Category: Astronomy

Image credit: UC Berkeley
Astronomers at the University of California, Berkeley, have discovered the nearest and youngest star with a visible disk of dust that may be a nursery for planets.

The dim red dwarf star is a mere 33 light years away, close enough that the Hubble Space Telescope or ground-based telescopes with adaptive optics to sharpen the image should be able to see whether the dust disk contains clumps of matter that might turn into planets.

“Circumstellar disks are signposts for planet formation, and this is the nearest and youngest star where we directly observe light reflected from the dust produced by extrasolar comets and asteroids – i.e., the objects that could possibly form planets by accretion,” said Paul Kalas, assistant research astronomer at UC Berkeley and lead author of a paper reporting the discovery.

“We’re waiting for the summer and fall observing season to go back to the telescopes and study the properties of the disk in greater detail. But we expect everyone else to do the same thing – there will be lots of follow-up.”

A paper announcing the discovery will be published online in Science Express this week, and will appear in the printed edition of the journal in March. Coauthors with Kalas are Brenda C. Matthews, a post-doctoral researcher with UC Berkeley’s Radio Astronomy Laboratory, and astronomer Michael C. Liu of the University of Hawaii. Kalas also is affiliated with the Center for Adaptive Optics at UC Santa Cruz.

The young M-type star, AU Microscopium (AU Mic), is about half the mass of the sun but only about 12 million years old, compared to the 4.6 billion year age of the sun. The team of astronomers found the star while searching for dust disks around stars emitting more than expected amounts of infrared radiation, indicative of a warm, glowing dust cloud.

The image of AU Mic, obtained last October with the University of Hawaii’s 2.2-meter telescope atop Mauna Kea, shows an edge-on disk of dust stretching about 210 astronomical units from the central star – about seven times farther from the star than Neptune is from the sun. One astronomical unit, or AU, is the average distance from the Earth to the sun, about 93 million miles.

“When we see scattered infrared light around a star, the inference is that this is caused by dust grains replenished by comets and asteroid collisions,” Kalas said. Because 85 percent of all stars are M-type red dwarfs, the star provides clues to how the majority of planetary systems form and evolve.

Other nearby stars, such as Gliese 876 at 16 light years and epsilon-Eridani at 10 light years, wobble, providing indirect evidence for planets. But images of debris disks around stars are rare. AU Mic is the closest dust disk directly imaged since the discovery 20 years ago of a dust disk around beta-Pictoris, a star about 2.5 times the mass of the sun and 65 light years away. Though the two stars are in opposite regions of the sky, they appear to have been formed at the same time and to be traveling together through the galaxy, Kalas said.

“These sister stars probably formed together in the same region of space in a moving group containing about 20 stars,” Kalas said. This represents an unprecedented opportunity to study stars formed under the same conditions, but of masses slightly larger and slightly smaller than the sun.

“Theorists are excited, too, at the opportunity to understand how planetary systems evolve differently around high-mass stars like beta-Pictoris and low-mass stars like AU Mic,” he said.

The pictures of AU Mic were obtained by blocking glare from the star with a coronagraph like that used to view the sun’s outer atmosphere, or corona. The eclipsing disk on the University of Hawaii’s 2.2-meter telescope blocked view of everything around the star out to about 50 AU. At this distance in our solar system, only the Kuiper Belt of asteroids and the more distant Oort cloud, the source of comets, would be visible.

Kalas said that sharper images from the ground or space should show structures as close as 5 AU, which means a Jupiter-like planet or lump in the dusty disk would be visible, if present.

“With the adaptive optics on the Lick 120-inch telescope or the Keck 10-meter telescopes, or with the Hubble Space Telescope, we can improve the sharpness by 10 to 100 times,” Kalas said.

In a companion paper accepted for publication in The Astrophysical Journal, the Berkeley-Hawaii team reports indirect evidence for a relatively dust-free hole within about 17 AU of the star. This would be slightly inside the orbit of Uranus in our own solar system.

“Potential evidence for the existence of planets comes from the infrared spectrum, where we notice an absence of warm dust grains,” he said. “That means that grains are depleted within about 17 AU radius from the star. One mechanism to clear out the dust disk within 17 AU radius is by planet-grain encounters, where the planet removes the grains from the system.”

