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SNe Ia are good distance indicators because the shape of their light curves,which can be measured independently of distance, varies smoothly withluminosity. This suggests that SNe Ia are a single family of events. Similarcorrelations are observed between luminosity and spectral properties. Inparticular, the ratio of the strengths of the SiII \lambda 5972 and \lambda6355 lines, known as R(SiII), was suggested as a potential luminosityindicator. Here, the physical reasons for the observed correlation areinvestigated. A Monte-Carlo code is used to construct a sequence of syntheticspectra resembling those of SNe with different luminosities near B maximum. Theinfluence of abundances and of ionisation and excitation conditions on thesynthetic spectral features is investigated. The ratio R(SiII) dependsssentially on the strength of SiII \lambda 5972, because SiII \lambda 6355 issaturated. In less luminous objects, SiII \lambda 5972 is stronger because of arapidly increasing SiII/SiIII ratio. Thus, the correlation between R(SiII) andluminosity is the effect of ionisation balance. The SiII \lambda 5972 lineitself may be the best spectroscopic luminosity indicator for SNe Ia, but allindicators discussed show scatter which may be related to abundancedistributions.

Spectral luminosity indicators in SNe Ia - Understanding the R(SiII) line strength ratio and beyond