Universe Today

Image credit: NOAO

The Pleiades star cluster has long been a favorite of astronomers, as it’s clearly visible with the naked eye, and looks even better in small telescopes and binoculars. The cluster’s wispy appearance comes from the fact that the stars are surrounded by a faint nebula. By tracking the motion of the stars and the cloud, a team of astronomers have discovered that the area is being formed by multiple clouds colliding together in the same region.

The naked-eye Pleiades star cluster has long been known to professional and amateur astronomers for the striking visible nebulosity that envelopes the cluster?s brightest stars, scattering their light like fog around a streetlamp.

Radio and infrared observations in the 1980s established that this nebulosity results from a chance encounter by the young stars of the Pleiades with an interstellar cloud, rather than being caused by debris from the cluster?s formation. New data obtained at Kitt Peak National Observatory suggest that the Pleiades are actually encountering two clouds, giving rise to an extraordinary and previously unknown occurrence: a three-body collision in the vast emptiness of interstellar space.

This new perspective on the motion of interstellar gas near the cluster comes from high-resolution spectra obtained at an adjunct facility of Kitt Peak?s 2.1-meter telescope known as the Coud? Feed. The investigator was Richard White of Smith College in Northampton, MA, who worked in collaboration with students from Smith College and Amherst College.

?The idea of the Pleiades and one gas cloud in an interstellar train wreck already made this nearby cluster an especially interesting region for astronomers seeking to understand the details of physical and chemical processes in the interstellar medium,? White says. ?The presence of a second cloud interacting with the first cloud and with the cluster creates a situation more like a three-car crash in a demolition derby, which makes the Pleiades altogether unique as natural laboratory.?

The time scale for the unfolding of the interstellar collisions in the Pleiades is several hundred thousand years. ?That is good news for those who enjoy the magnificent color images of the Pleiades images that grace textbooks and coffee table books, which suffer no danger of obsolescence,? White says. ?It is bad news for those who would like to see celestial fireworks unfolding from year to year.?

Known as the Seven Sisters for the seven stars said to be visible with the naked eye, the Pleiades (M45) consists of more than 500 stars roughly 100 million years old in a cluster located about 400 light-years from Earth.

Sodium atoms in gas found between Earth and the stars absorb two specific wavelengths of yellow starlight (the same wavelengths of yellow light emitted by low-pressure sodium streetlamps). Because of the Doppler effect (analogous to the shift in siren pitch produced when an ambulance is moving toward or away from a listener), the motion of the gas along our line of sight produces subtle shifts in the observed wavelengths.

In a paper published in the October 2003 Astrophysical Journal Supplement, White interprets the new observations of sodium atoms in the Pleiades region in the context of other recent observations of the Pleiades region. These observations include significant new optical images of the Pleiades from the Burrell Schmidt telescope on Kitt Peak, published earlier this year in the Astrophysical Journal by Steven Gibson of the University of Calgary and Kenneth Nordsieck of the University of Wisconsin, and radio maps of neutral hydrogen that formed part of Gibson?s doctoral thesis.

The orientation of features in the optical and radio imagery provides clues to gas and dust motions across the sky, which can be combined with the spectroscopically measured velocities from Kitt Peak to allow astronomers to reconstruct the three-dimensional configuration of the interstellar matter near the Pleiades.

The sodium absorption lines reveal that there always is one feature between Earth and the Pleiades stars, moving toward the cluster with a line of sight velocity of about 10 kilometers per second. White associates this feature with the Taurus-Auriga interstellar cloud complex, the bulk of which lies about 40 light-years to the east.

Toward some stars, however, there are two or more absorption features. White argues that a shock-wave from the collision between the Pleiades and gas associated with the Taurus-Auriga complex can account for splitting of one feature into three in some areas, primarily on the south and east sides of the Pleiades. However, the presence of an additional feature in the data, primarily on the west side and moving into the cluster at about 12 kilometers per second, defies understanding unless a second cloud also is converging on the Pleiades, he concludes.

The only previously known three-body collisions in interstellar space are inferred close encounters by a star and a neighboring binary or triple star system within a globular cluster or in the cores of galaxies.

Previously released images of the Pleiades from Kitt Peak that amply demonstrate the surrounding nebulosity are available in the NOAO Image Gallery (linked above).

Located southwest of Tucson, AZ, Kitt Peak National Observatory is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation.