“The dust missing from the inner regions of AU Mic is the telltale sign of an orbiting planet. The planet sweeps away any dust in the inner regions, keeping the dust in the outer region at bay,” said Liu.

Aside from further observations with the 2.2-meter telescope in Hawaii, Kalas and his colleagues plan to use the Spitzer Space Telescope, an infrared observatory launched last August by the National Aeronautics and Space Administration (NASA), to conduct a more sensitive search for gas.

The research was supported by the NASA Origins Program and the National Science Foundation’s Center for Adaptive Optics.

Original Source: UC Berkeley News Release

Image credit: CfA
Since its discovery in 1998, the “winking star” called KH 15D has baffled astronomers seeking to explain its long-lasting (24-day) eclipses. Many hypothesized that the eclipses were caused by intervening blobs of material within a protoplanetary disk surrounding a single, young Sun-like star.

By examining the past history of these eclipses and how they are changing with time, astronomer Joshua Winn (Harvard-Smithsonian Center for Astrophysics) and colleagues have overturned this hypothesis and devised a new theory that explains nearly everything about the system.

They found that the “winking star” is actually a double star system. Something in the foreground, possibly a dusty disk of material surrounding the binary, intermittently blocks the light from one or both stars, as the stars orbit each other. Eventually, both stars will be completely covered by the dust curtain, and the “winking star” system will disappear from view.

“These two stars have been playing hide and seek with us. The second star used to peek out briefly, but now is completely obscured. Soon, it will be joined by the first star and both will remain hidden for decades,” says Winn.

Archives Reveal The Truth
The vital clues to understanding the “winking star” were found in archival sky photographs from Harvard College Observatory, in Massachusetts, and Asiago Observatory, in Italy. Examination of the Harvard photographs showed that during the first half of the 20th century, there were none of the total eclipses that are observed today. Asiago photographs taken between 1967 and 1982 held evidence of eclipses, but with a key difference: the system was brighter than it is today, both during eclipses and outside of eclipses. This extra light must have come from a second star that was visible in the 1970s, but is completely hidden today.

This insight was the key to unlocking the mystery of KH 15D. Before 1960, neither star was being eclipsed. Then, a curtain of dust drifted into the foreground as seen from the Earth, blocking part of the orbit of one of the stars. Throughout the 1970s, that star underwent eclipses as its orbital motion carried it behind the curtain. By 1998, the curtain had advanced enough to completely hide one of the stars-and the other star periodically drops out of sight as its orbit takes it behind the curtain. By about 2012, both stars will be completely hidden from view.

Radial velocity measurements currently being made by John Johnson (UC Berkeley), a co-author of this study, will be able to test whether the visible star is moving back and forth, tugged by the gravity of a stellar-mass companion.

“The Asiago plates give very convincing evidence, but the radial velocity measurements will be the clincher,” says Johnson.

The New Picture of KH 15D
Assembling the observations of KH 15D like pieces of a jigsaw puzzle reveals two stars no older than 10 million years. (Our Sun, by contrast, is 5 billion years old.) They revolve around each other every 48 days in highly elliptical orbits, which explains the 48-day eclipse period. Their average distance apart is approximately 0.25 astronomical units (23 million miles), or two-thirds the distance from Mercury to the Sun. Yet their eccentric orbits take them as close to each other as only 0.07 AU (6.5 million miles).

“As binaries go, their orbit is not unusual” says co-author Krzysztof Stanek (CfA).

Winn agrees, adding, “The weird thing about this system is that there’s something blocking the light from these stars-something opaque, with a sharp edge.” The identity of this curtain is unknown, but it may be the edge of a disk of dust that surrounds both of the stars.

“Dust disks have been seen around other binary star systems,” says Matthew Holman (CfA), a co-author of the study. “We imagine that the disk in this system is inclined, relative to the plane of the orbit of the two stars. That would cause the disk to wobble, the way a Frisbee sometimes wobbles in the air after a bad throw.”

According to Holman’s calculations, the dust may exist in a ring located 2.6 AU (240 million miles) from the stars. The material in the ring itself makes one complete orbit about every 4 years, but the wobbling (or “precession”) of the ring has a much longer period of about 1000 years. A similar theory has been proposed independently by Eugene Chiang and Ruth Murray-Clay of UC Berkeley.

