Dust Shells Seen for the First Time Around Dying Stars

The Helix Nebula
The Helix Nebula - the fate of most stars including our Sun. These new results illuminate how nebulae like this are formed. Credit: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO).

[/caption]

Stars get pretty sloppy towards the end of their lives.  As the nuclear fuels start to wane, the star pulsates – expanding and contracting like a marathon runner catching her breath.  With each pulsation, the dying star belches out globs of gas into space that eventually get recycled into a new generation of stars and planets.  But accounting for all that lost material is difficult.  Like trying to see a wisp of smoke next to a stadium spotlight, observing these tenuous sheets of stellar material swirling just over the surface of the star is considerably challenging.  However, using an innovative technique to image starlight scattering off interstellar grains, astronomers have finally succeeded in seeing ripples of dust flowing off dying stars!

The stars – W Hydra, R Doradus, and R Leonis – are all highly variable red giants, stars that are no longer fusing hydrogen in their cores but have moved on to forming heavier elements.  Each is completely enveloped by a very thin dust shell most likely made up of minerals like forsterite and enstatite.  These grains can only form once the raw ingredients have flowed some distance from the star.  At distances roughly equal to the size of the star itself, the gas has cooled enough to allow atoms to start sticking together and forming more complex compounds.  Minerals like these will go on to seed asteroids and possibly rocky planets like the Earth in the continual cycle of death and rebirth playing out in the Galaxy.

The paper describing this discovery, accepted to the journal Nature, can be found here.

The astronomers who recently reported this discovery used the eight meter wide Very Large Telescope in the Chilean Atacama Desert – and a suite of clever tools – to tease out the subtle reflections off these dust shells.  The trick to seeing light bouncing off interstellar dust particles involves taking advantage of one of light’s wave properties.  Imagine you had a length of rope: one end is in your hand, the other tied to a wall.  You start to wiggle your end and waves travel down the cord.  If you move your arm up and down, the waves are perpendicular to the floor; if you move your arm from side to side, they are parallel to it.  The orientation of those waves is known as their “polarization”.  If you mixed things up by constantly changing the direction in which your arm was oscillating, the orientation of the waves would be similarly confused.  The rope would bounce in all directions.  With out a preferred direction of movement, the rope waves are said to be “unpolarized”.

Light waves emitted from the surface of star are just like your chaotic rope flinging. The oscillations in the electric and magnetic fields that make up the propagating light wave have no preferred direction of motion – they are unpolarized.  However, when light bounces off a dust grain, all that confusion drops away.  The waves now oscillate in roughly the same direction, just as if you decided to only bounce the rope up and down.  Astronomers call this light “polarized”.

A polarizing filter only allows light with a specific orientation to pass through.  Hold it one way, and only “vertically polarized” light – light where the electric field is oscillating up and down – will pass. Turn the filter ninety degrees, and you’ll only transmit “horizontally polarized” light.  If you have polarizing sunglasses, you can try this yourself by rotating the glasses and watching how the the scene through the lenses gets brighter and darker.  This is also a nice demonstration of how our atmosphere polarizes incoming sunlight.

A shell of dust around a star will polarize the light that bounces off it.  Just like the sky gets brighter and dimmer as you turn your sunglasses, looking at a such star through differently oriented polarizing filters will reveal a halo of polarized light surrounding it.  The different orientations will reveal different segments of the halo.  By combining polarimetric observations with interferometry – the beating together of light waves from widely separated spots on a telescope mirror to create very high-resolution images – a thin ring of scattered light reveals itself around these three stars.

These new observations represent a milestone in our understanding of not only a star’s end game but also the production of interstellar dust that follows. Like the smokestacks of great factories, red giant stars expel a soot of minerals into space, carried aloft by stellar winds.  With meticulous observation, results such as these can help tie together the death of one generation of stars with the birth of another.  Unraveling the mysteries of grain formation in space takes us one step closer to piecing together the many steps that lead from stellar death to the creation of rocky planets like our own.

Hidden Companion Responsible for Surprising Disk Around Strange Supergiant Star

3D images of HD 62623, obtained with the VLTI (left), compared to the model of a rotating disk (right). In the boxes, the gas kinematics is shown (3rd dimension): blue-coloured gas approaches the observer, while red-coloured gas recedes from the observer. The size of the inner gas disk of approx. 2 milli-arcseconds corresponds to 1.3 astronomical units (distance Earth-Sun), while the outer dust ring seen in the images has a radius corresponding to 4 astronomical units, assuming 2100 light years as distance to HD 62623. Images: F. Millour et al.

