Vikram V. Dwarkadas

The Evolution of Supernovae in Circumstellar Wind-Blown Bubbles. I. Introduction and One-Dimensional Calculations

Mass loss from massive stars (8 M☉) can result in the formation of circumstellar wind-blown cavities surrounding the star, bordered by a thin, dense, cold shell. When the star explodes as a core-collapse supernova (SN), the resulting shock wave will interact with this modified medium around the star, rather than the interstellar medium. In this work we first explore the nature of the circumstellar medium around massive stars in various evolutionary stages. This is followed by a study of the evolution of SNe within these wind-blown bubbles. The evolution depends primarily on a single parameter Λ, the ratio of the mass of the dense shell to that of the ejected material. We investigate the evolution for different values of this parameter. We also plot approximate X-ray surface brightness plots from the simulations. For very small values Λ 1 the effect of the shell is negligible, as one would expect. Values of Λ 1 affect the SN evolution, but the SN forgets about the existence of the shell in about 10 doubling times or so. The remnant density profile changes, and consequently the X-ray emission from the remnant will also change. The initial X-ray luminosity of the remnant is quite low, but interaction of the shock wave with the dense circumstellar shell can increase the luminosity by 2-3 orders of magnitude. As the reflected shock begins to move inward, X-ray images will show the presence of a double-shelled structure. Larger values result in more SN energy being expended to the shell. The resulting reflected shock moves quickly back to the origin, and the ejecta are thermalized rapidly. The evolution of the remnant is speeded up, and the entire remnant may appear bright in X-rays. If Λ 1, then a substantial amount of energy may be expended in the shell. In the extreme case the SN may go directly from the free expansion to the adiabatic stage, bypassing the Sedov stage. Our results show that in many cases the SNR spends a significant amount of time within the bubble. The low density within the bubble can delay the onset of the Sedov stage and may end up reducing the amount of time spent in the Sedov stage. The complicated density profile within the bubble makes it difficult to infer the mass-loss properties of the pre-SN star by studying the evolution of the resulting SNR.

The evolution of supernovae in circumstellar wind bubbles. II. Case of a wolf-rayet star

Mass loss from massive stars leads to the formation of circumstellar wind-blown bubbles surrounding the star, bordered by a dense shell. When the star ends its life in a supernova (SN) explosion, the resulting shock wave expands within this modified medium. Following up on an introductory paper (Dwarkadas), herein we study the evolution of a SN in the bubble formed by a 35 M☉ star that evolves through the phases O star, red supergiant, and Wolf-Rayet star. We model the evolution of the circumstellar medium, and the expansion of the SN shock wave within this medium. Our multidimensional simulations clearly reveal density and pressure fluctuations within the surrounding medium, the presence of hydrodynamic instabilities, the growth of vorticity, and the onset of turbulence. The SN shock interaction with this medium, and then with the dense shell, gives rise to transmitted and reflected shocks. Their effect on the X-ray emission is examined. In this particular case the shock wave is trapped in the dense shell for several doubling times. The turbulent interior, coupled with the density and pressure fluctuations, lead to a corrugated SN shock that impacts the dense shell. The impact occurs in a piecemeal fashion, with some parts of the shock wave interacting with the shell before others. As each interaction is accompanied by an increase in the X-ray and optical emission, different parts of the shell will light up at different times. The situation is resemblant of the scenario in SN 1987A. The reflected shock formed upon shell impact comprises several smaller shocks with different velocities, which are not necessarily moving radially inward. The spherical symmetry of the initial shock wave is completely destroyed.

