A Hubble Diagram of Distant Type Ia Supernovae

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Session 33 -- Supernovae
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[33.07] A Hubble Diagram of Distant Type Ia Supernovae

M.Hamuy, M.M.Phillips (CTIO), J.Maza (Univ.Chile), N.B.Suntzeff, R.A.Schommer, R.Avil\'es (CTIO)

Due to their extreme luminosities at maximum light, type Ia supernovae (SNe Ia) have been long considered among the most attractive cosmological standard candles. The Hubble diagram (HD) for SNe Ia has been studied by a number of authors who found a scatter in the peak B magnitudes of 0.3-0.5 mag. Leibundgut and Tammann (1990,A\&A,230,81) suggested that this scatter is caused solely by observational errors. However, Phillips (1993,ApJ,413, L105) found clear evidence for a significant intrinsic dispersion in SNe Ia absolute magnitudes of $\sim$0.8 mag in B and $\sim$0.6 mag in V, which appeared to be correlated with the initial decline rate of the B light curve.

In this paper we present HDs in B and V for 25 SNe Ia recently discovered in the course of the Cal\'an/Tololo Supernova Survey, covering an unprecedented range in redshift (0.01 $\leq$ z $\leq$ 0.1). As opposed to other published HDs, these are solely based on light curves obtained with CCDs, which have been carefully reduced in order to avoid background contamination. Special care was taken in the extrapolation of peak magnitudes for the SNe that were discovered after maximum light by using 5 different template light curves representing the range of observed decline rates of SNe Ia. The resulting HDs are characterized by dispersions of 0.4 mag in B and 0.3 mag in V, proving that SNe Ia are certainly not perfect standard candles, but still among the best distance indicators known. Our data confirm, in general terms, the peak luminosity-decline rate relationship, although the effect appears to be 2 to 3 times less pronounced than Phillips result. A dramatic decrease in the scatter in these diagrams is obtained when the data are corrected for such relation, leading to dispersions of 0.21 mag in B and 0.16 in V. When combined with the Cepheid distances recently determined with HST, our corrected HDs yield a new estimate for the Hubble constant.

This research has been partially supported by Grant 92/0312 from Fondo Nacional de Ciencias y Tecnolog\'{\i}a (FONDECYT-Chile).

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