A HUBBLE DIAGRAM OF DISTANT TYPE IA SUPERNOVAE

We have constructed Hubble diagrams in B and V for 13 Type Ia supernovae (SNe Ia) found in the course of the Calán/Tololo survey covering an unprecedented range in redshift (0.01<z<0.1). As opposed to other published Hubble diagrams, these are solely based on light curves obtained with CCDs, which have been carefully reduced in order to avoid background contamination. Special care was also taken in the extrapolation of peak magnitudes for the SNe that were discovered after maximum light by using five different template light curves representing the range of observed decline rates of SNe Ia. The resulting Hubble diagrams show clear evidence for a distance-dependent dispersion. Although some of the scatter could be due to the peculiar velocities of the host galaxies or to uncorrected dust absorption in the host galaxies, we argue that the dominant source is an intrinsic dispersion in the peak absolute magnitudes of SNe Ia of ∼0.8 mag in MB and ∼0.5 mag in MV. If low-luminosity events like SN 1991bg are actually a separate class of supernovae which arise from different progenitors, then the intrinsic dispersion in MB and MV for "normal" SNe Ia would decrease to ∼0.3-0.5 mag. This study confirms, in general terms, the finding by Phillips [ApJ, 413, L105 (1993)] from a sample of well observed nearby SNe Ia that the absolute B and V magnitudes are correlated with the initial decline rate of the B light curve, although this effect seems to be less pronounced in the Calán/Tololo SNe. Closer inspection shows that the peak luminosity-decline rate relation is well defined over a wide range of decline rates [0.8<Δm15(B)<1.5]; at faster decline rates the dispersion may increase, but further observations of more such events are required to verify this. Although the number of SNe studied here is relatively small, we find that galaxies having a younger stellar population appear to host the most luminous SNe Ia. This finding suggests that the progenitors of SNe Ia cover a range of masses with the most luminous events corresponding to the most massive progenitors. If ignored, this effect could introduce significant bias into determinations of both H0 and q0. We present Hubble diagrams in B and V for the subset of the Calán/Tololo SNe which have decline rates in the range 0.8<Δm15(B)<1.5. Adramatic decrease in the scatter in these diagrams is obtained when the data are "corrected" for the peak luminosity-decline rate relation derived from events in the Phillips sample of nearby SNe Ia covering the same range of decline rates. An analysis of published photometry for SN 1972E in NGC 5253 indicates that this supernova was a slow-declining event. Using the recently measured Cepheid distance to NGC 5253, a Hubble constant in the range H0≃62-67 km s-1 Mpc-1 is calculated when the peak luminosity-decline rate relation is taken into account. The result of ignoring this effect for SN 1972E is to underestimate H0 by ∼15%.