Radiatively Driven Winds and the Shaping of Bipolar Luminous Blue Variable Nebulae

Nebulae around luminous blue variable (LBV) stars are often characterized by a bipolar, prolate form. In the standard interpretation of the generalized interacting stellar winds model, this bipolar form is attributed to an asymmetry in the density structure of the ambient medium. However, there is limited observational evidence to suggest that such an asymmetric medium is present in most LBV nebulae. In this work we use scaling relations derived from the theory of radiatively driven winds to model the outflows from LBV stars, taking account of stellar rotation and the associated latitudinal variation of the stellar flux due to gravity darkening. We show that, for a star rotating close to its critical speed, the decrease in effective gravity near the equator and the associated decrease in the equatorial wind speed results naturally in a bipolar, prolate interaction front, even for a spherically symmetric ambient medium. Moreover, when gravity darkening is included, the resulting density of the outburst is also strongest over the prolate poles. We discuss the implications of these results for the formation of windblown nebulae in general.

On the Formation of the Homunculus Nebula around η Carinae

We have constructed an interacting winds scenario to account for the geometric and kinematic properties of the Homunculus in η Carinae as seen in recent Hubble Space Telescope observations. Winds from a giant eruption in 1840–1860 sweep into a small (1014 cm), dense (~1014 cm-3), 2 M⊙, near-nuclear toroidal ring. The external medium is uniform at ~2000 particles cm-3. The ring is all but destroyed by the winds in the eruption. Even so, it manages to provide a good deal of collimation to the mass ejected in the first 20 years. Subsequent weaker outflows ram into the outburst gas and initiate surface instabilities and wrinkles. Unlike earlier models, ours is in accordance with the observation that no large, extended disklike distribution is seen around the nebula that could have collimated the bipolar lobes. Models with cooling form essentially ballistic flows (that is, a pair of cones each with a spherical base) whose lateral edges become wrinkled by shear instabilities. A new aspect of the radiative models is the fragmentation of the dense ring, which may help to explain the thin, radial filamentary structure that is seen in the equatorial region of the Homunculus. Adiabatic models become very hot quickly and explode through the nascent cones into the confining gas before the dense collar is destroyed. A pair of spherical lobes form. After 150 years the lobe walls are corrugated by shearing instabilities. These lobes morph into a large, single balloon after about another 300 years.

On luminous blue variables as the progenitors of core‐collapse supernovae, especially Type IIn supernovae

Luminous blue variable (LBV) stars are very massive, luminous, unstable stars that suffer frequent eruptions. In the last few years, these stars have been proposed as the direct progenitors of some core-collapse supernovae (SNe), particularly Type IIn SNe, in conflict with stellar evolution theory. In this paper we investigate various scenarios wherein LBV stars have been suggested as the immediate progenitors of SNe. Many of these suggestions stem from the fact that the SNe appear to be expanding in a high-density medium, which has been interpreted as resulting from a wind with a high mass-loss rate. Others arise due to perceived similarities between the SN characteristics and those of LBVs. Only in the case of SN 2005gl do we find a valid possibility for an LBV-like progenitor. Other scenarios encounter various levels of difficulty. The evidence that points to LBVs as direct core-collapse SNe progenitors is far from convincing. High mass-loss rates are often deduced by making assumptions regarding the wind parameters, which are contradicted by the results themselves. A high density need not necessarily imply a high wind mass-loss rate: wind shocks sweeping up the surrounding medium may give a high-density shell with a low associated wind mass-loss rate. High densities may also arise due to wind clumps, or due to a previous LBV phase before the SN explodes as a WolfRayet (WR) star. Some Type IIn SNe appear to signify more a phase in the life of an SN than a class of SNe, and may arise from more than one type of progenitor. A WR phase that lasts for a few thousand years or less could be one of the more probable progenitors of Type IIns, and channels for creating short-lived WR phases are briefly discussed.

Interaction of Type Ia Supernovae with Their Surroundings: The Exponential Profile in Two Dimensions

