Supernovae and Stellar Wind Interaction with the Ambient Medium

Abstract

This thesis deals with understanding salient aspects of the interaction of stars (both low and high mass) and supernova remnants (SNRs) with the medium surrounding them. It has three main parts. The first part attempts to explain the wide variety of aspherical shapes observed in planetary nebulae (PNe). The interacting stellar winds model (S. Kwok, C. R. Purton, & P. M. Fitzgerald, ApJ, 219, L125 [1978]) is used to describe the formation of the nebula, with the further assumption that the evolution occurs in a self-similar manner. The asphericity arises from an asymmetry in the slow wind. Shapes of PNe are computed semianalytically, assuming an equatorially enhanced density in the slow wind, and later adding an asymmetrical velocity profile as well. Hydrodynamic simulations are carried out that substantiate our semianalytical results while providing some idea of their range of validity. The effects of the velocity of the ambient wind are included and found to be a significant parameter in determining the nebula morphology. The morphology is shown to depend on the pole-to-equator density contrast, steepness of the density profile and velocity of the ambient medium. If the external wind velocity is small compared to the expansion velocity of the nebula, the PNe tend to be more bipolar, even with a moderate density contrast. Our numerical simulations show that asymmetric PN shells are corrugated because of Kelvin-Helmholtz instabilities. The next part considers Type Ia supernovae (SNe). These are presumed to arise from white dwarf progenitors, which may not appreciably modify their ambient medium. We study the interaction of the resulting supernova remnants with a constantdensity interstellar medium. Density profiles obtained from detailed explosion models of Type Ia SN explosions can be complex, but an exponential profile gives the best approximate representation of a set of profiles and we emphasize this case. We describe the time evolution of dynamical quantities (such as radius, velocity, and expansion parameter) as a result of the interaction in terms of dimensionless variables and present the profiles of physical quantities. We compare our results to the power law and constant ejecta density cases; a characteristic feature of the exponential case is that the shocked ejecta have a relatively constant temperature. The effect of a possible circumstellar wind region close to the supernova is to create a dense, cool shell near the contact discontinuity between the shocked ejecta and the surrounding medium. The complex density structure found in some supernova models persists in the shocked layer, giving rise to density and temperature variations. We apply our results to two historical Type Ia SNe, SN 1006 and Tycho. The observed angular sizes and expansion rates are consistent with a distance of kpc and an ambient 1.95 5 0.4 H density of 0.05–0.1 cm for SN 1006. For Tycho’s SNR the results are not conclusive but do indicate a distance around 2.3 kpc for an ambient density of 0.6–1.1 cm. In both cases, the low expansion rate limits the extent of a possible circumstellar wind region. The evidence for temperature variations in the ejecta of Tycho’s remnant suggests that the supernova profile was more complex than an exponential and contained density inhomogeneities, or that there was early interaction with a circumstellar wind. The last part addresses circumstellar interaction in high-mass stars ( M,). The variation in wind properties during the M * 8 star’s evolution may result in the formation of large circumstellar bubbles surrounding the star, bordered by a dense shell of swept-up gas. When the star explodes as a SN, the shock wave interacts with the dense shell. The initial interaction of the shock with the shell can be explored analytically (R. A. Chevalier & E. Liang, ApJ, 344, 332 [1989]), but the subsequent evolution of the shock-shell structure must be studied numerically. Different regimes of the parameter L (ratio of shell mass to ejecta mass) are explored. As the value of L is increased, thermalization of the ejecta is speeded up. For higher L the reflected shock wave bounces back from the center and impacts the shell again. For low L the shock can easily overrun the shell, and the evolution reverts back to a self-similar structure once the outer shock “loses memory” of the shell. Analytical approximations are derived whenever possible. Interaction of the SN shock wave with a circumstellar shell is expected to occur in the nearby SN 1987A. We have studied the available data and find that many of the puzzling properties inferred from the X-ray and radio observations can be explained by postulating the existence of an H ii region interior to the circumstellar shell. Spherically symmetric simulations are presented that support this hypothesis. They indicate that shockshell interaction will occur in the year . 2005 5 3