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

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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.
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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