Nicholas B. Suntzeff

The High-Z Supernova Search: Measuring Cosmic Deceleration and Global Curvature of the Universe Using Type Ia Supernovae*

The High-Z Supernova Search is an international collaboration to discover and monitor Type Ia supernovae (SNe Ia) at z > 0.2 with the aim of measuring cosmic deceleration and global curvature. Our collaboration has pursued a basic understanding of supernovae in the nearby universe, discovering and observing a large sample of objects and developing methods to measure accurate distances with SNe Ia. This paper describes the extension of this program to z ≥ 0.2, outlining our search techniques and follow-up program. We have devised high-throughput filters that provide accurate two-color rest frame B and V light curves of SNe Ia, enabling us to produce precise, extinction-corrected luminosity distances in the range 0.25 M=-0.2 -->−0.8+1.0 if ΩΛ = 0. For a spatially flat universe composed of normal matter and a cosmological constant, we find Ω -->M=0.4 -->−0.4+0.5, Ω

Southern spectrophotometric standards. I

{Λ}

Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae

-->=0.6 -->−0.5+0.4. We demonstrate that with a sample of ~30 objects, we should be able to determine relative luminosity distances over the range 0 < z < 0.5 with sufficient precision to measure ΩM with an uncertainty of ±0.2.

An asymptotic-giant-branch star in the progenitor system of a type Ia supernova

We have obtained new observations of both secondary and tertiary spectrophotometric standards with the RC spectrographs and CCD cameras on the 1.5-m and 4-m telescopes at CTIO in the wavelength range of 3300-7550 angstroms, as well as UBV(RI)KC photometry for these stars. We have modified the monochromatic fluxes of the secondary spectrophotometric standards published by Taylor (1984) according to the new calibration of Vega provided by Hayes (1985). We have also uned the zero-point of the energy distribution of these stars by matching their V synthetic magnitudes to the observed magnitudes. We use these adjusted spectrophotometric standards in order to calculate ne fluxes for the tertiary standards of Stone and Baldwin (1983), as well as for three stars of the northern hemisphere from Stone (1977). We find that the synthetic magnitudes calculated from our spectra through the B and V bands agree extremely well with our photometry, to better than 1% on average. For the monochromatic fluxes, we find an internal precision better than 0.01 mag at all wavelengths, and a fair agreement with previous measurements of the tertiary standards. We present also a fine grid of averaged monochromatic fluxes (at continuous steps of 16 angstroms) for the ten secondary standards selected for our program, to be used in the flux calibration of high dispersion spectra.

Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M4

We have coordinated Hubble Space Telescope (HST) photometry with ground-based discovery for three supernovae: Type Ia supernovae near z ≈ 0.5 (SN 1997ce, SN 1997cj) and a third event at z = 0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. We use these light curves and relations between luminosity, light-curve shape, and color calibrated from low-z samples to derive relative luminosity distances that are accurate to 10% at z ≈ 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 matter alone is insufficient to produce a flat universe. Specifically, for Ωm+ΩΛ = 1, Ωm is less than 1 with more than 95% confidence, and our best estimate of Ωm is -0.1±0.5 if ΩΛ = 0. Although this result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST measurements for high-redshift supernovae.

SN 1991bg - A type Ia supernova with a difference

Stars that explode as supernovae come in two main classes. A type Ia supernova is recognized by the absence of hydrogen and the presence of elements such as silicon and sulphur in its spectrum; this class of supernova is thought to produce the majority of iron-peak elements in the Universe. They are also used as precise ‘standard candles’ to measure the distances to galaxies. While there is general agreement that a type Ia supernova is produced by an exploding white dwarf star, no progenitor system has ever been directly observed. Significant effort has gone into searching for circumstellar material to help discriminate between the possible kinds of progenitor systems, but no such material has hitherto been found associated with a type Ia supernova. Here we report the presence of strong hydrogen emission associated with the type Ia supernova SN2002ic, indicating the presence of large amounts of circumstellar material. We infer from this that the progenitor system contained a massive asymptotic-giant-branch star that lost several solar masses of hydrogen-rich gas before the supernova explosion.