“Starting around 1960, the edge of this precessing disk happened to start blocking our view of the stars,” says Holman. “After another decade, the disk will precess a little further and completely block our view.” Some time after that, depending on how thick the ring is, the process will reverse itself as the stars are gradually uncovered, and the eclipses will stop.

Many questions about KH 15D still remain. For example, what is the nature of the disk? Why is it inclined to the orbital plane of the binaries? Why does it have such a sharp edge? The winking stars of KH 15D are likely to confound astronomers with these and other riddles for years to come.

This research will be published in the March 1, 2004 issue of The Astrophysical Journal Letters. The authors of the study are Joshua Winn (CfA), Matthew Holman (CfA), John Johnson (UC Berkeley), Krzysztof Stanek (CfA), and Peter Garnavich (University of Notre Dame).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

The European Space Agency’s Rosetta mission is now on the launch pad, and it should be heading up into space early tomorrow. If you’ve got some free time and a connection to the Internet, why not watch the launch live on your computer? I really enjoy watching the launches live, since there’s still a quite a bit of suspense. You also get a sense of the various tasks that need to be performed to get a spacecraft off the ground. The launch is scheduled for early Thursday, February 26 at 0736 UTC (2:36 am EST). I know, that’s the middle of the night for a lot of you, but if you’re a night owl, tune it in.

Click here to access the live feed.

Enjoy!

Fraser Cain
Publisher
Universe Today

Image credit: NASA
Scientists at the Canadian Institute for Theoretical Astrophysics (CITA) and NASA have captured unprecedented details of the swirling flow of gas hovering just a few miles from the surface of a neutron star, itself a sphere only about ten miles across.

A massive and rare explosion on the surface of this neutron star – pouring out more energy in three hours than the Sun does in 100 years – illuminated the area and allowed the scientists to spy on details of the region never before revealed. They could see details as fine as the ring of gas swirling around and flowing onto the neutron star as this ring buckled from the explosion and then slowly recovered its original form after approximately 1,000 seconds.

All of this was occurring 25,000 light years from Earth, captured second-by-second in movie-like fashion through a process called spectroscopy with NASA’s Rossi X-ray Timing Explorer.

Dr. David Ballantyne of CITA at the University of Toronto and Dr. Tod Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Md., present this result in an upcoming issue of Astrophysical Journal Letters. The observation provides new insight into the flow of a neutron star’s (and perhaps a black hole’s) “accretion disk,” usually far too minute to resolve with even the most powerful telescopes.

“This is the first time we have been able to watch the inner regions of an accretion disk, in this case literally a few miles from the neutron star’s surface, change its structure in real-time,” said Ballantyne. “Accretion disks are known to flow around many objects in the Universe, from newly forming stars to the giant black holes in distant quasars. Details of how such a disk flows could only be inferred up to now.”

A neutron star is the dense, core remains of an exploded star at least eight times more massive than the Sun. The neutron star contains about a sun’s worth of mass packed in a sphere no larger than Toronto. An accretion disk refers to the flow of hot gas (plasma) swirling around neutron stars and black holes, attracted by the strong gravity of the region. This gas is often supplied by a neighboring star.

As matter crashes down on the neutron star it builds up a 10- to 100-meter layer of material comprised mostly of helium. The fusion of the helium into carbon and other heavier elements releases enormous energy and powers a strong burst of X-ray light, far more energetic than visible light. (Nuclear fusion is the same process that powers the Sun.) Such bursts can occur several times a day on a neutron star and last for about 10 seconds.

What Ballantyne and Strohmayer observed on this neutron star, named 4U 1820-30, was a “superburst”. These are much more rare than ordinary, helium-powered bursts and release a thousand times more energy. Scientists say these superbursts are caused by a buildup of nuclear ash in the form of carbon from the helium fusion. Current thinking suggests that is takes several years for the carbon ash to buildup to such an extent that it begins to fuse.

The superburst was so bright and long that it acted like a spotlight beamed from the neutron star surface and onto the innermost region of the accretion disk. The X-ray light from the burst illuminated iron atoms in the accretion disk, a process called fluorescence. The Rossi Explorer captured the characteristic signature of the iron fluorescence — that is, its spectrum. This, in turn, provided information about the iron’s temperature, velocity and location around the neutron star.

