SPY - the ESO Supernovae type Ia Progenitor survey

Abstract

Supernovae (SNe) mark the violent termination of a star’s life in an explosion. They are classified according to their light curve as type I or II, with the type I SNe producing very similar light curves, while the SNe type II are more diverse. Spectroscopic observations reveal the presence of hydrogen in SNe type II, while no hydrogen lines are detectable in SNe type I. According to their spectral appearance the type I class can be further subdivided into Ia, Ib, and Ic. SNe type II and Ib,c are observed only in spiral galaxies and irregular galaxies containing young stellar populations. This indicates that their progenitors are short-lived massive stars (masses above 8 M ). Indeed, the occurrence of SN explosions and the formation of a neutron star remnant at the end of the nuclear lifetime of a massive star are now relatively well understood processes. However, the question of SN Ia progenitors is not yet settled (e.g. Livio 2000). SN Ia are observed in all types of galaxies, including elliptical galaxies containing only old stellar populations. The light curves of SN Ia are dominated by the decay of the radioactive material synthesized in the explosion (mainly nickel). The 56Ni isotope sits at the top of a decay chain leading to 56Co (halflife 6.1 days) and to stable 56Fe (half-life 77 days). The rapid evolution of SN Ia light curves indicates that the precursors of these supernovae must be compact objects of small mass with very little mass holding back the gamma-rays produced by the radioactive decay. The only candidate, which can fulfill the observational constraints, is the thermonuclear explosion of a white dwarf. Since type Ia supernovae were identified as excellent distance indicators for cosmology and have provided indications of cosmic acceleration, it is extremely important to have a better understanding of their explosions and the systems that lead up to them. While it is possible to test the quality of the distance indicator in the nearby universe by checking the linear Hubble expansion, one has to rely on the accuracy of the distance indicator to go beyond the linear Hubble flow and probe the redshift regime, where the cosmological models differ in their predictions. At this point, other signatures of the reliability of the distance indicator have to be secured. With lookback times of about half