[/caption]

For several years, astronomers have been trying to get a good look at a peculiar supergiant star that is surrounded by a disk of gas and dust. The star, HD 62623, is one of the very few known supergiant stars to have such a disk. These disks are generally only associated with smaller, young stars, as supergiants have strong stellar winds that would blow away any surrounding plasma and debris. Now, using long-baseline stellar interferometry with the “Amber” instrument at ESO’s Very Large Telescope interferometer, a team of astronomers were able to capture, for the first time, a 3-D view of this strange star and its surrounding environment, which revealed a hidden secret: a companion star is likely responsible for the surrounding disk.

“Thanks to our interferometric observations with Amber we could synthetize a 3-D image of HD 62623 as seen through a virtual 130 m-diameter telescope”, says Florentin Millour, leading author of the study, from Observatoire de la Côte d’Azur. “The resolution is an order of magnitude higher compared with the world’s largest optical telescopes of 8-10 m diameter.”

HD 62623 is an exotic, hot, supergiant star. Supergiants are the most massive stars out there, ranging between 10 to 70 solar masses, and can range in brightness from 30,000 to hundreds of thousands of times the output of our Sun. They have very short lifespans, living from 30 million down to just a few hundred thousand years. Supergiants seem to always detonate as Type II supernovae at the end of their lives.

“Our new 3D image locates the dust-forming region around HD 62623 very precisely, and it provides evidence for the rotation of the gas around the central star,” said co-author Anthony Meilland from Max Planck Institute for Radio Astronomy. “This rotation is found to be Keplerian, the same way the Solar system planets rotate around the Sun.”

The companion star, although not seen directly because its light couldn’t be resolved among the brightness of HD62623, was detected by a central cavity between the gas disk and HD 62623. The companion is thought to be approximately the mass of our Sun, and its presence would explain the exotic characteristics of HD 62623, which has many similar characteristics to a monster among the old stars within our Galaxy, Eta Carinae.

HD 62623 is located in the constellation Cygnus near another bright supergiant, Deneb of the summer triangle. Deneb however, like most other supergiants, has no surrounding disk.

Four domes of the 1.8 m Auxiliary Telescopes (AT), utilized for the Very Large Telescope Interferometer (VLTI). ESO, Cerro Paranal, Chile. Image: F. Millour, OCA, Nice, France.

The images obtained with the Amber instrument combines spatial and velocity information, showing not only the shape of the close environment of HD 62623, but also its kinematics or motion. Up to now, the necessary kinematics information was missing in such images.

The astronomers were able to “disentangle” the dust and gas emission in the HD 62623 circumstellar disc, and measure the dusty disc inner rim. They also constrained the inclination angle and the position angle of the major-axis of the disc.

The new 3D imaging technique used by the team is equivalent to integral-field spectroscopy, but gives access to a 15 times larger angular resolution or capacity to detect fine details in the images. “With these new capacities, the VLTI will be able to provide a better comprehension of many sky targets, too small to be resolved by the largest telescopes,” said Millour. “We could aim at young stellar disks or jets, or even the central regions of active galaxies.”

Read the team’s paper: “Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD62623,” Florentin Millour, Anthony Meilland, Olivier Chesneau, Philippe Stee, Samer Kanaan, Romain Petrov, Denis Mourard, Stefan Kraus, 2011, Astronomy & Astrophysics, Vol. 526, A107.

Source: Max Planck Institute for Astronomy

Big or Small, All Stars Form the Same Way

IRAS 13481-6124 (upper left is about twenty times the mass of our sun and five times its radius. It is surrounded by its pre-natal cocoon. Image credit: NASA/JPL-Caltech/ESO/Univ. of Michigan

[/caption]
How do massive stars form? This has been one of the more hotly debated questions in astronomy. Do big stars form by accretion like low-mass stars or do they form through the merging of low mass protostars? Since massive stars tend to be quite far away and usually are surrounded by a shroud of dust, they are difficult to observe, said Stefan Kraus from the University of Michigan. But Kraus and his team have obtained the first image of a dusty disc closely encircling a massive baby star, providing direct evidence that, big or small, all stars form the same way.

“Our observations show a disc surrounding an embryonic young, massive star, which is now fully formed,” said Kraus. “It’s the first time something like this has been observed, and the disk very much resembles what we see around young stars that are much smaller, except everything is scaled up and more massive.”

Not only that, but Kraus and his team found hints at a potential planet-forming region around the nascent star.

Using ESO’s Very Large Telescope Interferometer Kraus and his team focused on IRAS 13481-6124, a star located about 10,000 light-years away in the constellation Centaurus, and about 20 times more massive than our sun. “We were able to get a very sharp view into the innermost regions around this star by combining the light of separate telescopes,” Kraus said, “basically mimicking the resolving power of a telescope with an incredible 85-meter (280-foot) mirror.”

