Author: Fraser Cain

A scientifically accurate model of Beta Pictoris and its disk. Image credit: NAOJ. Click to enlarge
The disks of gas and dust that surround newborn stars are known as proto-planetary disks; which are thought to be regions where planets will eventually form. These disks disappear as the stars mature, but some stars can still be seen with a cloud of material around them called debris disks. One of the most famous of these is the disk surrounding Beta Pictoris, located only 60 light years away.

Planets form in disks of gas and dust that surround new born stars. Such disks are called proto-planetary disks. The dust in these disks become rocky planets like Earth and the inner cores of giant gas planets like Saturn. This dust is also a repository of elements that form the basis of life.

Proto-planetary disks disappear as stars mature, but many stars have what are called debris disks. Astronomers hypothesize that once objects such as asteroids and comets are born from the proto-planetary disk, collisions among them can produce a secondary dust disk.

The most well-known example of such dust disks is the one surrounding the second brightest star in the constellation Pictor, meaning “painter’s easel”. This star, known as Beta Pictoris or Beta Pic, is a very close neighbor of the Sun, only sixty light years away, and therefore easy to study in great detail.

Beta Pic is twice as bright as the Sun, but the light from the disk is much fainter. Astronomers Smith and Terrile were the first to detect this faint light in 1984, by blocking the light from the star itself using a technique called coronagraphy. Since then, many astronomers have observed the Beta Pic disk using ever better instruments and ground and space-based telescopes to understand in detail the birth place of planets, and hence life.

A team of astronomers from the National Astronomical Observatory of Japan, Nagoya University and Hokkaido University combined several technologies for the first time to obtain an infrared polarization image of the Beta Pic disk with better resolution and higher contrast than ever before: a large aperture telescope (the Subaru telescope, with its large 8.2 meter primary mirror), adaptive optics technology, and a coronagraphic imager capable of taking images of light with different polarizations (Subaru’s Coronagraphic Imager with Adaptive Optics,CIAO).

A large aperture telescope, especially with Subaru’s great imaging quality, allows faint light to be seen at high resolution. Adaptive optics technology reduces Earth’s atmosphere’s distorting effects on light, allowing higher resolution observations. Coronagraphy is a technique for blocking light from a bright object such as a star, to see fainter objects near it, such as planets and dust surrounding a star. By observing polarized light, reflected light can be distinguished from light coming directly from its original source. Polarization also contains information about the size, shape, and alignment of dust reflecting light.

With this combination of technologies, the team succeeded in observing Beta Pic in infrared light two micrometers in wavelength at a resolution of a fifth of an arcsecond. This resolution corresponds to being able to see an individual grain of rice from one mile away or a mustard seed from a kilometer away. Achieving this resolution represents a huge improvement over comparable previous polarimetric observations from the 1990’s that had only resolutions of about one and a half arcseconds.

The new results strongly suggest that Beta Pic’s disk contains planetesimals, asteroid or comet-like objects, that collide to generate dust that reflects starlight.

The polarization of the light reflected from the disk can reveal the physical properties of the disk such as composition, size, and distribution. An image of all the two micrometer wavelength light shows the long thin structure of the disk seen nearly edge on. The polarization of the light shows that ten percent of the two micrometer light is polarized. The pattern of polarization indicates that the light is a reflection of light that originated from the central star.

An analysis of how the brightness of the disk changes with distance from the central shows a gradual decrease in brightness with a small oscillation. The slight oscillation in brightness corresponds to variations in the density of the disk. The most likely explanation is that denser regions correspond to where planetesimals are colliding. Similar structures have been seen closer to the star in earlier observations at longer wavelengths using Subaru’s COoled Mid-Infrared Camera and Spectrograph (COMICS) and other instruments.

A similar analysis of how the amount of polarization changes with distance from the star shows a decrease in polarization at a distance of one hundred astronomical units (an astronomical unit is the distance between Earth and the Sun). This corresponds to a location where the brightness also decreases, suggesting that at this distance from the star there are fewer planetesimals.