“The Rossi Explorer can get a good measurement of the fluorescence spectrum of the iron atoms every few seconds,” Strohmayer said. “Adding up all this information, we get a picture of how this accretion disk is being deformed by the thermonuclear blast. This is the best look we can hope to get, because the resolution needed to actually see this action as an image, instead of spectra, would be a billion times greater than what the Hubble Space Telescope offers.”

The scientists said the bursting neutron stars serve as a laboratory to study accretion disks, which are seen (but in less detail) through the Universe around nearby stellar black holes and exceedingly distant quasar galaxies. Stellar black holes with accretion disks do not produce X-ray bursts.

The Rossi Explorer was launched in December 1995 to observe fast-changing, energetic and rapidly spinning objects, such as supermassive black holes, active galactic nuclei, neutron stars and millisecond pulsars.

Original Source: NASA News Release

Image credit: Caltech
Planetary scientists at the California Institute of Technology and Yale University on Tuesday night discovered a new planetoid in the outer fringes of the solar system.

The planetoid, currently known only as 2004 DW, could be even larger than Quaoar–the current record holder in the area known as the Kuiper Belt–and is some 4.4 billion miles from Earth.

According to the discoverers, Caltech associate professor of planetary astronomy Mike Brown and his colleagues Chad Trujillo (now at the Gemini North observatory in Hawaii), and David Rabinowitz of Yale University, the planetoid was found as part of the same search program that discovered Quaoar in late 2002. The astronomers use the 48-inch Samuel Oschin Telescope at Palomar Observatory and the recently installed QUEST CCD camera built by a consortium including Yale and the University of Indiana, to systematically study different regions of the sky each night.

Unlike Quaoar, the new planetoid hasn’t yet been pinpointed on old photographic plates or other images. Because its orbit is therefore not well understood yet, it cannot be given an official name.

“So far we only have a one-day orbit,” said Brown, explaining that the data covers only a tiny fraction of the orbit the object follows in its more than 300-year trip around the sun. “From that we know only how far away it is and how its orbit is tilted relative to the planets.”

The tilt that Brown has measured is an astonishingly large 20 degrees, larger even than that of Pluto, which has an orbital inclination of 17 degrees and is an anomaly among the otherwise planar planets.

The size of 2004 DW is not yet certain; Brown estimates a size of about 1,400 kilometers, based on a comparison of the planetoid’s luminosity with that of Quaoar. Because the distance of the object can already be calculated, its luminosity should be a good indicator of its size relative to Quaoar, provided the two objects have the same albedo, or reflectivity.

Quaoar is known to have an albedo of about 10 percent, which is slightly higher than the reflectivity of our own moon. Thus, if the new object is similar, the 1,400-kilometer estimate should hold. If its albedo is lower, then it could actually be somewhat larger; or if higher, smaller.

According to Brown, scientists know little about the albedos of objects this large this far away, so the true size is quite uncertain. Researchers could best make size measurements with the Hubble Space Telescope or the newer Spitzer Space Telescope. The continued discovery of massive planetoids on the outer fringe of the solar system is further evidence that objects even farther and even larger are lurking out there. “It’s now only a matter of time before something is going to be discovered out there that will change our entire view of the outer solar system,” Brown says.

The team is working hard to uncover new information about the planetoid, which they will release as it becomes available, Brown adds. Other telescopes will also be used to better characterize the planetoid’s features.

Original Source: Caltech News Release

Image credit: Chandra
Imagine making a natural telescope more powerful than any other telescope currently operating. Then imagine using it to view closer to the edge of a black hole where its mouth is like a jet that forms super-hot charged particles and spits them millions of light-years into space. The task would seem to take one to the edge of no-return, a violent spot four billion light-years from Earth. That place is called a quasar named PKS 1257-326. Its faint twinkle in the sky is given the more catchy name of a ‘blazar’, meaning it is a quasar that varies dramatically in brightness, and may mask an even more mysterious, inner black hole of enormous gravitational power.

The length of a telescope needed to peer into the mouth of the blazar would have to be gigantic, about a million kilometers wide. But just such a natural lens has been found by a team of Australian and European astronomers; its lens is remarkably, a cloud of gas. The idea of a vast, natural telescope seems too elegant to avoid peering into.

The technique, dubbed ‘Earth-Orbit Synthesis’, was first outlined by Dr Jean-Pierre Macquart of the University of Groningen in The Netherlands and CSIRO’s Dr David Jauncey in a paper published in 2002. The new technique promises researchers the ability to resolve details about 10 microarcseconds across – equivalent to seeing a sugar cube on the Moon, from Earth.

