results : evolution, absorption, selection bias, a local void, weak lensing, and sample contamination. Our conclusions are summarized in ° 6.
2

. OBSERVATIONS

2.1. Discovery We have designed a search program to ïnd supernovae in the redshift range 0.3 \ z \ 0.6 with the purpose of measuring luminosity distances to constrain cosmological parameters (Schmidt et al. 1998). Distances are measured with the highest precision from SNe Ia observed before maximum brightness and in the redshift range of 0.35 \ z \ 0.55, where our set of custom passbands measures the supernova light emitted in rest-frame B and V . By imaging ïelds near the end of a dark run, and then again at the beginning of the next dark run, we ensure that the newly discovered supernovae are young (Nòrgaard-Nielsen et al. 1989 ; Hamuy et al. 1993a ; Perlmutter et al. 1995). Observing a large area and achieving a limiting magnitude of m B 23 mag yields many SN Ia candidates in the desired R redshift range (Schmidt et al. 1998). By obtaining spectra of these candidates with 4 m to 10 m telescopes, we can identify the SNe Ia and conïrm their youth using the spectral feature aging technique of Riess et al. (1997). The 10 new SNe Ia presented in this paper (SN 1995ao, SN 1995ap, SN 1996E, SN 1996H, SN 1996I, SN 1996J, SN 1996K, SN 1996R, SN 1996T, and SN 1996U) were discovered using the CTIO 4 m Blanco Telescope with the facility prime-focus CCD camera as part of a three-night program in 1995 OctoberõNovember and a six-night program in 1996 FebruaryõMarch. This instrument has a pixel scale of 0A43, and the Tek 2048 ] 2048 pixel CCD . frame covers 0.06 deg2. In each of the search programs, multiple images were combined after removing cosmic rays, dierenced with "" template îî images, and searched for new objects using the prescription of Schmidt et al. (1998). The data on 1995 October 27 and November 17 were gathered under mediocre conditions, with most images having seeing worse than 1A . The resulting dierenced images were suffi.5 cient to ïnd new objects brighter than m \ 22.5 mag. The R data acquired in 1996 had better image quality (D1A5), and . the dierenced images were sufficient to uncover new objects brighter than m \ 23 mag. R In total, 19 objects were identiïed as possible supernovaeõtwo new objects were detected on each of 1995 November 17 and 1995 November 29, ïve new objects on 1996 February 14 õ15, two on 1996 February 20 õ21, and eight on 1996 March 15 õ16 (Kirshner et al. 1995 ; Garnavich et al. 1996a, 1996b).


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RIESS ET AL. were SNe Ia, one was a SN II, and two were active galactic nuclei or SNe II (Kirshner et al. 1995 ; Garnavich et al. 1996a, 1996b). The remaining six candidates were observed, but the spectra did not have sufficient signal to allow an unambiguous classiïcation. The identiïcation spectra for the 10 new SNe Ia are summarized in Table 1 and shown in Figure 1. In addition we include the spectral data for three previously analyzed SNe : SN 1997ce, SN 1997cj, and SN 1997ck (Garnavich et al. 1998a). The spectral data for SN 1995K are given by Schmidt et al. (1998). The spectrum of SN 1997ck shows only an [O II] emission line at 7328.9 ñ in four separate exposures (Garnavich et al. 1998a). The equivalent R-band magnitude of the exposure was 26.5, which is more than 1.5 mag dimmer than the supernova would have been in R, suggesting that the SN was not in the slit when the host galaxy was observed. Most of the host galaxies showed emission lines of [O II], [O III], or Ha in the spectrum, and the redshift was easily measured for these. For the remainder, the redshift was found by matching the broad features in the high-redshift supernovae to those in local supernova spectra. The intrinsic dispersion in the expansion velocities of SNe Ia (Branch et al. 1988 ; Branch & van den Bergh 1993) limits the precision of this method to 1 pB 2500 km s~1 independent of the signal-to-noise ratio of the SN spectrum. The method used to determine the redshift for each SN is given in Table 1. Following the discovery and identiïcation of the SNe Ia, photometry of these objects was obtained from observatories scheduled around the world. The SNe were primarily observed through custom passbands designed to match the wavelength range closest to rest-frame Johnson B and V passbands. Our "" B45,îî "" V45,îî "" B35,îî and "" V35 îî ïlters are speciïcally designed to match Johnson B and V redshifted by z \ 0.45 and z \ 0.35, respectively. The characteristics of these ïlters are described by Schmidt et al. (1998). A few observations were obtained through standard bandpasses as noted in Table 2, where we list the photometric observations for each SN Ia. Photometry of local standard stars in the supernova ïelds in the B35, V35, B45, V45 (or "" supernova îî) photometric system were derived from data taken on three photometric nights. The method has been described in Schmidt et

