An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae

Abstract

Type Ia supernovae forman observationally uniform class of stellar explosions, in that more luminous objects have smaller declinerates 1. This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological ‘standard candles’, and led to the discovery of an accelerating Universe2,3. Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested4–8 that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion4. Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity– decline-rate relation is an unresolved issue9,10. Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Ourfindings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.