Annu. Rev. Astron. Astrophys. 1998. 36: 17-55 Copyright © 1998 by Annual Reviews. All rights reserved

4.3. Light Curves

Harkness (1991b) first carried out light-curve calculations that took into account the dependence of the opacity on temperature, density, and composition, and he found that the light curve of model W7 was about right for normal SNe Ia. Machinery developed for calculating gamma-ray deposition (Höflich et al 1992) and bolometric and monochromatic light curves (Höflich et al 1993) led to a major computational effort by Höflich & Khokhlov (1996), who calculated local thermodynamic equilibrium (LTE) light curves for 37 explosion models encompassing each of the kinds mentioned above. The light curves and colors of Chandrasekhar-mass carbon-ignitor models depend mainly on the amount of ⁵⁶Ni that is ejected: Models with more ⁵⁶Ni are hotter and brighter, and, because the opacity increases with temperature in the range of interest, they have broader light curves. Carbon-ignitor models can account reasonably well for the photometric properties of both normal and peculiar weak SNe Ia (Wheeler et al 1995, Höflich et al 1996, 1997), with normal SNe Ia requiring M_Ni 0.6 M (Figure 12) and SN 1991bg requiring only about 0.1 M. The differences between the calculated light curves for different kinds of carbon-ignitor models that eject similar amounts of ⁵⁶Ni are fairly subtle, so deciding just which kind of explosion model applies to any particular SN Ia on the basis of its light curves and colors alone is difficult.

Figure 12. Observed light curves of SN 1994D are compared with calculated light curves for model M36, a delayed detonation that ejected MNi = 0.6 M and reached MV = -19.4. From Höflich (1995).

Discriminating between carbon ignitors and helium ignitors is more straightforward because they have such different compositions. Höflich & Khokhlov (1996) found their helium-ignitor models to be inferior to the carbon ignitors in producing the light curve and colors of a normal bright SN Ia. The helium ignitors do not work at all for subluminous SN Ia like SN 1991bg because the ⁵⁶Ni in the outer layers keeps the photosphere hot and the colors too blue.