2.2. Data Spectra of the supernova candidates were obtained to classify the SNe and obtain redshifts of their host galaxies. For this purpose, the Keck Telescope, Multiple Mirror Telescope (MMT), and the European Southern Observatory 3.6 m (ESO 3.6 m) were utilized following the fall of 1995 and spring of 1996 search campaigns. Some galaxy redshifts were obtained with the Keck Telescope in the spring of 1998. The Keck spectra were taken with the Low Resolution Imaging Spectrograph (LRIS ; Oke et al. 1995), providing a resolution of 6 ñ full width at half-maximum (FWHM). Exposure times were between 3 ] 900 and 5 ] 900 s, depending on the candidate brightness. The MMT spectra were obtained with the Blue Channel Spectrograph and 500 line mm~1 grating, giving a resolution of 3.5 ñ FWHM. Exposure times were 1200 s and repeated ïve to seven times. The MMT targets were placed on the slit using an oset from a nearby bright star. The ESO 3.6 m data were collected with the ESO Faint Object Spectrograph Camera (EFOSC1) at a nominal resolution of 18 ñ FWHM. Single 2700 s exposures were made of each target. Using standard reduction packages in IRAF, the CCD images were bias-subtracted and divided by a ÿat-ïeld frame created from a continuum lamp exposure. Multiple images of the same object were shifted where necessary and combined using a median algorithm to remove cosmic-ray events. For single exposures, cosmic rays were removed by hand using the IRAF/IMEDIT routine. Sky emission lines were problematic, especially longward of 8000 ñ. The spectra were averaged perpendicular to the dispersion direction, and that average was subtracted from each line along the dispersion. However, residual noise from the sky lines remains. The one-dimensional spectra were then extracted using the IRAF/APSUM routine and wavelength-calibrated either from a comparison lamp exposure or the sky emission lines. The ÿux was calibrated using observations of standard stars and the IRAF/ONEDSTDS database. The candidates were classiïed from visual inspection of their spectra and comparison with the spectra of wellobserved supernovae (see ° 5.7). In all, 10 of the candidates

TABLE 1 HIGH-z SUPERNOVA SPECTROSCOPY SN 1995ao ...... 1995ap ...... 1996E ....... 1996H ...... 1996I ....... 1996J ....... 1996K ...... 1996R ...... 1996T ....... 1996U ...... 1997ce ...... 1997cj ...... 1997cj ...... 1997ck ...... UT Date 1995 1995 1996 1996 1996 1996 1996 1996 1996 1996 1997 1997 1997 1997 Nov 23 Nov 23 Feb 23 Feb 23 Feb 23 Feb 23 Feb 23 Mar 18 Mar 18 Mar 18 May 4 May 2 May 4 May 4 Telescope Keck I Keck I ESO 3.6 ESO 3.6 ESO 3.6 ESO 3.6 ESO 3.6 MMT MMT MMT Keck II MMT Keck II Keck II Spectral Range (nm) 510 510 600 600 600 600 600 400 400 400 570 400 570 570 õ1000 õ1000 õ990 õ990 õ990 õ990 õ990 õ900 õ900 õ900 õ940 õ900 õ940 õ940 Redshift 0.24b 0.30c 0.43b 0.62b 0.57c 0.30b 0.38c 0.16b 0.24b 0.43b 0.44c 0.50b 0.50c 0.97b Comparison a 1996X([4) 1996X([4) 1989B(]9) 1996X(]5) 1996X(]5) 1995D(]0) 1995D(]0) 1989B(]12) 1996X([4) 1995D(]0) 1995D(]0) ... 1995D(]0) ...

m m m m m

a Supernova and its age (relative to B maximum) used for comparison spectrum in Fig. 1. b Derived from emission lines in host galaxy. c Derived from broad features in SN spectrum.


FIG. 1.õIdentiïcation spectra (in f ) of high-redshift SNe Ia. The spectra obtained for the 10 new SNe of the high-redshift sample are shown in the rest j frame. The data are compared to nearby SN Ia spectra of the same age as determined by the light curves (see Table 1). The spectra of the three objects from Garnavich et al. (1998a) are also displayed.