Kraus added that the resulting resolution is about 2.4 milliarcseconds, which is equivalent to picking out the head of a screw on the International Space Station from Earth, or more than ten times the resolution possible with current visible-light telescopes in space.

They also made complementary observations with the 3.58-meter New Technology Telescope at La Silla. The team chose this region by looking at archived images from the Spitzer Space Telescope as well as from observations done with the APEX 12-meter submillimeter telescope, where they discovered the presence of a jet.

“Such jets are commonly observed around young low-mass stars and generally indicate the presence of a disc,” says Kraus.

Astronomers have obtained the first clear look at a dusty disk closely encircling a massive baby star, providing direct evidence that massive stars do form in the same way as their smaller brethren -- and closing an enduring debate. This artist's concept shows what such a massive disk might look like. Image credit: ESO/L. Calçada

From their observations, the team believes the system is about 60,000 years old, and that the star has reached its final mass. Because of the intense light of the star — 30,000 times more luminous than our Sun — the disc will soon start to evaporate. The disc extends to about 130 times the Earth–Sun distance — or 130 astronomical units (AU) — and has a mass similar to that of the star, roughly twenty times the Sun. In addition, the inner parts of the disc are shown to be devoid of dust, which could mean that planets are forming around the star.

“In the future, we might be able to see gaps in this and other dust disks created by orbiting planets, although it is unlikely that such bodies could survive for long,” Kraus said. “A planet around such a massive star would be destroyed by the strong stellar winds and intense radiation as soon as the protective disk material is gone, which leaves little chance for the development of solar systems like our own.”

Kraus looks forward to observations with the Atacama Large Millimeter/submillimeter Array (ALMA), currently under construction in Chile, which may be able to resolve the disks to an even sharper resolution.

Previously, Spitzer detected dusty disks of planetary debris around more mature massive stars, which supports the idea that planets may form even in these extreme environments. (Read about that research here.) .

Sources: ESO, JPL

Astronomers Image Mysterious Dark Object That Eclipses Epsilon Aurigae

Screenshot of the eclipse movie.

Epsilon Aurigae has baffled astronomers since the 1800’s, but new images are providing insight into this very unusual eclipsing binary star. While eclipsing binary stars aren’t unique in themselves, the way this star fades and then regains its brightness is inimitable and has not been fully understood, even after over 175 years of study. One theory has been that a large opaque disk seen nearly edge-on eclipses the primary star. The new images from an instrument developed at the University of Michigan appear to confirm that theory. “It kind of blows my mind that we could capture this,” said John Monnier from U-M. “There’s no other system like this known. On top of that, it seems to be in a rare phase of stellar life. And it happens to be so close to us. It’s extremely fortuitous.”

Epsilon Aurigae has a two-year-long eclipse that occurs every 27 years. The current eclipse started in August 2009 and amateur and professional astronomers have taken this opportunity to train as many telescopes on the event as possible.

Monnier led the development of the Michigan Infra-Red Combiner (MIRC) instrument, which uses interferometry to combine the light entering four telescopes at the CHARA array at Georgia State University and amplify it so that it seems to be coming through a device 100 times larger than the Hubble Space Telescope. MIRC allowed astronomers to “see” the eclipsing object for the first time.

The object that eclipses the primary star is dark — almost invisible — and is only seen as it passes in front of Epsilon Aurigae, the fifth brightest star in the northern constellation Auriga. Because astronomers hadn’t observed much light from it, one theory is the object was a stellar mass black hole. But the prevailing theory labeled it a smaller star orbited edge-on by a thick disk of dust. The theory held that the disk’s orbit must be in precisely the same plane as the dark object’s orbit around the brighter star, and all of this had to be occurring in the same plane as Earth’s vantage point. As unlikely as this alignment would be, it explained the observations.

The new images show that this is indeed the case. A geometrically thin, dark, dense, but partially translucent cloud can be seen passing in front of Epsilon Aurigae.

“This really shows that the basic paradigm was right, despite the slim probability,” Monnier said, and the disk appears much flatter than recent modeling from the Spitzer Space Telescope suggests. “It’s really flat as a pancake,” he said.
[/caption]

While the “movie” of the disk passing in front of the star looks eerily like Saturn’s rings, Monnier doesn’t think the object is like a ring system.

