Instabilities and Clumping in Type Ia Supernova Remnants

Abstract

We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant-density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density proÐle. The interaction gives rise to a double-shocked structure susceptible to hydrodynamical instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. Provided the simulation is initiated early in the evolution, with a perturbation the instability reaches its fully Z1%, developed nonlinear strength within 5 doubling times. The further nonlinear evolution does not depend on the initial conditions. Considering the small initial radii of Type Ia supernovae, they are likely to reach this fully developed phase. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the Ñatter part of the supernova density distribution. On the basis of observations of X-ray knots and the protrusion in the southeast outline of TychoÏs supernova remnant, we include clumping in the ejecta because these features cannot be explained by instabilities growing from small perturbations. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging e †ect on the forward shock, the clumpÏs initial density ratio to the surrounding ejecta must be at least 100 for the conditions in TychoÏs remnant. The 56Ni bubble e †ect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a nonradioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-rayÈemitting Si and S ejecta from the remnant center indicates that they were initially in clumps.

Thermal and Nonthermal X‐Ray Emission from the Forward Shock in Tycho’s Supernova Remnant

“…It is not yet clear which of the radio or X-ray expansion values should be used, although the radio measurement is made with higher angular resolution data. Also, if the ejecta currently located near the forward shock have high velocities, they might bias the X-ray expansion measurement, as suggested by Wang & Chevalier (2001). This question will, hopefully, be better resolved with a new study of the X-ray proper motion using higher spatial resolution data.…”

“…1b) shows low-level emission throughout the remnant. This emission might be associated with a faint, smooth component of Si ejecta (see Wang & Chevalier 2001). To estimate the contribution of such a component, we smoothed the Si line image with an adjustable beam containing at least 25 counts, subtracted the off-source background, and determined the lowest intensity level inside the remnant.…”

Strong evidence for hadron acceleration in Tycho’s supernova remnant

“…This value has, however, to be taken with a grain of salt, sinceit may be enhanced by Rayleigh-Taylor instabilities at the CD. For instance, purely gaseous 1D hydrodynamical simulations predict a significantly lower value for this ratio, and namely R CD /R sh = 0.77 (Wang & Chevalier 2001). In the same work the authors also performed a 2D simulation and showed that the Rayleigh-Taylor instability at the CD allows fingers of ejecta to protrude well beyond the average CD radius, also inferring the maximum extent of these fingers in about the 87% of the FS radius.…”

ChandraCCD Imagery of the Northeast and Northwest Limbs of SN 1006

“…The physical scale is similar, and there is evidence of ejecta extending to the forward shock. Wang & Chevalier (2001) have recently carried out twodimensional calculations of the expansion of Type Ia SNRs into a homogeneous medium. If SN 1006 was the 10 51 erg explosion of a Chandrasekhar mass expanding into an ISM with density between 0.1 and 0.5 cm À3 , then SN 1006 has reached a stage corresponding to the Wang & Chevalier simulations for a SNR in which the reverse shock has just begun, or will shortly begin, moving toward the center of the SNR.…”

“…At this point in a one-dimensional calculation, the reverse shock and ISM-ejecta contact discontinuity are expected at about 60% and 70% of the forward shock radius, respectively. In the Wang & Chevalier (2001) calculation, small inhomogeneities in the exploding material are amplified by Rayleigh-Taylor and Kelvin-Helmholtz instabilities in the contact discontinuity. Existing as they do between the primary and reverse shock, the density enhancements associated with the instabilities might be expected to have a clumpy appearance at X-ray wavelengths, but the calculations suggest that these structures never reach the edge of the SNR but are confined to at most 80% of the radius.…”