TABLE 2 SN Ia IMAGING JDa UT Date B45 V45 SN 1996E 127.6 128.6 132.1 134.6 135.5 138.7 139.6 157.6 163.7 ...... ...... ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 1996 1996 Feb Feb Feb Feb Feb Feb Feb Mar Mar 14 15 19 21 22 25 26 15 21 22.30(0.09) 22.27(0.04) 22.46R(0.11) 22.66(0.10) 22.68(0.13) 23.04(0.12) 22.89(0.15) 24.32(0.18) ... ... 21.86(0.08) ... 21.99(0.26) 22.09(0.06) 22.29(0.15) 22.72(0.33) 23.51(0.77) 22.87(0.50) SN 1996H 127.6 128.6 132.1 134.6 135.5 136.6 138.7 139.6 140.6 141.6 142.6 157.6 161.6 164.6 ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Feb Mar Mar Mar 14 15 19 21 22 23 25 26 27 28 29 15 19 22 22.78(0.13) 22.81(0.06) 22.71R(0.29) 22.85(0.08) 22.83(0.18) 22.84(0.13) 22.85(0.09) 22.88(0.15) 22.96(0.16) 23.05(0.08) 23.21(0.20) 23.98(0.22) 24.16(0.22) ... ... 22.25(0.14) 22.40I(0.37) 22.48(0.19) 22.28(0.10) ... 22.58(0.15) 22.52(0.25) 23.10(0.10) ... 22.69(0.16) 23.18(0.28) ... 24.01(0.30) SN 1996I 128.6 132.1 134.6 135.5 136.6 138.7 140.6 142.6 157.6 161.6 ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 Feb Feb Feb Feb Feb Feb Feb Feb Mar Mar 15 19 21 22 23 25 27 29 15 19 22.77(0.05) 22.95(0.22) 22.95(0.05) 22.92(0.05) 22.88(0.09) 23.12(0.13) 23.64(0.36) 23.48(0.10) 24.83(0.17) 24.70(0.31) ... 22.30(0.22) 22.65(0.15) 22.64(0.20) 22.74(0.28) 22.86(0.17) 22.67(0.36) 23.06(0.22) 23.66(0.30) ... SN 1996J 127.6 128.6 134.6 135.6 135.6 139.7 140.7 157.6 161.8 166.6 ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 Feb Feb Feb Feb Feb Feb Feb Mar Mar Mar 14 15 21 22 22 26 27 15 19 24 22.01(0.02) 21.95(0.03) 21.57(0.03) 21.62(0.04) ... 21.63(0.04) ... 22.77(0.05) ... ... ... 21.95(0.07) 21.59(0.05) 21.61(0.04) ... 21.46(0.07) ... 22.06(0.12) ... ... SN 1996K 128.5 135.5 135.5 135.7 136.6 138.6 138.7 139.6 140.8 157.5 157.5 161.7 162.6 165.6 168.5 169.7 ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 1996 Feb Feb Feb Feb Feb Feb Feb Feb Feb Mar Mar Mar Mar Mar Mar Mar 15 22 22 22 23 25 25 26 27 15 15 19 20 23 26 27 23.74(0.04) 22.49(0.07) 22.52(0.07) 22.56(0.03) 22.48(0.05) 22.15(0.10) 22.18(0.07) 22.37(0.05) ... 22.83(0.07) 22.81(0.09) 23.20(0.16) 23.17(0.06) ... ... 24.05(0.26) ... ... ... 22.48(0.06) 22.26(0.16) 22.47(0.11) ... 22.42(0.13) ... ... ... 22.45(0.13) 22.79(0.12) ... ... ... ... ... ... ... ... ... ... ... 22.23(0.10) 22.93(0.12) 22.86(0.10) 23.17(0.17) ... 23.58(0.16) ... 24.42(0.25) ... ... ... ... ... ... ... ... 22.06(0.11) 22.61(0.19) 22.45(0.10) 22.69(0.15) ... 23.17(0.14) 23.20(0.19) ... CTIO 4 m ESO 3.6 m ESO 3.6 m ESO 3.6 m ESO 1.5 m ESO 1.5 m ESO 1.5 m ESO 1.5 m ESO 1.5 m CTIO 4 m CTIO 4 m CTIO 4 m WIYN CTIO 1.m CTIO 1.m MDM ... ... ... 21.84(0.03) 21.89(0.04) ... 21.90(0.07) 23.69(0.07) 24.34(0.19) ... ... ... ... 21.46(0.06) 21.47(0.02) ... 21.77(0.05) 21.83(0.04) 22.05(0.05) 22.76(0.07) CTIO 4 m CTIO 4 m CTIO 4 m ESO 3.6 m ESO 3.6 m ESO 1.5 m ESO 1.5 m CTIO 4 m CTIO 4 m CTIO 1.5 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CTIO 4 m ESO NTT CTIO 4 m ESO 3.6 m ESO 3.6 m ESO 1.5 m ESO 1.5 m WIYN CTIO 4 m CTIO 4 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CTIO 4 m CTIO 4 m ESO NTT CTIO 4 m ESO 3.6 m ESO 3.6 m ESO 1.5 m ESO 1.5 m ESO 1.5 m WIYN WIYN CTIO 4 m CTIO 4 m WIYN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CTIO 4 m CTIO 4 m ESO NTT CTIO 4 m CTIO 4 m ESO 1.5 m ESO 1.5 m CTIO 4 m WIYN B35 V35 Telescope