As the team investigated models of the Beta Pic disk that can explain both the new and old observations, they found that the dust in Beta Pic’s disk is more than ten times larger than typical grains of interstellar dust. Beta Pics dust disk is probably made of micrometer sized loose clumps of dust and ice like miniscule bacteria-size dust bunnies.

Together, these results provide very strong evidence that the disk surrounding Beta Pic is generated by the formation and collision of planetesimals. The level of detail of this new information solidifies our understanding of the environment in which planets form and develop.

Motohide Tamura who leads the team says “few people have been able to study the birth place of planets by observing polarized light with a large telescope. Our results show that this is a very rewarding approach. We plan on extending our research to other disks, to get a comprehensive picture of how dust transforms into planets.”

These results were published in the April 20, 2006, edition of the Astrophysical Journal.

Team Members: Motohide Tamura, Hiroshi Suto, Lyu Abe (NAOJ), Misato Fukagawa (Nagoya University, California Institute of Technology), Hiroshi Kimura, Tetsuo Yamamoto (Hokkaido University)

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan through a Grant-in-Aid for Scientific Research on Priority Areas for the “Development of Extra-solar Planetary Science.”

Original Source: NAOJ News Release

Massive star forming region as seen by ISO. Image credit: ESA. Click to enlarge
Astronomers have used ESA’s Infrared Space Observatory to peer at the biggest stars being born. These stars form from the collapse of enormous clouds of gas, and can shine more than 100,000 times as bright as our own Sun. The images were captured as a bonus, taken while the space observatory was slowly turning from one target to another. A team of astronomers built up a vast web of images comprised from 10,000 of these telescope repositionings, and then identified potential star forming regions from the data.

Scientists have secured their first look at the birth of monstrous stars that shine 100 000 times more brightly than the Sun, thanks to ESA’s Infrared Space Observatory (ISO).

The discovery allows astronomers to begin investigating why only some regions of space promote the growth of these massive stars.

Space is littered with giant clouds of gas. Occasionally, regions within these clouds collapse to form stars. “One of the major questions in the field of study is why do some clouds produce high- and low-mass stars, whilst others form only low-mass stars?” asks Oliver Krause, Max-Planck-Institut fur Astronomie, Heidelberg and Steward Observatory, Arizona.

The conditions necessary to form high-mass stars are difficult to deduce because such stellar monsters form far away and are shrouded behind curtains of dust. Only long wavelengths of infrared radiation can escape from these obscuring cocoons and reveal the low temperature dust cores that mark the sites of star formation. This radiation is exactly what ISO’s ISOPHOT far-infrared camera has collected.

Stephan Birkmann, Oliver Krause and Dietrich Lemke, all of the Max-Planck-Institut f?r Astronomie, Heidelberg, used ISOPHOT’s data to zero-in on two intensely cold and dense cores, each containing enough matter to form at least one massive star. “This opens up a new era for the observations of the early details of high-mass star formation,” says Krause.

The data was collected in the ISOPHOT Serendipity Survey (ISOSS), a clever study pioneered by Lemke. He realised that when ISO was turning from one celestial object to another, valuable observing time was being lost. He organised for ISOPHOT’s far-infrared camera to continuously record during such slews and beam this data to Earth.

During the ISO mission, which lasted for two and a half years during 1995?98, the spacecraft made around 10 000 slews, providing a web of data across the sky for the previously unexplored window of infrared emission at 170 micrometres. This wavelength is 310 times longer than optical radiation and reveals cold dust down to just 10K (-263 degrees Celsius). A catalogue was produced of the cold sites in the survey.

Birkmann and his colleagues investigated this catalogue and found fifty potential places of high-mass stellar birth. A campaign of follow-up observations using ground-based telescopes revealed that object ISOSS J18364-0221 was in fact two cold dense cores that looked suspiciously like those associated with the birth of low-mass stars, but containing much more mass.

The first core is at 16.5 Kelvin (?256.5 degrees Celsius). It contains seventy-five times the mass of the Sun and shows signs of gravitational collapse. The second one is around 12K (?261 degrees Celsius) and contains 280 solar masses. The team are currently studying the other potential sites.

