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The Red Rectangle


This page describes recent work on the bizarre object known as the Red Rectangle, code name HD 44179. An interplay between observations and theory reveals the nature of the "space weather" in and around a double star, producing a beautiful and fascinating view of the processes accompanying stellar death.

ObservSimulation

Hans Van Winckel (Catholic University of Leuven, Belgium), the principal investigator for the Hubble observation team, says: “The structure of the Red Rectangle as seen with Hubble, is surprisingly complex. What impresses me most are what looks like rungs on a ladder. In reality these are projections of nested 'wine glasses', filled to their brim with gas.”

Van Winckel and his co-workers observed exquisite features in the outflow from the central double star. I predicted this double-cone pattern in the 1981 Astrophysical Journal. The development of extremely fast computers has enabled me to revisit my work. The computed flow patterns match the observations in great detail.

The attached page of links to Scientific Details goes into the subject more deeply.



What For?

The final blast of a star is a normal part of the stellar life-and-death cycle. In the beginning, the embryo of a star collects hydrogen gas and interstellar dust, from which the star and its attendant planets form. During the "main sequence" phase (the "adult life" of the star), hydrogen turns into helium and heavier elements by nuclear fusion, liberating energy that makes the star shine. In the end, the final explosion returns some of these elements to interstellar space, and the cycle starts again elsewhere. Life on Earth, and probably all over the Universe, is closely linked to the life of the stars: we are made of starstuff, and run on stellar energy. The violent final phases of stars are a vital part of the life of the stellar population, and deserve close study.



What Physics?

The final blast of a star, for all its violence, is a delicate process. Understanding it requires knowledge and application of many branches of physics: the behaviour of hot gas moving at supersonic speed (gas dynamics), heating and cooling of gas (thermodynamics), formation and destruction of dust (physical chemistry), and the generation and propagation of light (radiative transfer). All of these effects, and sometimes more (such as gravity), are incorporated into a carefully crafted computer code. This code, or set of programmes, is run many times, simulating a suitably chosen set of circumstances. By comparison with known physical situations, we can judge whether the code does what it is supposed to do. It is as if we use a computer programme that correctly predicts the weather on Earth, and apply it to the atmosphere of Mars.

RedRectDiagramNoText
click to enlarge

In the physical models, I assume that the central object of the Red Rectangle is surrounded by a gas disk. Every few hundred years, the disk becomes unstable and drives off its upper layers, causing a beamed two-sided outflow in the form of two oppositely directed cones. I specify the density of the gas and the ejection velocity on the basis of the available observations. I introduce specific rules for the heating and the cooling of the gas, based on the known properties of cosmic gas and dust. The computer code solves a complicated set of equations to produce a series of models. These give an idea of what nebulae of this type will do.



What History?

Research in the physical sciences roughly follows a critical cycle, in which the observation of a physical phenomenon is interpreted in a theory. Part of this theory is a prescription about calculating its observable consequences. Ideally, this results in a prediction about properties of the observations, or suggestions for specific new observations. Example: an astronomer discovers a new fuzzy object in the sky. Supposing it's a comet, he or she computes the probable orbit, and predicts that it will re-appear on a particular day in a certain spot. If the object is indeed found in the predicted place and time, the theory fits, is "right" (for the time being). If not, it's back to the drawing board.

In the case of the Red Rectangle, the first observation was made during a rocket flight in the early 1970's, in which astronomers searched for strong sources of infrared radiation. This source lies about 2300 light-years from Earth in the direction of the constellation Monoceros. Stars surrounded by clouds of small particles, called "dust" by astronomers, are often strong infrared sources because the dust is heated by the starlight and radiates long-wavelength light. Studies of HD 44179 with ground-based telescopes revealed a rectangular shape in the dust surrounding the star in the centre, leading to the name Red Rectangle that was coined in 1973 by astronomers Martin Cohen and Mike Merrill. Their colleague, the late Ed Ney, a professor of astronomy at the University of Minnesota, half-jokingly asked me (a visiting assistant professor at the time): "Can you make rectangular nebulae?" I replied, also half-jokingly: "Of course!"