EVIDENCE FOR AN ACCELERATING UNIVERSE
TABLE 2õContinued JDa UT Date B45 V45 SN 1996R 157.7 158.7 167.7 191.7 ...... ...... ...... ...... 1996 1996 1996 1996 Mar Mar Mar Apr 15 16 25 18 20.48(0.01) 20.59(0.03) ... 22.41(0.09) ... 20.70(0.03) ... ... SN 1996T 161.7 167.6 191.7 212.6 ...... ...... ...... ...... 1996 1996 1996 1996 Mar 19 Mar 25 Apr 18 May 9 20.83R(0.03) 20.95R(0.04) ... 22.52R(0.08) . . . . . . . . . . . . 20.86V(0.02) 20.96V(0.03) 22.37V(0.17) 22.99V(0.31) . . . . . . . . . . . . CTIO 4 m CTIO 1.5 m ESO 1.5 m WIYN . . . . . . . . . . . . . . 21.62 . . . V . . . (0.04) . CTIO 4 m CTIO 4 m CTIO 1.5 m ESO 1.5 m B35 V35 Telescope

1015

SN 1996U 158.7 160.7 161.7 165.7 167.7 186.7 188.7 ...... ...... ...... ...... ...... ...... ...... 1996 1996 1996 1996 1996 1996 1996 Mar Mar Mar Mar Mar Apr Apr 16 18 19 23 25 13 15 22.16(0.04) 22.00(0.11) 22.04(0.05) ... 22.19(0.10) 23.33R(0.17) 23.51(0.17) ... 22.03(0.18) 22.23(0.26) 22.35(0.28) ... 22.64I(0.28) 22.96(0.36) SN 1995ao 39.6 ....... 46.6 ....... 51.6 ....... 1995 Nov 18 1995 Nov 25 1995 Nov 30 21.42(0.05) 21.30(0.03) 21.24(0.05) ... 21.10(0.13) ... SN 1995ap 39.6 46.6 48.6 51.6 ....... ....... ....... ....... 1995 1995 1995 1995 Nov Nov Nov Nov 18 25 27 30 22.41(0.14) 21.13(0.08) 21.04(0.11) 21.04(0.11) ... 21.40(0.10) ... ... ... ... ... 21.65(0.09) ... ... ... 20.92(0.07) CTIO 4 m WIYN WIYN CTIO 4 m ... ... 21.52(0.05) ... ... 21.12(0.03) CTIO 4 m WIYN CTIO 4 m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CTIO 4 m MDM CTIO 4 m CTIO 1.5 m CTIO 1.5 m Las Campanas WIYN

NOTE.õUncertainties in magnitudes are listed in parentheses. a Actually JD [ 2,450,000.

al. (1998) but we summarize it here. The supernova photometric system has been deïned by integrating the ÿuxes of spectrophotometric standards from Hamuy et al. (1994) through the supernova bandpass response functions (based on the ïlter transmissions and a typical CCD quantum efficiency function) and solving for the photometric coefficients that would yield zero color for these stars and monochromatic magnitudes of 0.03 for Vega. This theoretically deïned photometric system also provides transformations between the Johnson/Kron-Cousins system and the supernova system. We use theoretically derived transformations to convert the known V , R, and I magnitudes of Landolt (1992) standard ïelds into B35, V35, B45, V45 photometry. On nights that are photometric, we observe Landolt standard ïelds with the B35, V35, B45, V45 ïlters and measure the starsî instrumental magnitudes from apertures