Original Source: ESA Portal

Effects of the extragalactic background light on the gamma emission by a quasar before reaching Earth. Image credit: HESS Collaboration. Click to enlarge
When astronomers look into the sky, they see bright objects, but also a diffuse glow coming from objects across the Universe in many different wavelengths. This glow could serve like a fossil record, to help astronomers untangle the different stages the Universe went through from the beginning, to the current day. Research teams are using very high energy gamma rays, generated in the most violent objects in the Universe – quasars – as a probe to understand this background light.

The light emitted from all objects in the Universe during its entire history – stars, galaxies, quasars etc. forms a diffuse sea of photons that permeates intergalactic space, referred to as “diffuse extragalactic background light” (EBL). Scientists have long tried to measure this fossil record of the luminous activity in the Universe in their quest to decipher the history and evolution of the Cosmos, but its direct determination from the diffuse glow of the night sky is very difficult and uncertain.

Very high energy (VHE) gamma-rays, some 100,000,000,000 times more energetic than normal light, offer an alternative way to probe this background light, and UK researchers from Durham University in collaboration with international partners used the High Energy Stereoscopic System (HESS) gamma-ray telescopes in the Khomas Highlands of Namibia to observe several quasars (the most luminous VHE gamma-ray sources known) with this goal in mind. The results, to be published in the April 20 issue of Nature, turned out to be rather striking.

Gamma-rays, which are produced in the most violent objects in the Universe, are absorbed in their journey from distant objects to Earth if they happen to hit a photon of “normal” background light. This fog of light in which the Universe is bathed is a fossil record of all the light emitted in the Universe over its lifetime, from the glare of the first stars and galaxies up to the present time. So, using the distant quasars as a probe and studying the effect of the fossil light on the energy distribution of the initial gamma-rays, astrophysicists used HESS to derive a limit on the maximum amount of this ‘extragalactic background light’, which is remarkably lower than what previous estimates had suggested.

This result, published in the April 20 issue of Nature, has important consequences for our understanding of galaxy formation and evolution, and expands the horizon of the gamma-ray Universe which is clearly more transparent to gamma-rays than previously believed

Commenting in the findings, Dr Lowry McComb of Durham University, said, “HESS has in the last few years achieved a number of important discoveries concerning high-energy gamma-ray sources in our own Galaxy and has revolutionized high-energy gamma-ray astronomy. These new HESS results illustrate the power of the instrument for extragalactic astronomy and cosmology. The discovery of lower levels of intergalactic starlight has the interesting side effect that the Universe becomes more transparent to gamma rays and that the telescopes can look deeper into the cosmos, increasing their reach for further discoveries!”

Original Source: PPARC News Release

The afterglow of GRB 030329 (white dot in center of image). Image credit: ESA/NASA. Click to enlarge
Since gamma ray bursts release a torrent of radiation visible across the Universe, it goes without saying that we wouldn’t one to blow up near us. Well, don’t worry. According to researchers at Ohio State University, our Milky Way is the just wrong type of galaxy for potential bursts – they almost always happen within small, misshapen galaxies that lack heavy chemical elements. That’s good news, since a burst within 3,000 light years of the Earth would give us a lethal dose of radiation.

Are you losing sleep at night because you’re afraid that all life on Earth will suddenly be annihilated by a massive dose of gamma radiation from the cosmos?

Well, now you can rest easy.

Some scientists have wondered whether a deadly astronomical event called a gamma ray burst could happen in a galaxy like ours, but a group of astronomers at Ohio State University and their colleagues have determined that such an event would be nearly impossible.

Gamma ray bursts (GRBs) are high-energy beams of radiation that shoot out from the north and south magnetic poles of a particular kind of star during a supernova explosion, explained Krzysztof Stanek, associate professor of astronomy at Ohio State. Scientists suspect that if a GRB were to occur near our solar system, and one of the beams were to hit Earth, it could cause mass extinctions all over the planet.

The GRB would have to be less than 3,000 light years away to pose a danger, Stanek said. One light year is approximately 6 trillion miles, and our galaxy measures 100,000 light years across. So the event would not only have to occur in our galaxy, but relatively close by, as well.