During my previous studies of the interaction between stars and surrounding gas clouds, it had occurred to me that a disk of gas around a star (a so-called "accretion disk") might radiate so brightly, that the resulting pressure drives off the upper layers of the disk, causing a beamed two-sided outflow. The astrophysicists Shakura and Sunyaev, in one of the most influential papers ever in this field, predicted that such a disk should have a bright ring at some distance from the star. I computed the outflow driven by this ring at the National Center for Atmospheric Research in Boulder (CO), and found a very remarkable outflow pattern: the gas rushed out from the disk in the form of two oppositely directed cones, a "bi-conical" outflow. I reported the results in "Are Bipolar Nebulae Biconical?", 1981 Astrophysical Journal Vol. 247, page 152.



What News?


In a fast-moving science such as astronomy, twenty years is an eternity. When I made the first theoretical models for the Red Rectangle in 1981, I couldn't foresee the dramatic improvements in astronomical instruments that has occurred in the past two decades. In 2002, the spectacular observations by Van Winckel and co-workers showed that the Red Rectangle is, indeed, a problem worthy of attack. The unprecedented detail of the new observations prompted me to re-do my earlier computations, using an improved version of my computer code, and taking advantage of twenty years of development in computer hardware. Some of the results are shown here, and on an accompanying web page showing more detail. This is the new work I'm presenting here; the paper I wrote about it gives full technical details. A selection of links for those interested in the full story is listed on this page.




Astrophysics

What follows is a series of paragraphs describing various aspects of the Red Rectangle research.



Stormy Weather

Most of the matter in the Universe is in the form of gas. Thus, "space weather forecasting" is important business. Galaxies, stars, and planets (including our Earth) formed from immense clouds of gas, contracting due to their own gravity. In my research group, we study theoretical gas dynamics, applied to a great variety of cosmic objects.

The weather on Earth gives rise to spectacular patterns, such as thunderclouds, tornadoes, and hurricanes. Likewise, "space weather" can be fascinating. When stars form, giant clouds of gas and dust collapse to create new suns and planets. At the end of its life, a star blows up again, sometimes in an extremely violent explosion, a supernova; low-mass stars, such as our Sun, pop in a more gentle fashion, forming a planetary nebula. The name has nothing to do with planets, but refers to the fact that in the telescopes of a century ago, these objects looked a little like the disks of planets.

Under certain circumstances, this final phase can be very surprising. The central object of the Red Rectangle is a binary star, a sort of Siamese twin. The outflow from the central object is not spherical, as in an ordinary explosion, but has the form of a double cone. Due to processes around the binary, this outflow is pulsed: every hundred years or so, the Red Rectangle spews a burst of cool gas up and down a fixed direction in space.



Published Paper

My original paper on two-cone outflow was published long ago, at a time when the superb HST images were not even dreamed of. If you want to track it down, the reference is: Vincent Icke, Are bipolar nebulae biconical?, 1981 Astrophysical Journal Vol. 247, p.152. My recent work was written up for IAU Symposium 209, edited by S. Kwok et al., Astronomical Society of the Pacific, 2003. A colour version of the paper is here in PDF format.



Density, Pressure,
Temperature and Velocity


Weather maps mostly show the things with which you make direct contact: the temperature and the velocity of the air. But there's more than just how hot or cold it is, and how fast the wind blows. Air pressure is important, too; and so is the mass of air in a given volume, the gas density. On Earth, the density in the atmosphere drops to half the sea-level value when you're about 5 kilometers high. In space, the density can be extremely low.

When computing the weather on Earth and in space, scientists specify these four quantities, and calculate what happens by using a complicated set of expressions, known as the hydrodynamic equations. This is a very demanding process, possible only on fast computers using carefully crafted scientific computer code. The weather predictions you see on the evening tv news are made in this way.

Mostly, the resulting maps are extremely simplified, showing only a few numbers for the temperature and some cartoon wind vanes for the velocity of the air. This is not good enough in astrophysics! In the maps presented here, I have used a colour coding. The red colour in a map indicates the gas density, i.e. the mass of gas in a given volume. The green colour shows the pressure. The blue colour indicates the speed of the gas. Thus, a region in a picture that is yellow (i.e. red plus green) has a high density and a high pressure. If you see a magenta colour (i.e. red plus blue), the gas is dense and fast.

densmodCe-DPV100
click to enlarge

This image shows the upper half of the nebula (the bottom half is the same, mirror-imaged) at an intermediate stage of the outburst. Below I will show a sampling of the computational results of my Red Rectangle model.



