Luminous Blue Variables and their Circumstellar Nebulae

 

Where is the Disk? The Shaping of Eta Carinae


Frank, A.; Dwarkadas, V.; Balick, B


American Astronomical Society, 191st AAS Meeting, #34.07; Bulletin of the American Astronomical Society, Vol. 29, p.1261


12/1997


1997AAS...191.3407F

Abstract

In this talk we address the issue of the origin of Eta Carinae's bipolar bubble. We focus on two variations of a hydrodynamic interacting wind mechanism for producing the bubbles. Our first model for the formation of the Homunculus nebula is a variant of the popular interacting stellar winds scenario. This model assumes that we have a fast, spherically symmetric wind emanating from the central star at the onset of the 1840 eruption, interacting with a constant, low density pre-eruptive ambient medium. The bipolarity is caused by the presence of a dense and cold torus of material surrounding the star. The bipolar nebula sculpted by this dense torus expands almost ballistically, eventually dwarfing the circumstellar torus, which is entrained in the equatorial material and hardly visible. This model, still under investigation, appears to reproduce fairly well the observed shape and size of the nebula. In our second model we use an aspherical fast wind expanding into a previously deposited isotropic slow wind. For this case we have completed a series of numerical experiments to test if and how aspherical fast winds effect wind blown bubble morphologies. These experiments explore a variety of models for the latitudinal variations of fast wind flow parameters. The simulations demonstrate that aspherical fast winds can produce strongly bipolar outflows similar to what is seen in the Homunculus of Eta Car.

On the Formation of the Homunculus Nebula around eta Carinae


Dwarkadas, Vikram V.; Balick, Bruce



The Astronomical Journal, Volume 116, Issue 2, pp. 829-839. (AJ Homepage)


08/1998


1998AJ....116..829D

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_ȯ, 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.

A New Hydrodynamic Model for the Formation of the Homunculus Nebula around η Carinae


Dwarkadas, V.; Balick, B.


Eta Carinae At The Millennium, ASP Conference Series #179, ed. Jon A. Morse, Roberta M. Humphreys, and Augusto Damineli. Astronomical Society of the Pacific (San Francisco), ISBN 1-58381-003-X (1999)., p.337


00/1999


1999ASPC..179..337D

On the Formation of the Homunculus Nebula around η Carinae



Dwarkadas, Vikram V.; Balick, Bruce



Unsolved Problems in Stellar Evolution: Poster Papers from the Space Telescope Science Institute Symposium, May 1998. Edited by Mario Livio. Space Telescope Science Institute Symposium Series v. 12. Baltimore, Md. : Space Telescope Science Institute, 1999, p.21 00/1999



1999upse.conf...21D

Radiatively Driven Winds and the Shaping of Bipolar LBV Nebulae


Dwarkadas, V. V.; Owocki, S. P.


American Astronomical Society, 199th AAS Meeting, #135.09; Bulletin of the American Astronomical Society, Vol. 33, p.1507  12/2001


2001AAS...19913509D

Abstract

Nebulae around Luminous Blue Variable (LBV) stars are characterized by asymmetric, often bipolar shapes. A classic example is the Homunculus nebula around the massive star η Carinae. In the standard interpretation of the generalized interacting stellar winds models, the asymmetry in shape is attributed to an asymmetry in the density structure of the ambient medium. However the observational evidence does not support this. In this work we use scaling relations derived from the theory of radiatively driven winds to model the outflows from LBV stars. Rotation of the star, and the latitudinal variation of the stellar flux due to gravity darkening is taken into account. It is shown that a star rotating close to its critical velocity will emit a wind whose velocity is higher at the poles than the equator, which can give rise to an asymmetric, bipolar wind-blown nebula. Inclusion of gravity darkening shifts the relative density toward the poles, but does not change the overall shape of the interaction front. We discuss the implications of these results for the general formation of wind-blown nebulae. VVD is supported by NASA grant NAG5-3530, and by a grant from NASA administered by the American Astronomical Society.

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


Dwarkadas, Vikram V.; Owocki, Stanley P.


The Astrophysical Journal, Volume 581, Issue 2, pp. 1337-1343.   12/2002



2002ApJ...581.1337D

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.

On Luminous Blue Variables as the Progenitors of Core-Collapse Supernovae, especially Type IIn Supernovae


Dwarkadas, Vikram V


eprint arXiv:1011.3484   11/2010



Accepted to MNRAS. 13 pages, 1 figure


2010arXiv1011.3484D

Abstract

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 Wolf-Rayet star. Some Type IIn SNe appear to signify more a phase in the life of a SN than a class of SNe, and may arise from more than one type of progenitor. A Wolf-Rayet 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 W-R phases are briefly discussed.