In the new study, which Stanek and his coauthors submitted to the Astrophysical Journal, they found that GRBs tend to occur in small, misshapen galaxies that lack heavy chemical elements (astronomers often refer to all elements other than the very lightest ones — hydrogen, helium, and lithium — as metals). Even among metal-poor galaxies, the events are rare — astronomers only detect a GRB once every few years.

But the Milky Way is different from these GRB galaxies on all counts — it’s a large spiral galaxy with lots of heavy elements.

The astronomers did a statistical analysis of four GRBs that happened in nearby galaxies, explained Oleg Gnedin, a postdoctoral researcher at Ohio State. They compared the mass of the four host galaxies, the rate at which new stars were forming in them, and their metal content to other galaxies catalogued in the Sloan Digital Sky Survey.

Though four may sound like a small sample compared to the number of galaxies in the universe, these four were the best choice for the study because astronomers had data on their composition, Stanek said. All four were small galaxies with high rates of star formation and low metal content.

Of the four galaxies, the one with the most metals — the one most similar to ours — hosted the weakest GRB. The astronomers determined that the odds of a GRB occurring in a galaxy like that one to be approximately 0.15 percent.

And the Milky Way’s metal content is twice as high as that galaxy, so our odds of ever having a GRB would be even lower than 0.15 percent.

“We didn’t bother to compute the odds for our galaxy, because 0.15 percent seemed low enough,” Stanek said.

He figures that most people weren’t losing sleep over the possibility of an Earth-annihilating GRB. “I wouldn’t expect the stock market to go up as a result of this news, either,” he said. “But there are a lot of people who have wondered whether GRBs could be blamed for mass extinctions early in Earth’s history, and our work suggests that this is not the case.”

Astronomers have studied GRBs for more than 40 years, and only recently determined where they come from. In fact, Stanek led the team that tied GRBs to supernovae in 2003.

He and Gnedin explained that when a very massive, rapidly rotating star explodes in a supernova, its magnetic field directs gamma radiation to flow only out of the star’s north and south magnetic poles, forming high-intensity jets.

Scientists have measured the energies of these events and assumed — rightly so, Stanek said — that such high-intensity radiation could destroy life on a planet. That’s why some scientists have proposed that a GRB could have been responsible for a mass extinction that occurred on Earth 450 million years ago.

Now it seems that gamma ray bursts may not pose as much a danger to Earth or any other potential life in the universe, either, since they are unlikely to occur where life would develop.

Planets need metals to form, Stanek said, so a low-metal galaxy would probably have fewer planets, and fewer chances for life.

He added that he didn’t originally intend to address the question of mass extinctions. The study grew out of a group discussion during the Ohio State Department of Astronomy’s “morning coffee” — a daily half-hour where faculty and students review new astronomy journal articles that have been posted to Internet preprint servers overnight. In February, Stanek published a paper on a GRB he had observed, and during coffee someone asked whether he thought it was just a coincidence that these events seem to happen in small, metal-poor galaxies.

“My initial reaction was that it’s not a coincidence, and everyone just knows that GRBs happen in metal-poor galaxies. But then people asked, ‘Is it really that well known? Has anybody actually proven it to be true?’ And we realized that nobody had.”

As a result, the list of coauthors on the paper includes astronomers across a broad range of expertise, which Stanek said is somewhat unusual in these days of specialized research. The coauthors were among faculty gathered for coffee that day, plus a few friends they recruited to help them: Stanek and Gnedin; John Beacom, assistant professor of physics and astronomy; Jennifer Johnson, assistant professor of astronomy; Juna Kollmeier, a graduate student; Andrew Gould, Marc Pinsonneault, Richard Pogge, and David Weinberg, all professors of astronomy at Ohio State; and Maryam Modjaz, a graduate student at the Harvard-Smithsonian Center for Astrophysics.

This work was sponsored by the National Science Foundation.

Original Source: Ohio State University

Pulsar RX J0720.4-3125 captured by XMM-Newton. Image credit: ESA/MPE. Click to enlarge
ESA’s orbiting X-ray telescope, the XMM-Newton space observatory, has located a neutron star that’s out of control. Researchers found that its temperature rose steadily for more than four years, but now it’s starting to decrease again. The object’s overall temperature isn’t changing, it’s just tumbling, and slowly displaying different areas to observers here on Earth – like a wobbling top. These observations will help astronomers understand some of the internal processes that govern these kinds of objects.

