Annu. Rev. Astron. Astrophys. 1998. 36: 17-55
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2. OBSERVATIONAL PROPERTIES OF SNe Ia

Among the observational advances of recent years, some of the highlights have been as follows.

Several SNe Ia in relatively nearby galaxies have been well observed. Normal events include SN 1989B in NGC 3627 of the Leo group of galaxies (Barbon et al 1990, Wells et al 1994), SN 1990N in NGC 4639 of the Virgo complex (Leibundgut et al 1991a, Jeffery et al 1992, Mazzali et al 1993), SN 1992A in NGC 1380 of the Fornax cluster (Kirshner et al 1993, Hamuy et al 1996d), and SN 1994D in NGC 4526 in Virgo (Richmond et al 1995, Patat et al 1996, Meikle et al 1996, Vacca & Leibundgut 1996). The two most notoriously peculiar events, SN 1991T in NGC 4527 (Ruiz-Lapuente et al 1992, Filippenko et al 1992a, Phillips et al 1992, Jeffery et al 1992, Mazzali et al 1995) and SN 1991bg in NGC 4374 (Filippenko et al 1992b, Leibundgut et al 1993, Turatto et al 1996, Mazzali et al 1997), were in the Virgo complex. SN 1986G in NGC 5128 ident Centaurus A (Phillips et al 1987, Cristiani et al 1992) also was peculiar, in the sense of SN 1991bg but less extreme. It is noteworthy that some of these events were discovered closer to their times of explosion than to their times of maximum light, that ultraviolet spectra of SN 1992A were obtained by the Supernova Intensive Study (SINS) collaboration using the Hubble Space Telescope (HST) (Kirshner et al 1993), and that infrared spectra of SN 1994D (Meikle et al 1996) and several other SNe Ia (Bowers et al 1997) have been observed.

Accurate CCD light curves have been measured for dozens of SNe Ia out in the Hubble flow (z > 0.01), where the recession velocities of their parent galaxies should be reliable indicators of their relative distances. The discovery and photometry of such events have been accomplished mainly by the Calán-Tololo collaboration (Hamuy et al 1993, 1995b, 1996c; see also Riess 1996).

SNe Ia at high redshift by SN standards, z > 0.3, are being discovered in even larger numbers (Perlmutter et al 1997b, c, Schmidt et al 1997). Scheduled discoveries of whole batches of such events have become routine (which is not to say easy), thus allowing the opportunity for follow-up spectroscopy and photometry to be arranged in advance. The primary purpose of the search for high-z SNe Ia is to use them as indicators of relative distances for the determination of Omegam and OmegaLambda, but they also have been used to constrain the ratio of the global and local values of H0 to be near (Kim et al 1997, Tripp 1997) but perhaps slightly less than (Zehavi et al 1998) unity.

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