SN 1992A : ultraviolet and optical studies based on HST, IUE, and CTIO observations

We present a chemical composition analysis of 36 giants in the nearby mildly metal-poor ( = -1.18) CN-bimodal globular cluster M4. The stars were observed at the Lick and McDonald Observatories using high-resolution echelle spectrographs and at the Cerro Tololo Inter-American Observatory using the multiobject spectrometer. Confronted with a cluster having interstellar extinction that is large and variable across the cluster face, we combined traditional spectroscopic abundance methods with modifications to the line depth ratio technique pioneered by Gray to determine the atmospheric parameters of our stars. We derive a total-to-selective extinction ratio of 3.4 ± 0.4 and an average E(B-V) reddening of 0.33 ± 0.01, which is significantly lower than that estimated by using the dust maps made by Schlegel and coworkers. We determine abundance ratios typical of halo field and cluster stars for scandium, titanium, vanadium, nickel, and europium with star-to-star variations in these elements of less than ±0.1. Silicon, aluminum, barium, and lanthanum are overabundant with respect to what is seen in other globular clusters of similar metallicity. These overabundances confirm the results of an earlier study by Brown & Wallerstein based on a much smaller sample of M4 giants. Superposed on the primordial abundance distribution is evidence for the existence of proton capture synthesis of carbon, oxygen, neon, and magnesium. We recover some of the C, N, O, Na, Mg, and Al abundance swings and correlations found in other more metal-poor globular clusters, but the range of variation is muted. In the case of Mg and Al, this is compatible with the idea that the Al enhancements are derived from the destruction of 25,26Mg, not 24Mg. We determine that the C + N + O abundance sum is constant to within the observational errors and agrees with the C + N + O total that might be expected for M4 stars at birth. The asymptotic giant branch (AGB) stars in M4 have C, N, and O abundances that show less evidence for proton capture nucleosynthesis than is found in the less evolved stars of the red giant branch (RGB). Deeply mixed stars of the RGB, subsequent to the helium core flash, might take up residence on the blue end of the horizontal branch and thus fail to evolve back to the AGB, but reasons for skepticism concerning this scenario are noted.

Optical and Infrared Photometry of the Nearby Type Ia Supernova 2001el

We present 13 spectra and 31 photometric observations covering the first 150 days of SN 1991bg in NGC 4374 (M 84). Although SN 1991bg was a type Ia supernova displaying the characteristic Si II absorption at 6150 A near maximum and the Fe emission lines at late phases, it varied from the well-defined norm for SNe Ia in several important respects. The peculiarities include faster declines in the B and V light curves after maximum, a distinct color evolution, a very red B−V color near maximum, relatively faint peak luminosity, a distinct spectral evolution, and a short peak phase

Testing LMC Microlensing Scenarios: The Discrimination Power of the SuperMACHO Microlensing Survey

The Type Ia supernova SN 1992A in the SO galaxy NGC 1380 was observed as a target of opportunity by the International Ultrauiolet Explorer (IUE) and with great alacrity by the Hubble Space Telescope (HST). Here we present the HST and IUE spectra and photometry that we obtained, as well as optical spectra obtained at the Cerro Tololo Inter-American Observatory (CTIO). The HST Faint Object Spectrograph (FOS) spectra, from 5 and 45 days past maximum light, are the best UV spectra of a Type Ia supernova and reveal for the first time with good signal-to-noise ratio the Type Ia spectral region blueward of ∼2650 A

A New Method to Calibrate the Magnitudes of Type Ia Supernovae at Maximum Light

We present well-sampled optical (UBVRI) and infrared (JHK) light curves of the nearby (≈18.0 Mpc) Type Ia supernova SN 2001el, from 11 days before to 142 days after the time of B-band maximum. The data represent one of the best sets of optical and infrared photometry ever obtained for a Type Ia supernova (SN). Based on synthetic photometry using optical spectra of SN 2001el and optical and infrared spectra of SN 1999ee, we were able to devise filter corrections for the BVJHK photometry of SN 2001el, which to some extent resolve systematic differences between SN 2001el data sets obtained with different telescope/filter/instrument combinations. We also calculated V-minus-infrared color curves on the basis of a delayed detonation model and show that the theoretical color curves match the unreddened loci for Type Ia SNe with midrange decline rates to within 0.2 mag. Given the completeness of the light curves and the elimination of filter-oriented systematic errors to some degree, the data presented here will be useful for the construction of photometric templates, especially in the infrared. On the whole the photometric behavior of SN 2001el was quite normal. The second H-band maximum being brighter than the first H-band maximum is in accord with the prediction of Krisciunas et al. for Type Ia SNe with midrange decline rates. The photometry exhibits nonzero host extinction, with total AV = 0.57 ± 0.05 mag along the line of sight. NGC 1448, the host of SN 2001el, would be an excellent target for a distance determination using Cepheids.