THE CARNEGIE SUPERNOVA PROJECT: ANALYSIS OF THE FIRST SAMPLE OF LOW-REDSHIFT TYPE-Ia SUPERNOVAE

Abstract

An analysis of the first set of low-redshift (z<0.08) Type Ia supernovae monitored by the Carnegie Supernova Project between 2004 and 2006 is presented. The data consist of well-sampled, high-precision optical (ugriBV ) and near-infrared (NIR; Y JHK s ) light curves in a well-understood photometric system. Methods are described for deriving light-curve parameters, and for building template light curves which are used to fit Type Ia supernova data in the ugriBV Y JH bands. The intrinsic colors at maximum light are calibrated using a subsample of supernovae assumed to have suffered little or no reddening, enabling color excesses to be estimated for the full sample. The optical-NIR color excesses allow the properties of the reddening law in the host galaxies to be studied. A low average value of the total-to-selective absorption coefficient, R V ≈ 1.7, is derived when using the entire sample of supernovae. However, when the two highly reddened supernovae (SN 2005A and SN 2006X) in the sample are excluded, a value R V ≈ 3.2 is obtained, similar to the standard value for the Galaxy. The red colors of these two events are well matched by a model where multiple scattering of photons by circumstellar dust steepens the effective extinction law. The absolute peak magnitudes of the supernovae are studied in all bands using a two-parameter linear fit to the decline 1 This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.rates and the colors at maximum light, or alternatively, the color excesses. In both cases, similar results are obtained with dispersions in absolute magnitude of 0.12-0.16 mag, depending on the specific filter-color combination. In contrast to the results obtained from the comparison of the color excesses, these fits of absolute magnitude give R V ≈ 1-2 when the dispersion is minimized, even when the two highly reddened supernovae are excluded. This discrepancy suggests that, beyond the "normal" interstellar reddening produced in the host galaxies, there is an intrinsic dispersion in the colors of Type Ia supernovae which is correlated with luminosity but independent of the decline rate. Finally, a Hubble diagram for the best-observed subsample of supernovae is produced by combining the results of the fits of absolute magnitude versus decline rate and color excess for each filter. The resulting scatter of 0.12 mag appears to be limited by the peculiar velocities of the host galaxies as evidenced by the strong correlation between the distance-modulus residuals observed in the individual filters. The implication is that the actual precision of Type Ia supernovae distances is 3-4%.Subject headings: distance scale -dust, extinction -galaxies: distances and redshiftsgalaxies: ISM -supernovae: general of the low-redshift data. 9 Over the five-year period ending in May 2009, the CSP has obtained densely sampled optical and near-infrared (NIR) light curves of ∼100 SNe Ia in a well-understood, homogeneous photometric system (Hamuy et al. 2006, hereafter H06)....

The Standardized Candle Method for Type Ii Plateau Supernovae

“…There have also been several studies on this subject using SNe Ia. Most recently, Folatelli et al (2010) solved for R V in the same manner as our approach (i.e., by minimizing the dispersion in the HD) and obtained a value of R V ≈ 1.5, remarkably consistent with our value. Further evidence for low R λ values using SNe Ia were reported by others applying a similar procedure (Phillips et al 1999;Altavilla et al 2004;Reindl et al 2005;Hicken et al 2009;Wang et al 2009).…”

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