“Ring systems are generally (always) quite sparsely populated and not optically thick,” Monnier said in an email to Universe Today. “Also ring systems have virtually no gas and settle into *extremely* thin layers. Both of these facts make it highly unlikley that the dust Eps Aur is in a “ring” because it wouldn’t be able to completely absorb so much of the star light during eclipse. That said, we don’t know much about the distribution — there might be a bit of a central hole as indicated by brightening of the star during mid-eclipse seen in the past.”

As to why this object is so dark, Monnier said, “At this epoch we are seeing the back side that can’t do any reflecting. We would expect some light to scatter off at other times in the orbit and would be worth looking for but requires very high angular resolution and high dynamic range. Note that the disk is not completely dark — the infrared glow of the cool dust grains have been seen in the 1980s and most recently in a Spitzer space telescope paper by Hoard et al.” (See the paper, “Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared.”

MIRC has also allowed astronomers to see the shape and surface characteristics of stars for the first time. Previously, stars were mere points of light even with the largest telescopes.

“Interferometry has made high resolution imaging of distant objects a reality,” said Fabien Baron, a post-doctoral researcher at U-M who helped with the imaging in this study. “It most probably will solve many mysteries but also raise many new questions.”

The new findings will be published in the April 8 edition of Nature. Researchers from the University of Denver and Georgia State University also contributed to the research.

Sources: EurekAlert, email exchange with John Monnier

Unprecedented Images Show Betelgeuse Has Sunspots

Caption:The surface of Betelgeuse in near infrared at 1.64 micron in wavelength, obtained with the IOTA interferometer (Arizona). The image has been re-constructed with two different algorithms, which yield the same details, of 9 milliarcseconds (mas). The star diameter is about 45 milliarcseconds. Credit: Copyright 2010 Haubois / Perrin (LESIA, Observatoire de Paris)

An international team of astronomers has obtained an unprecedented image of the surface of the red supergiant Betelgeuse, in the constellation Orion. The image reveals the presence of two giant bright spots, which cover a large fraction of the surface. Their size is equivalent to the Earth-Sun distance. This observation provides the first strong and direct indication of the presence of the convection phenomenon, transport of heat by moving matter, in a star other than the Sun. This result provides a better understanding of the structure and evolution of supergiants.

Betelgeuse is a red supergiant located in the constellation of Orion, and is quite different from our Sun. First, it is a huge star. If it were the center of our Solar System it would extend to the orbit of Jupiter. At 600 times larger than our Sun, it radiates approximately 100,000 times more energy. Additionally, with an age of only a few million years, the Betelgeuse star is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight.

But we now know Betelgeuse has some similarities to the Sun, as it also has sunspots. The surface has bright and dark spots, which are actually regions that are hot and cold spots on the star. The spots appear due to convection, i.e., the transport of heat by matter currents. This phenomenon is observed every day in boiling water. On the surface of the Sun, these spots are rather well-known and visible. However, it is not at all the case for other stars and in particular supergiants. The size, physical characteristics, and lifetime of these dynamical structures remain unknown.

Betelgeuse is a good target for interferometry because its size and brightness make it easier to observe. Using simultaneously the three telescopes of the Infrared Optical Telescope Array (IOTA) interferometer on Mount Hopkins in Arizona (since removed), and the Paris Observatory (LESIA) the astronomers were able to obtain a numerous high-precision measurements. These made it possible to reconstruct an image of the star surface thanks to two algorithms and computer programs.

Two different algorithms gave the same image. One was created by Eric Thiebaut from the Astronomical Research Center of Lyon (CRAL) and the other was developed by Laurent Mugnier and Serge Meimon from ONERA. The final image reveals the star surface with unprecedented, never-before-seen details. Two bright spots clearly show up next to the center of the star.

The analysis of the brightness of the spots shows a variation of 500 degrees compared to the average temperature of the star (3,600 Kelvin). The largest of the two structures has a dimension equivalent
to the quarter of the star diameter (or one and a half the Earth-Sun distance). This marks a clear difference with the Sun where the convection cells are much finer and reach hardly 1/20th of the solar radius (a few Earth radii). These characteristics are compatible with the idea of luminous spots produced by convection. These results constitute a first strong and direct indication of the presence of convection on the surface of a star other than the Sun.

Convection could play an important role in the explanation of the mass-loss phenomenon and in the gigantic plume of gas that is expelled from Betelgeuse. The latter has been discovered by a team led by Pierre Kervella from Paris Observatory (read our article about this discovery). Convection cells are potentially at the origin of the hot gas ejections.

The astronomers say this new discovery provides new insights into supergiant stars, opening up a new field of research.

Sources: Abstract: arXiv, Paper: “Imaging the spotty surface of Betelgeuse in the H band,” 2009, A&A, 508, 923″. Paris Observatory