Gas Density

The mass of air in a given volume is called the gas density. On Earth, the density in the atmosphere drops to half the sea-level value when you're about 5 kilometers high. In space, the density can be extremely low, but because the speed of the gas can be very high (sometimes thousands of kilometers per second, much faster than the speed of sound) this tenuous gas packs a wallop. Below I show  a snapshot of the gas density in the outflow cones of the Red Rectangle.

CeLogDens4
click to enlarge

This image is in fact a cross section through the nebula. Almost all of the images on this page are such cross-sections. It shows a number of curious patterns that are responsible for the observed appearance of the Red Rectangle.



Three Snapshots

densmodCe-DPV050  densmodCe-DPV100  densmodCe-DPV150
click to enlarge

In the maps presented here, I have used a colour coding. The red colour indicates the gas density. The green colour shows the pressure. The blue colour indicates the speed of the gas. Thus, a region in a picture that is yellow (i.e. red plus green) has a high density and a high pressure. If you see a magenta colour (i.e. red plus blue), the gas is dense and fast. These images show the upper half of the nebula only. In the equatorial plane (bottom edge of the images), we see a yellowish colour (high density and high pressure). The cones show magenta blobs or streaks; these are ejected pieces of gas that are dense and fast. The images are snapshots taken equal times apart; the spacing is roughly 600 years.



Density, Temperature and Velocity

densmodCe-DTV4-125
click to enlarge

This image is like the one on the right above, but it is coded in a different way. The red channel shows the gas density and the blue one the wind speed, as before. But in this case, the green colour shows the temperature of the gas. This is important, because it turns out that the stuff ejected by the central binary star in the Red Rectangle loses a lot of energy through radiation. In other words, the gas cools very quickly. This turns out to be crucial: if the gas would not become cool, the outflow pattern would look drastically different. This is one of the main findings of my work.



Velocity


densmodCe-velo-125L
click to enlarge

The velocity of the gas is the most difficult item to show. In a weather map, the direction of the air is indicated by a few wind vanes, and the wind strength is given in just a few places. As you can see from the above maps and the movie below, the flow pattern in the Red Rectangle is much too complex for that. Therefore, I have chosen to indicate only the strength of the wind, which is the blue colour in most of the images. The white colour shows the high speed regions, red is intermediate, and yellow to black are the slow-moving parts. Note the enormous variations in the flow pattern. The direction of the motion is best seen in the movie.



What Do We See?

densmodCe-sRGB4-125
click to enlarge

Computer simulations allow a detailed look at the inner workings of an exploding star. But astronomers are not so lucky. Most of the key items in the "space weather" are not directly observable. Even the gas density (the amount of stuff in space) cannot be measured directly. Thus, a theorist is obliged to try and go a step further, and compute what the nebula would actually look like. To do this, I have put a source of light in the centre of the model nebula, and have computed how that light bounces off the gas and the dust in the outflow cones. This requires the computation of radiative transfer. The above image shows a cross section through the nebula, where the colouring indicates what the colour of the light would be. In the reddish regions, the light is strongly scattered; this is similar to what happens in the atmosphere of Earth, causing the rising or setting Sun to look red, and the sky to look blue.



Predicted Observations

CeObservAppear
click to enlarge

Such a cross section is not yet what we actually see. To make a full image, I must spin the model nebula around its axis, and make a sum total of all the light travelling in the direction of the observer. That is what I have done in these images, which correspond to the three flow cross sections shown above.



Movie of the Outflow

MovieFrame
QuickTime movie, 11.4 Mb

QuickTime movie, 1.4 Mb

In the movie, I have used a colour coding. The red colour indicates the gas density. The green colour shows the pressure. The blue colour indicates the speed of the gas. Thus, a region in a picture that is yellow (i.e. red plus green) has a high density and a high pressure. If you see a magenta colour (i.e. red plus blue), the gas is dense and fast. These images show the upper half of the nebula only. In the equatorial plane (bottom edge of the images), we see a yellowish colour (high density and high pressure).




Key Words

NumerHydroSpecs
click to enlarge

For those who know a little more about this type of theoretical astrophysics, the above image summarizes the key properties of my computer simulations.


More Technical Details on
Planetary Nebula Hydrodynamics


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