Using data from ESA’s XMM-Newton X-ray observatory, an international group of astrophysicists discovered that one spinning neutron star doesn’t appear to be the stable rotator scientists would expect. These X-ray observations promise to give new insights into the thermal evolution and finally the interior structure of neutron stars.

Spinning neutron stars, also known as pulsars, are generally known to be highly stable rotators. Thanks to their periodic signals, emitted either in the radio or in the X-ray wavelength, they can serve as very accurate astronomical ‘clocks’.

The scientists found that over the past four and a half years the temperature of one enigmatic object, named RX J0720.4-3125, kept rising. However, very recent observations have shown that this trend reversed and the temperature is now decreasing.

According to the scientists this effect is not due to a real variation in temperature, but instead to a changing viewing geometry. RX J0720.4-3125 is most probably ‘precessing’, that is it is slowly tumbling and therefore, over time, it exposes to the observers different areas of the surface.

Neutron stars are one of the endpoints of stellar evolution. With a mass comparable to that of our Sun confined into a sphere of 20-40 km diameter, their density is even somewhat higher than that of an atomic nucleus – a billion tonnes per cubic centimetre. Soon after their birth in a supernova explosion their temperature is of the order of 1 000 000 degrees celsius and the bulk of their thermal emission falls in the X-ray band of the electromagnetic spectrum. Young isolated neutron stars are slowly cooling down and it takes a million years before they become too cold to be observable in X-rays.

Neutron stars are known to possess very strong magnetic fields, typically several trillion times stronger than that of the Earth. The magnetic field can be so strong that it influences the heat transport from the stellar interior through the crust leading to hot spots around the magnetic poles on the star surface.

It is the emission from these hotter polar caps which dominates the X-ray spectrum. There are only a few isolated neutron stars known from which we can directly observe the thermal emission from the surface of the star. One of them is RX J0720.4-3125, rotating with a period of about eight and a half seconds. “Given the long cooling time scale it was therefore highly unexpected to see its X-ray spectrum changing over a couple of years,” said Frank Haberl from the Max-Planck-Institute for Extraterrestrial Physics in Garching (Germany), who led the research group.

“It is very unlikely that the global temperature of the neutron star changes that quickly. We are rather seeing different areas of the stellar surface at different times. This is also observed during the rotation period of the neutron star when the hot spots are moving in and out of our line of sight, and so their contribution to the total emission changes,” Haberl continued.

A similar effect on a much longer time scale can be observed when the neutron star precesses (similarly to a spinning top). In that case the rotation axis itself moves around a cone leading to a slow change of the viewing geometry over the years. Free precession can be caused by a slight deformation of the star from a perfect sphere, which may have its origin in the very strong magnetic field.

During the first XMM-Newton observation of RX J0720.4-3125 in May 2000, the observed temperature was at minimum and the cooler, larger spot was predominantly visible. On the other hand, four years later (May 2004) the precession brought into view mostly the second, hotter and smaller spot, that made the observed temperature increase. This likely explains the observed variation in temperature and emitting areas, and their anti-correlation.

In their work Haberl and colleagues developed a model for RX J0720.4-3125 which can explain many of the peculiar characteristics which have been a challenge to explain so far. In this model the long-term change in temperature is produced by the different fractions of the two hot polar caps which enter into view as the star precesses with a period of about seven to eight years.

In order for such a model to work, the two emitting polar regions need to have different temperatures and sizes, as it has been recently proposed in the case of another member of the same class of isolated neutron stars.

According to the team, RX J0720.4-3125 is probably the best case to study precession of a neutron star via its X-ray emission directly visible from the stellar surface. Precession may be a powerful tool to probe the neutron star interior and learn about the state of matter under conditions which we can not produce in the laboratory.

Additional XMM-Newton observations are planned to further monitor this intriguing object. “We are continuing the theoretical modelling from which we hope to learn more about the thermal evolution, the magnetic field geometry of this particular star and the interior structure of neutron stars in general,” Haberl concluded.

Original Source: ESA Portal