“That’s a hundred times finer detail than we can see with any other current technique in astronomy,” says Dr. Hayley Bignall, who recently completed her PhD at the University of Adelaide and is now at JIVE, the Joint Institute for Very Long Baseline Interferometry in Europe. “It’s ten thousand times better than the Hubble Space Telescope can do. And it’s as powerful as any proposed future space-based optical and X-ray telescopes.”

Bignall made the observations with the CSIRO Australia Telescope Compact Array radio telescope in eastern Australia. When she refers to a microarcsecond, that is a measure of angular size, or how big an object looks. If for instance the sky were divided by degrees as a hemisphere, the unit is about a third of a billionth of one degree.

How does the largest telescope work? Using the clumpiness inside a cloud of gas is not entirely unfamiliar to night-watchers. Like atmospheric turbulence makes the stars twinkle, our own galaxy has a similar invisible atmosphere of charged particles that fill the voids between stars. Any clumping of this gas naturally can form a lens, just like the density change from air-to-glass bent and focused the light in what Galileo first saw when he pointed his first telescope towards the star. The effect is also called scintillation, and the cloud acts like a lens.

Seeing better than anyone else may be remarkable, but how to decide where to look first? The team is particularly interested using ‘Earth-Orbit Synthesis’ to peer close to black holes in quasars, which are the super-bright cores of distant galaxies. These quasars subtend such small angles on the sky as to be mere points of light or radio emission. At radio wavelengths, some quasars are small enough to twinkle in our Galaxy’s atmosphere of charged particles, called the ionized interstellar medium. Quasars twinkle or vary much more slowly than the twinkling one might associate with visible stars. So observers have to be patient to view them, even with the help of the most powerful telescopes. Any change in less than a day is considered to be fast. The fastest scintillators have signals that double or treble in strength in less than an hour. In fact the best observations made so far benefit from the annual motion of the Earth, since the yearly variation gives a complete picture, potentially allowing astronomers to see the violent changes in the mouth of a black-hole jet. That’s one of the team’s goals: “to see to within a third of a light-year of the base of one of these jets,” according to CSIRO’s Dr David Jauncey. “That’s the ‘business end’ where the jet is made.”

It is not possible to “see” into a black hole, because these collapsed stars are so dense, that their overpowering gravity doesn’t even allow light to escape. Only the behavior of matter outside a horizon some distance away from a black-hole can signal that they even exist. The largest telescope may help the astronomers understand the size of a jet at its base, the pattern of magnetic fields there, and how a jet evolves over time. “We can even look for changes as matter strays near the black hole and is spat out along the jets, ” says Dr Macquart.

Astrobiology Magazine had the opportunity to talk with Hayley Bignall about how to make a telescope from gas clouds, and why peering deeper than anyone before may offer insight into remarkable events near black holes. Astrobiology Magazine (AM): How did you first become interested in using gas clouds as part of a natural focus for resolving very distant objects?

Hayley Bignall (HB): The idea of using interstellar scintillation (ISS), a phenomenon due to radio wave scattering in turbulent, ionized Galactic gas “clouds”, to resolve very distant, compact objects, really represents the convergence of a couple of different lines of research, so I will outline a little of the historical background.

In the 1960s, radio astronomers used another kind of scintillation, interplanetary scintillation, due to scattering of radio waves in the solar wind, to measure sub-arcsecond (1 arcsecond = 1/3600 degrees of arc) angular sizes for radio sources. This was higher resolution than could be achieved by other means at the time. But these studies largely fell by the wayside with the advent of Very Long Baseline Interferometry (VLBI) in the late 1960s, which allowed direct imaging of radio sources with much higher angular resolution – today, VLBI achieves resolution better than a milliarcsecond.

I personally became interested in potential uses of interstellar scintillation through being involved in studies of radio source variability – in particular, variability of “blazars”. Blazar is a catchy name applied to some quasars and BL Lacertae objects – that is, Active Galactic Nuclei (AGN), probably containing supermassive black holes as their “central engines”, which have powerful jets of energetic, radiating particles pointed almost straight at us.

