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.

Effects of Magnetic Fields on the Propagation of Nuclear Flames in Magnetic White Dwarfs

“…The double degenerate scenario, in which a binary system composed of double WD merges with a massive WD with a mass exceeding the Chandrasekhar limit to result in an explosion, naturally yields a variety of explosions depending on the total mass (Iben & Tutukov 1984). Another possibility is the effect of a magnetic field in a progenitor WD (Ghezzi et al 2004). The propagation of expanding flow generated by deflagration may be suppressed in the direction perpendicular to the magnetic field, which will cause an asymmetric explosion.…”

“…Since the mass of observed magnetic white dwarfs (MWDs) are significantly less than the Chandrasekhar limit (the average mass is ∼0.9 M ), increasing their masses toward the Chandrasekhar limit, their fields are likely to become much stronger when these MWDs explode as SNe Ia. Ghezzi et al (2004) studied effects of the magnetic field on the turbulent combustion front and calculated resulting asymmetry of the explosion by evaluating the growth rate of the Rayleigh-Taylor instability of the turbulent front suppressed by the existence of magnetic fields.…”

The Data Big Bang and the Expanding Digital Universe: High‐Dimensional, Complex and Massive Data Sets in an Inflationary Epoch

“…Examples include star formation on galactic scales [87], the stellar initial mass function [88], galaxy distributions in space [89,90], cloud distributions in active galactic nuclei [91], the cosmic microwave background [92], thermonuclear flame velocities in Type Ia supernovae [93], and the mass spectrum of interstellar clouds [94]. Such data sets exhibit a dimensionality which is often much lower than the dimension of the Euclidian space they are embedded into.…”

Optical polarimetry of SN 2004bv

“…If our observed polarization of SN 2004bv is representative of its intrinsic polarization, it would imply a deviation from spherical symmetry of less than 10% (Höflich 1991). However, calculating the polarization from new SN models (e.g., Ghezzi et al 2004;Kasen et al 2003Kasen et al , 2004 as well as new polarimetric data are needed to better ascertain the importance of asymmetries in Type Ia SN explosions. Time evolution of SN polarization is another important topic to be explored as shown by Wang et al (2003) in SN 2001el when a polarization decrease was observed through a monitoring campaign that covered from day −4 to day 41 around maximum.…”