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

摘要

An analysis of the first set of low-redshift (z < 0.08) Type Ia supernovae (SNe Ia) monitored by the CarnegieSupernova Project between 2004 and 2006 is presented. The data consist of well-sampled, high-precision optical(ugriBV) and near-infrared (NIR; YJHKS) light curves in a well-understood photometric system. Methods aredescribed for deriving light-curve parameters, and for building template light curves which are used to fit SN Iadata in the ugriBVYJH bands. The intrinsic colors at maximum light are calibrated using a subsample of supernovae(SNe) 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 lowaverage value of the total-to-selective absorption coefficient, Rv ≈ 1.7, is derived when using the entire sample ofSNe. However, when the two highly reddened SNe (SN 2005A and SN 2006X) in the sample are excluded, a valueRv ≈ 3.2 is obtained, similar to the standard value for the Galaxy. The red colors of these two events are wellmatched by a model where multiple scattering of photons by circumstellar dust steepens the effective extinction law.The absolute peak magnitudes of the SNe are studied in all bands using a two-parameter linear fit to the decline ratesand the colors at maximum light, or alternatively, the color excesses. In both cases, similar results are obtained withdispersions in absolute magnitudes of 0.12-0.16 mag, depending on the specific filter—color combination. In contrastto the results obtained from the comparison of the color excesses, these fits of absolute magnitude give Rv 1-2when the dispersion is minimized, even when the two highly reddened SNe are excluded. This discrepancy suggeststhat, beyond the "normal" interstellar reddening produced in the host galaxies, there is an intrinsic dispersion in thecolors of SNe Ia which is correlated with luminosity but independent of the decline rate. Finally, a Hubble diagramfor the best-observed subsample of SNe is produced by combining the results of the fits of absolute magnitudeversus decline rate and color excess for each filter. The resulting scatter of 0.12 mag appears to be limited bythe peculiar velocities of the host galaxies as evidenced by the strong correlation between the distance-modulusresiduals observed in the individual filters. The implication is that the actual precision of SNe Ia distances is 3-4.