We then see effects of relativistic beaming in the radiation from the jet, including rapid variability in intensity across the whole electromagnetic spectrum, from radio to high-energy gamma rays. Most of the observed variability in these objects could be explained, but there was a problem: some sources showed very rapid, intra-day radio variability. If such short time-scale variability at such long (centimeter) wavelengths were intrinsic to the sources, they would be far too hot to stay around for years, as many were observed to do. Sources that hot should radiate all their energy away very quickly, as X-rays and gamma-rays. On the other hand, it was already known that interstellar scintillation affects radio waves; so the question of whether the very rapid radio variability was in fact ISS, or intrinsic to the sources, was an important one to resolve.

During my PhD research I found, by chance, rapid variability in the quasar (blazar) PKS 1257-326, which is one of the three most rapidly radio variable AGN ever observed. My colleagues and I were able to show conclusively that the rapid radio variability was due to ISS [scintillation]. The case for this particular source added to mounting evidence that intra-day radio variability in general is predominantly due to ISS.

Sources which show ISS must have very small, microarcsecond, angular sizes. Observations of ISS can in turn be used to “map” source structure with microarcsecond resolution. This is much higher resolution than even VLBI can achieve. The technique was outlined in a 2002 paper by two of my colleagues, Dr Jean-Pierre Macquart and Dr David Jauncey.

The quasar PKS 1257-326 proved to be a very nice “guinea pig” with which to demonstrate that the technique really works.

AM: The principles of scintillation are visible to anyone even without a telescope, correct–where a star twinkles because it covers a very small angle in the sky (being so far away), but a planet in our solar system doesn’t scintillate visibly? Is this a fair comparison of the principle for estimating distances visually with scintillation?

HB: The comparison with seeing stars twinkle as a result of atmospheric scintillation (due to turbulence and temperature fluctuations in the Earth’s atmosphere) is a fair one; the basic phenomenon is the same. We don’t see planets twinkle because they have much larger angular sizes – the scintillation gets “smeared out” over the planet’s diameter. In this case, of course, it is because the planets are so close to us that they subtend larger angles on the sky than stars.

Scintillation is not really useful for estimating distances to quasars, however: objects that are further away do not always have smaller angular sizes. For example, all pulsars (spinning neutron stars) in our own Galaxy scintillate because they have very tiny angular sizes, much smaller than any quasar, even though quasars are often billions of light-years away. In fact, scintillation has been used to estimate pulsar distances. But for quasars, there are many factors besides distance which affect their apparent angular size, and to complicate matters further, at cosmological distances, the angular size of an object no longer varies as the inverse of distance. Generally the best way of estimating the distance to a quasar is to measure the redshift of its optical spectrum. Then we can convert measured angular scales (e.g. from scintillation or VLBI observations) to linear scales at the redshift of the source

AM: The telescope as described offers a quasar example that is a radio source and observed to vary over an entire year. Are there any natural limits to the types of sources or the length of observation?

HB: There are angular size cut-offs, beyond which the scintillation gets “quenched”. One can picture the radio source brightness distribution as a bunch of independently scintillating “patches” of a given size, so that as the source gets larger, the number of such patches increases, and eventually the scintillation over all the patches averages out so that we cease to observe any variations at all. From previous observations we know that for extragalactic sources, the shape of the radio spectrum has a lot to do with how compact a source is – sources with “flat” or “inverted” radio spectra (i.e. flux density increasing towards shorter wavelengths) are generally the most compact. These also tend to be “blazar”-type sources.

As far as the length of observation goes, it is necessary to obtain many independent samples of the scintillation pattern. This is because scintillation is a stochastic process, and we need to know some statistics of the process in order to extract useful information. For fast scintillators like PKS 1257-326, we can get an adequate sample of the scintillation pattern from just one, typical 12-hour observing session. Slower scintillators need to be observed over several days to get the same information. However, there are some unknowns to solve for, such as the bulk velocity of the scattering “screen” in the Galactic interstellar medium (ISM). By observing at intervals spaced over a whole year, we can solve for this velocity – and importantly, we also get two-dimensional information on the scintillation pattern and hence the source structure. As the Earth goes around the Sun, we effectively cut through the scintillation pattern at different angles, as the relative Earth/ISM velocity varies over the course of the year. Our research group dubbed this technique “Earth Orbital Synthesis”, as it is analogous to “Earth rotation synthesis”, a standard technique in radio interferometry.

