|Annu. Rev. Astron. Astrophys. 1997. 35:
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3.4. Cosmological Uses of SNe Ia
Despite not being truly "standard" candles (cf Branch & Tammann 1992, and references therein), SNe Ia are still exceptionally useful for cosmological studies (e.g. Riess et al 1995a, b, 1996, Hamuy et al 1995, 1996a, b, Perlmutter et al 1995, 1997, Goobar & Perlmutter 1995, Leibundgut et al 1996, Goldhaber et al 1997, Filippenko 1997b, Kim et al 1997, Schmidt 1997). The key point is that luminosity correlates strongly with light curve shape; given enough high-quality observations, the luminosity of each object can therefore be calibrated. Even the extinction can be determined reliably from observations through several filters (Riess et al 1996), obviating the need to use other, much more uncertain methods such as those based on the strength of the Na I D interstellar absorption lines (e.g. Richmond et al 1994).
There are additional, potentially tight correlations that could enable the peak luminosity of individual SNe Ia to be calibrated even more accurately. Fisher et al (1995) found that the velocity of the red edge of the Ca II H&K absorption line at t 60 days in spectra of SNe Ia correlates reasonably well with absolute visual magnitude. Nugent et al (1995) discuss other spectral trends and begin to explore their physical basis. For example, the ratio of the Si II absorption line at 5750 Å to that at 6150 Å increases with decreasing luminosity, as does the ratio of the two peaks on either side of the Ca II H&K absorption trough. The latter could be especially useful when classifying high-redshift SNe Ia. Similarly, Branch et al (1996) found that the U-B color at maximum brightness correlates with absolute magnitude; intrinsically luminous SNe Ia generally exhibit the largest UV excess. Further work is required to see whether at least two independent methods yield the same corrections (within the uncertainties) to the derived peak luminosities of SNe Ia.