Asymmetric explosions of thermonuclear supernovae

ABSTRACT A Type Ia supernova explosion starts in a white dwarf as a laminar deflagration at the centre of the star and soon several hydrodynamic instabilities – in particular, the Rayleigh–Taylor (R–T) instability – begin to act. In previous work, we addressed the propagation of an initially laminar thermonuclear flame in the presence of a magnetic field assumed to be dipolar. We were able to show that, within the framework of a fractal model for the flame velocity, the front is affected by the field through the quenching of the R–T instability growth in the direction perpendicular to the field lines. As a consequence, an asymmetry develops between the magnetic polar and the equatorial axis that gives a prolate shape to the burning front. We have here computed numerically the total integrated asymmetry as the flame front propagates outward through the expanding shells of decreasing density of the magnetized white dwarf progenitor, for several chemical compositions. We have found that a total asymmetry of about 50 per cent is produced between the polar and equatorial directions for progenitors with a surface magnetic field B ∼ 5 × 10 7 G , and a composition 12 C = 0.2 and 16 O = 0.8 (in this case, the R–T instability saturates at scales ∼20 times the width of the flame front). This asymmetry is in good agreement with the inferred asymmetries from spectropolarimetric observations of very young supernova remnants, which have recently revealed intrinsic linear polarization interpreted as evidence of an asymmetric explosion in several objects, such as SN1999by, SN1996X and SN1997dt. Larger magnetic field strengths will produce even larger asymmetries. We have also found that for lighter progenitors (i.e. progenitors with smaller concentrations of 16 O and larger concentrations of 12 C) the total asymmetry is larger.