Annu. Rev. Astron. Astrophys. 1997. 35: 309-355
Copyright © 1997 by . All rights reserved

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3.2. Homogeneity

It was noticed long ago that the optical spectra of SNe Ia are usually quite homogeneous, if care is taken to compare objects at similar times relative to maximum brightness (Oke & Searle 1974, and references therein). One can even deduce a fairly accurate age of a "normal" SN Ia at the time of observation by comparison of its spectrum with a series of template spectra, such as those of SNe 1937C, 1972E, 1981B, 1989B, and 1994D mentioned above. A good example of this homogeneity is provided by spectra (Figure 6) of SN 1987D, SN 1987N, and SN 1990N, each about one week after maximum brightness. Especially impressive are the small "notches" visible in all three spectra near 4550 Å, 4650 Å, and 5150 Å.

Figure 6

Figure 6. Spectra of SNe Ia about one week past maximum brightness. The parent galaxies and their redshifts (kilometers per second) are as follows: SN 1990N (NGC 4639; 970), SN 1987N (NGC 7606; 2171), and SN 1987D (MCG+00-32-01; 2227).

The optical light curve shapes of many SNe Ia also closely resemble each other (e.g. Hamuy et al 1991). By examining a large quantity of data, Leibundgut (1988) constructed "template" light curves in several bandpasses. It is striking that the B and V light curves of SN 1990N (Leibundgut et al 1991a), discovered two weeks before maximum brightness, very closely match the previously determined templates.

Branch & Miller (1993), Vaughan et al (1995), and others have quantified the dispersion in the peak absolute magnitude of unreddened SNe Ia. By including only those SNe Ia whose B-V color at maximum brightness is within the range -0.25 to 0.25 mag, Vaughan et al found that <MB > = -18.54 ± 0.06 + 5 log H0 / 85) mag and < MV> = -18.54 ± 0.06 + 5 log H0 / 85) mag, where the units of H0 are kilometers per second per megaparsec. Much of the measured dispersion (only 0.30 mag in both cases) may be due to observational errors, incorrect relative distances, and residual reddening; the intrinsic dispersion is certainly smaller.

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