Wind-Blown Nebulae, LBV Nebulae

                  On the Formation of the Homunculus Nebula around Eta Carinae

                                            Vikram V. Dwarkadas & Bruce Balick. , 1998, AJ, 116, 829

Abstract: We have constructed an interacting winds scenario to account for the geometric and kinematic properties of the Homunculus in eta Carinae as seen in recent Hubble Space Telescope observations. Winds from a giant eruption in 1840-1860 sweep into a small (10^14 cm), dense (~10^14 cm^-3), 2 M_&sun;, 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.


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

                                           Vikram V. Dwarkadas & Stanley Owocki, ApJ, 2002, 581, 1337

Abstract: 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.

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

Vikram V Dwarkadas, 2005, ApJ

Abstract: Mass loss from massive stars (>~8 Msolar) 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, 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 <<1 the effect of the shell is negligible, as one would expect. Values of <~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 ?>>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.

       Hydrodynamics of Supernova Evolution in the Winds of Massive Stars

                        Vikram V Dwarkadas, 2007, Astrophysics and Space Science

Abstract: Core-Collapse supernovae arise from stars greater than 8 $\msun$. 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.

The Evolution of Supernovae in Circumstellar Wind Bubbles II: Case of a Wolf-Rayet star

Vikram V Dwarkadas, 2007, ApJ, 667, 226

Abstract: 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 will interact with this modified medium. In a previous paper \citep{d05} we discussed the basic parameters of this interaction with idealized models. In this paper we go a step further and study the evolution of SNe in the wind blown bubble formed by a 35 $\msun$ star that starts off as an O star, goes through a red supergiant phase, and ends its life as a Wolf-Rayet star. We model the evolution of the circumstellar medium throughout its lifetime, and then the expansion of the SN shock wave within this medium. Our simulations clearly reveal fluctuations in density and pressure within the surrounding medium, due to the changing mass-loss parameters over the star's evolution. The SN shock interacting with these fluctuations, and then with the dense shell surrounding the wind-blown cavity, gives rise to a variety of transmitted and reflected shocks in the wind bubble. The interactions between these various shocks and discontinuities is examined, and its effects on the emission from the remnant, especially in the X-ray regime, is noted. In this particular case the shock wave is trapped in the dense shell for a large number of doubling times, and the remnant size is restricted by the size of the surrounding circumstellar bubble. Our multi-dimensional simulations reveal the presence of several hydrodynamic instabilities. They show that the turbulent interior, coupled with the large fluctuations in density and pressure, gives rise to an extremely corrugated SN shock wave. The shock shows considerable wrinkles as it impacts the dense shell, and the impact occurs in a piecemeal fashion, with some parts of the shock wave interacting with the shell before the 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 reflected shock that is formed upon shell impact will comprise of several smaller shocks with different velocities, and which are not necessarily moving radially inwards. The non-spherical nature of the interaction means that it will occur over a prolonged period of time, and the spherical symmetry of the initial shock wave is completely destroyed by the end of the simulation.

Winds From Massive Stars and Implications for Supernovae and Gamma-Ray Bursts

                                                                                        Vikram V Dwarkadas

AIP Conf. Proc. -- August 21, 2007 -- Volume 924, pp. 249-254

We review the effects of winds from massive O and B stars on the surrounding medium over the various stages of stellar evolution. Furthermore we discuss some of the implications for SNe and GRB evolution within this wind-blown medium.

                 Turbulence in wind-blown bubbles around massive stars

                                                      V . V Dwarkadas

                                                                        Physica Scripta, Volume 132, Issue , pp. 014024 (2008)

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.

Theoretical Studies of Wind Blown Nebulae around Massive Stars

                                                                V. V Dwarkadas

  2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #17.25; Bulletin of the American Astronomical Society, Vol. 38, p.923

The combined action of winds and ionizing radiation from massive stars leads to the formation of large wind-blown nebulae surrounding the star. The properties of these nebulae depend on the initial mass of the star and the stellar and wind parameters, as well as the structure and density of the surrounding medium. Using analytic and semi-analytic calculations, supplemented by numerical simulations where necessary, we study the structure, evolution and properties of these nebulae for stars of different initial mass. These results are compared with observational data for stars in various evolutionary phases.

VVD is supported by award AST-0319261 from the National Science Foundation, and by NASA through grant HST-AR-10649 awarded by the Space Science Telescope Institute.

Wind Blown Bubbles Around Massive Stars: Ionization-Gasdynamics Modelling and X-ray Emission Calculations

V. V Dwarkadas & Duane Rosenberg

AAS Meeting 217, Seattle WA, Jan 2011

Using a code that employs a self-consistent method for computing the effects of photo-ionization on circumstellar gas dynamics, we model the formation of wind-driven nebulae around massive stars. Our algorithm incorporates a simplified model of the photo-ionization source, computes the fractional ionization of hydrogen due to the photo-ionizing flux and recombination, and determines self-consistently the energy balance due to ionization, photo-heating and radiative cooling. We take into account changes in stellar properties and mass-loss over the star's evolution. Our multi-dimensional simulations clearly reveal the presence of strong ionization front instabilities, similar to those seen in galactic ionization fronts. In this poster we describe the code, and show how inclusion of photo-ionization affects the wind bubble structure and dynamics. Using various X-ray emission models, we compute the X-ray flux and spectra from our wind bubble models, and compare to observed data.

VVD's research is supported by grant TM9-0001X provided by NASA through the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060

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

Dwarkadas, V. V.; Rosenberg, D. L

High Energy Density Physics, Volume 9, Issue 1, p. 226-230

Using an ionization gas dynamics code, we simulate a model of the wind-blown bubble around a 40 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.