SNE IA and the Formation History of Early-Type Galaxies

Abstract

Type Ia supernovae (SNe Ia) describe stellar explosions whose spectra show lines of elements of intermediate mass such as silicon, and of the iron group, but no hydrogen lines. The observed properties of SNe Ia’s suggest a binary model in which at least one of the stars is a white dwarf that reaches the Chandrasekhar limit (∼ 1.4M _⊙) by accretion or by merging with another white dwarf. The timescale for the explosion is thereby limited by the lifetimes of stars which end up as white dwarfs, i.e., with masses M > 8M _⊙. On the other hand, Type II supernovae (SNe II) show hydrogen lines in their spectra and arise from the core collapse of a single, massive (M > 8M _⊙) star at the end of its lifetime, which occurs between 1 – 50 Myrs after the core hydrogen burning phase started. Hence, the timescales of either type of supernovae are remarkably different. Furthermore, the yields of chemical elements are also in sharp contrast, since SNe Ia produce much more iron than SNe IIs, so that stars born during the first phases of star formation — when the contribution from SNe Ia to the interstellar medium was negligible — display an enhancement of α elements (such as Mg) over iron, with respect to the younger generations of stars, such as the Sun, which are born in an environment polluted by both types of supernovae.