Original Source: NOAO News Release

Image credit: UA

Astronomers from the University of Arizona have used a new technique called “nulling interferometry” to reveal the planetary disk around a newly-forming star. Incredibly, they discovered a gap in the disk, where a Jupiter-like planet is probably forming. This nulling technique works by combining the light from the central star in such a way that it gets canceled out. This allows fainter objects, such as dust and planets to be observed. The planet is likely several times the mass of Jupiter and orbits its star at about 1.5 billion kilometers.

University of Arizona astronomers have used a new technique called nulling interferometry to probe a dust disk around a young nearby star for the first time. They not only confirmed that the young star does have a protoplanetary disk — the stuff from which solar systems are born — but discovered a gap in the disk, which is strong evidence of a forming planet.

“It’s very exciting to find a star that we think should be forming planets, and actually see evidence of that happening,” said UA astronomer Philip Hinz.

“The bottom line is, we not only confirmed the hypothesis that this young star has a protoplanetary disk, we found evidence that a giant, Jupiter-like protoplanet is forming in this disk,” said Wilson Liu, a doctoral student and research assistant on the project.

“There’s evidence that this star is right on the cusp of becoming a main-sequence star,” Liu added. “So basically, we’re catching a star that is right at the point of becoming a main-sequence star, and it looks like it’s caught in the act of forming planets.”

Main-sequence stars are those like our sun that burn hydrogen at their cores.

Earlier this year, Hinz and Liu realized that observations of HD 100546 at thermal, or mid-infrared, wavelengths showed that the star had a dust disk.

Finding faint dust disks is “analogous to finding a lighted flashlight next to Arizona Stadium when the lights are on,” Liu said.

The nulling technique combines starlight in such a way that it is canceled out, creating a dark background where the star’s image normally would be. Because HD 100546 is such a young star, its dust disk is still relatively bright, about as bright as the star itself. The nulling technique is needed to distinguish what light comes from the star, which can be suppressed, and what comes from the extended dust disk, which nulling does not suppress.

Hinz and UA astronomers Michael Meyer, Eric Mamajek, and William Hoffmann took the observations in May 2002. They used BLINC, the only working nulling interferometer in the world, along with MIRAC, a state-of-the-art mid-infrared camera, on the 6.5-meter (21-foot) diameter Magellan telescope in Chile to study the roughly 10-million-year-old star in the Southern Hemisphere sky.

Typically, dust in disks around stars is uniformly distributed, forming a continuous, flattened, orbiting cloud of material that is hot on the inner edge but cold most of the distance to the frigid outer edge.

“The data reduction was complicated enough that we didn’t realize until later that there was an inner gap in the disk,” Hinz noted.

“We realized the disk appeared about the same size at warmer (10 micron) wavelengths and at colder (20 micron) wavelengths. The only way that could be is if there’s an inner gap.”

The most likely explanation for this gap is that it is created by the gravitational field of a giant protoplanet =AD an object that could be several times more massive than Jupiter. The researchers believe the protoplanet may be orbiting the star at perhaps 10 AU. (An AU, or astronomical unit, is the distance between Earth and the sun. Jupiter is about 5 AU from the sun.)

Astronomers from the Netherlands and Belgium had previously used the Infrared Space Observatory to study HD 100546, which is 330 light-years from Earth. They detected comet-like dust around the star and concluded that it might be a protoplanetary disk. But the European space telescope was too small to clearly see dust surrounding the star.

Hinz, who developed BLINC, has been using the nulling interferometer with two 6.5-meter telescopes for the past three years for his survey of nearby stars in search of protoplanetary systems. In addition to the Magellan telescope that covers the Southern Hemisphere, Hinz uses the 6.5-meter UA/Smithsonian MMT atop Mount Hopkins, Ariz., for the Northern Hemisphere sky.=20

Hinz developed BLINC as a technology demonstration for the Terrestrial Planet Finder mission, which is managed for NASA by the Jet Propulsion Laboratory, Pasadena, Calif. NASA, which funds Hinz’ survey, supports research on solar-system formation under its Origins program and is developing nulling interferometry for Terrestrial Planet Finder.

“Nulling interferometry is very exciting because it is one of only a few technologies that can directly image circumstellar environments,” Liu said.