The evolution of Type Ia supernovae in the surrounding medium is studied using 2-dimensional numerical hydrodynamic simulations. The ejecta are assumed to be described by an exponential density profile, following the work of Dwarkadas & Chevalier (1998). The case of a circumstellar (CS) region formed by mass loss from the progenitor or a companion star is also considered. The decelerating contact discontinuity is found to be Rayleigh-Taylor (R-T) unstable, as expected, and the nature of the instability is studied in detail for 2 cases: 1) a constant density ambient medium, and 2) a CS medium whose density goes as r^{-2}. The nature of the instability is found to be different in both cases. In the case of a CS medium the instability is much better resolved, and a fractal-like structure is seen. In the case of a constant density medium the extent of growth is less, and the R-T fingers are found to be limited by the presence of Kelvin-Helmholtz mushroom caps at the tips of the fingers. The unstable region is far enough away from the reverse shock that the latter is not affected by the mixing taking place in the interaction region. In contrast the reverse shock in the case of a CS medium is found to be rippled due to the formation of instabilities. In neither case is the outer shock front affected. These results are consistent with similar studies of power-law ejecta profiles conducted by Chevalier, Blondin & Emmering (1992). Our results are then applied to Tychos supernova remnant.

Simulated Radio Images and Light Curves of Young Supernovae

We present calculations of the radio emission from supernovae based on high-resolution simulations of the hydrodynamics and radiation transfer, using simple energy density relations that link the properties of the radiating electrons and the magnetic field to the hydrodynamics. As a specific example we model the emission from SN 1993J, which cannot be adequately fitted with the often-used analytic minishell model, and present a good fit to the radio evolution at a single frequency. Both free-free absorption and synchrotron self-absorption are needed to fit the light curve at early times, and a circumstellar density profile of ρ ~ r-1.7 provides the best fit to the later data. We show that the interaction of density structures in the ejecta with the reverse supernova shock may produce features in the radio light curves such as have been observed. We discuss the use of high-resolution radio images of supernovae to distinguish between different absorption mechanisms and determine the origin of specific light curve features. Comparisons of VLBI images of SN 1993J with synthetic model images suggest that internal free-free absorption completely obscures emission at 8.4 GHz passing through the center of the supernova for the first few tens of years after explosion. We predict that at 8.4 GHz the internal free-free absorption is currently declining, and that over the next ~40 yr the surface brightness of the center of the source should increase relative to the bright ring of emission seen in VLBI images. Similar absorption in a nearby supernova would make the detection of a radio pulsar at 1 GHz impossible for ~150 yr after explosion.

Simulated X-ray spectra from ionized wind-blown nebulae around massive stars

Abstract Using an ionization gas dynamics code, we simulate a model of the wind-blown bubble around a 40xa0 M ⊙ star. We use this to compute the X-ray spectra from the bubble, which can be directly compared to observations. We outline our methods and techniques for these computations, and contrast them with previous calculations. Our simulated X-ray spectra compare reasonably well with observed spectra of Wolf–Rayet bubbles. They suggest that X-ray nebulae around massive stars may not be easily detectable, consistent with observations.

Hydrodynamics of Supernova Evolution in the Winds of Massive Stars

Core-Collapse supernovae arise from stars greater than 8 M⊙. These stars lose a considerable amount of mass during their lifetime, which accumulates around the star forming wind-blown bubbles. Upon the death of the star in a spectacular explosion, the resulting SN shock wave will interact with this modified medium. We study the evolution of the shock wave, and investigate the properties of this interaction. We concentrate on the evolution of the SN shock wave in the medium around a 35 solar mass star. We discuss the hydrodynamics of the resulting interaction, the formation and growth of instabilities, and deviations from sphericity.

Turbulence in wind-blown bubbles around massive stars

Winds from massive stars (> 8 solar masses) result in the formation of wind-blown bubbles around these stars. In this paper we study, via two-dimensional (2D) numerical hydrodynamic simulations, the onset and growth of turbulence during the formation and evolution of these wind-blown bubbles. Our simulations reveal the formation of vortex rolls during the main-sequence stage of the evolution, and Rayleigh?Taylor instabilities in the subsequent stages due to accelerating and/or decelerating wind-blown shells. The bubble shows a very turbulent interior just prior to the death of the star, with a significant percentage of the internal energy expended in non-radial motions. This would affect the subsequent evolution of the resultant supernova shock wave. We discuss the implications of these results, show how the ratio of kinetic energy in radial versus non-radial motions varies throughout the evolution, and discuss how these results would carry over to 3D.