AM: A recent estimate for the number of stars in the sky estimated that there are ten times more stars in the known universe than grains of sand on Earth. Can you describe why jets and black holes are interesting as difficult-to-resolve objects, even using current and future space telescopes like Hubble and Chandra?

HB: The objects we are studying are some of the most energetic phenomena in the universe. AGN can be up to ~1013 (10 to the power of 13, or 10,000 trillion) times more luminous than the Sun. They are unique “laboratories” for high energy physics. Astrophysicists would like to fully understand the processes involved in forming these tremendously powerful jets close to the central supermassive black hole. Using scintillation to resolve the inner regions of radio jets, we are peering close to the “nozzle” where the jet forms – closer to the action than we can see with any other technique!

AM: In your research paper, you point out that how fast and how strongly the radio signals vary depends on the size and shape of the radio source, the size and structure of the gas clouds, the Earth’s speed and direction as it travels around the Sun, and the speed and direction in which the gas clouds are travelling. Are there built-in assumptions about either the shape of the gas cloud ‘lens’ or the shape of observed object that is accessible with the technique?

The Ring Nebula, although not useful imaging through, has the suggestive look of a far-away telescope lens. 2,000 light years distant in the direction of the constellation, Lyra, the ring is formed in the late stages of the inner star’s life, when it sheds a thick and expanding outer gas layer. Credit: NASA Hubble HST

HB: Rather than think of gas clouds, it is perhaps more accurate to picture a phase-changing “screen” of ionized gas, or plasma, which contains a large number of cells of turbulence. The main assumption which goes into the model is that the size scale of the turbulent fluctuations follows a power-law spectrum – this seems to be a reasonable assumption, from what we know about general properties of turbulence. The turbulence could be preferentially elongated in a particular direction, due to magnetic field structure in the plasma, and in principle we can get some information on this from the observed scintillation pattern. We also get some information from the scintillation pattern about the shape of the observed object, so there are no built-in assumptions about that, although at this stage we can only use quite simple models to describe the source structure.

AM: Are fast scintillators a good target for expanding the capabilities of the method?

HB: Fast scintillators are good simply because they don’t require as much observing time as slower scintillators to get the same amount of information. The first three “intra-hour” scintillators have taught us a lot about the scintillation process and about how to do “Earth Orbit Synthesis”.

AM: Are there additional candidates planned for future observations?

HB: My colleagues and I have recently undertaken a large survey, using the Very Large Array in New Mexico, to look for new scintillating radio sources. The first results of this survey, led by Dr Jim Lovell of the CSIRO’s Australia Telescope National Facility (ATNF), were recently published in the Astronomical Journal (October 2003). Out of 700 flat spectrum radio sources observed, we found more than 100 sources which showed significant variability in intensity over a 3-day period. We are undertaking follow-up observations in order to learn more about source structure on ultra-compact, microarcsecond scales. We will compare these results with other source properties such as emission at other wavelengths (optical, X-ray, gamma-ray), and structure on larger spatial scales, such as that seen with VLBI. In this way we hope to learn more about these very compact, high brightness temperature sources, and also, in the process, learn more about properties of the interstellar medium of our own Galaxy.

It seems that the reason for very fast scintillation in some sources is that the plasma “scattering screen” causing the bulk of the scintillation is quite nearby, within 100 light-years of the solar system. These nearby “screens” are apparently quite rare. Our survey found very few fast scintillators, which was somewhat surprising as two of the three fastest known scintillators were discovered serendipitously. We thought that there might be many more such sources!

Original Source: Astrobiology Magazine

Image credit: ESA
Unlike conventional reflecting telescopes, whose mirrors are made from special glass or sometimes metal, Herschel’s telescope mirrors are being made from a novel ceramic material.

The manufacture of the shaped blank that will be used to create the flight model primary mirror for Herschel’s telescope was completed late last year. The blank is 3.5 metres in diameter and was fabricated by brazing together twelve segments. The segments were formed by isostatic pressing and sintering of silicon carbide. The mirror blank is the biggest silicon carbide structure in the world and, when completed, the mirror will be the largest single-component telescope reflector ever made for use in space. Larger mirrors are planned for future missions but they will be composed of multiple, deployable sections.

The silicon carbide segment manufacture and blank fabrication was performed by Boostec (Tarbes, France). The prime contractor for the Herschel telescope is EADS Astrium SAS (Toulouse, France).