Using MIRAC, the camera developed by William Hoffmann and others, was important because it is sensitive to mid-infrared wavelengths, Hinz said. Astronomers will have to look in mid-infrared wavelengths, which correspond to room temperatures, to find planets with liquid water and possible life, he said.

Hinz’ survey includes HD 100546 and other “Herbig Ae” stars, which are nearby young stars generally more massive than our sun, but are not yet main sequence stars powered by nuclear fusion.

Hinz and Liu plan to observe increasingly mature star systems, searching for ever-fainter circumstellar dust disks and planets, as they continue to improve nulling interferometry and adaptive optics technologies. Adaptive optics is a technique that eliminates the effects of Earth’s shimmering atmosphere from starlight.

Hinz and others at UA Steward Observatory are designing a nulling interferometer for the Large Binocular Telescope, which will view the sky with two 8.4-meter (27-foot) diameter mirrors on Mount Graham, Ariz., in 2005.

Original Source: UA News

Image credit: ESA

The European Space Agency’s Integral gamma-ray observatory has produced a new map of the Milky Way in the gamma-ray spectrum. Integral is looking for traces of radioactive aluminum, which gives off gamma rays with a specific wavelength. But the question is, what’s producing all this aluminum? Some astronomers believe these could be created by specific objects in the Milky Way, like Red Giant stars or hot blue stars. Another possibility is that it’s produced as part of supernova explosions. Integral will help get to the bottom of this mystery.

ESA’s gamma-ray observatory Integral is making excellent progress, mapping the Galaxy at key gamma-ray wavelengths.

It is now poised to give astronomers their truest picture yet of recent changes in the Milky Way’s chemical composition. At the same time, it has confirmed an ‘antimatter’ mystery at the centre of the Galaxy.

Since its formation from a cloud of hydrogen and helium gas, around 12 000 million years ago, the Milky Way has gradually been enriched with heavier chemical elements. This has allowed planets and, indeed, life on Earth to form.

Today, one of those heavier elements – radioactive aluminium – is spread throughout the Galaxy and, as it decays into magnesium, gives out gamma rays with a wavelength known as the ‘1809 keV line.’ Integral has been mapping this emission with the aim of understanding exactly what is producing all this aluminium.

In particular, Integral is looking at the aluminium ‘hot spots’ that dot the Galaxy to determine whether these are caused by individual celestial objects or the chance alignment of many objects.

Astronomers believe that the most likely sources of the aluminium are supernovae (exploding high-mass stars) and, since the decay time of the aluminium is around one million years, Integral’s map shows how many stars have died in recent celestial history. Other possible sources of the aluminium include ‘red giant’ stars or hot blue stars that give out the element naturally.

To decide between these options, Integral is also mapping radioactive iron, which is only produced in supernovae. Theories suggest that, during a supernova blast, aluminium and iron should be produced together in the same region of the exploding star. Thus, if the iron’s distribution coincides with that of the aluminium, it will prove that the overwhelming majority of aluminium comes indeed from supernovae.

These measurements are difficult and have not been possible so far, since the gamma-ray signature of radioactive iron is about six times fainter than that of the aluminium. However, as ESA’s powerful Integral observatory accumulates more data in the course of the next year, it will finally be possible to reveal the signature of radioactive iron. This test will tell astronomers whether their theories of how elements form are correct.

In addition to these maps, Integral is also looking deeply into the centre of the Galaxy, to make the most detailed map ever of ‘antimatter’ there.

Antimatter is like a mirror image to normal matter and is produced during extremely energetic atomic processes: for example, the radioactive decay of aluminium. Its signature is known as the ‘511 keV line.’ Even though Integral’s observations are not yet complete, they show that there is too much antimatter in the centre of the Galaxy to be coming from aluminium decay alone. They also show clearly that there must be many sources of antimatter because it is not concentrated around a single point.

There are many possible sources for this antimatter. As well as supernovae, old red stars and hot blue stars, there are jets from neutron stars and black holes, stellar flares, gamma-ray bursts and interaction between cosmic rays and the dusty gas clouds of interstellar space.

Chris Winkler, Integral’s Project Scientist, says: “We have collected excellent data in the first few months of activity but we can and will do much more in the next year. Integral’s accuracy and sensitivity have already exceeded our expectations and, in the months to come, we could get the answers to some of astronomy’s most intriguing questions.”

Original Source: ESA News Release