All the major telescope components – the primary and secondary mirrors and the hexapod that supports the secondary mirror – are made from silicon carbide, allowing the telescope mass to be reduced to 300 kg rather than the 1.5 tonnes that would have resulted from using conventional materials. In addition to substantial mass savings, silicon carbide also offers the excellent structural stability and thermal properties needed to maintain a mirror location accuracy of better than 10 ?m.

The mirror blank will now be machined by Boostec to remove the internal stiffeners, used to provide mechanical stability during manufacture, and reduce the shell thickness to 2.5 mm. After machining, the mirror will be polished by Opteon (Turku, Finland) to obtain a parabolic surface with a roughness of less than 30 nm. The mirror accuracy will be such that the completed telescope will have a total wavefront error of less than 6 ?m RMS.

The polished mirror will be coated by vacuum deposition at the Calar Alto Observatory (Almer?a, Spain), first with a 10 nm thick adhesive layer of nickel-chrome and then with a 300 nm reflective layer of aluminium.

Original Source: ESA News Release

What is the biggest telescope in the world?

Image credit: Sky and Telescope
Treat yourself to an eye-catching celestial treat on Monday evening, February 23, 2004. At dusk, just look to the west. There you’ll find Venus, the magnificently brilliant “Evening Star,” blazing to the right of the crescent Moon. Provided it’s clear, you can’t miss this celestial splendor. Venus will outshine every star in the sky and may even appear about as bright as the 3?-day-old Moon itself.

The Moon will have only about 15 percent of its disk illuminated by the Sun. But look closer and you will likely make out the rest of the lunar disk glowing faintly ? you may even see the Man in the Moon. This effect is known as earthshine. What you are seeing is reflected sunlight from Earth shining onto the Moon’s night landscape, providing a dull illumination.

Venus is so brilliant for two reasons: it’s closer to the Sun than Earth so it gets lit more brightly, and its white clouds reflect sunlight very well. Although the Moon and Venus look close together, they’re actually at very different distances. On the night of the 23rd, the Moon is 244,000 miles (392,000 kilometers) from Earth, but Venus is 370 times farther: 90 million miles (145 million km) away. Venus is sometimes called our sister planet because it is similar in size to Earth.

For members of the broadcast media, the timing of this celestial pairing will make for a perfect opportunity to take your cameras outside for a live shot during the evening news. Be sure to take advantage.

Original Source: Sky and Telescope News Release

Image credit: Hubble
An international team of astronomers may have set a new record in discovering what is the most distant known galaxy in the universe. Located an estimated 13 billion light-years away, the object is being viewed at a time only 750 million years after the big bang, when the universe was barely 5 percent of its current age.

The primeval galaxy was identified by combining the power of NASA’s Hubble Space Telescope and CARA’s W. M. Keck Telescopes on Mauna Kea in Hawaii. These great observatories got a boost from the added magnification of a natural “cosmic gravitational lens” in space that further amplifies the brightness of the distant object.

The newly discovered galaxy is likely to be a young galaxy shining during the end of the so-called “Dark Ages” ? the period in cosmic history which ended with the first galaxies and quasars transforming opaque, molecular hydrogen into the transparent, ionized universe we see today.

The new galaxy was detected in a long exposure of the nearby cluster of galaxies Abell 2218, taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. This cluster is so massive that the light of distant objects passing through the cluster actually bends and is amplified, much as a magnifying glass bends and magnifies objects seen through it. Such natural gravitational “telescopes” allow astronomers to see extremely distant and faint objects that could otherwise not be seen. The extremely faint galaxy is so far away its visible light has been stretched into infrared wavelengths, making the observations particularly difficult.

“As we were searching for distant galaxies magnified by Abell 2218, we detected a pair of strikingly similar images whose arrangement and color indicate a very distant object,” said astronomer Jean-Paul Kneib (Observatoire Midi-Pyrenees and Caltech), who is lead author reporting the discovery in a forthcoming article in the Astrophysical Journal.

Analysis of a sequence of Hubble images indicate the object lies in between a redshift of 6.6 and 7.1, making it the most distant source currently known. However, long exposures in the optical and infrared taken with spectrographs on the 10-meter Keck telescopes suggest that the object has a redshift towards the upper end of this range, around redshift 7.

Redshift is a measure of how much the wavelengths of light are shifted to longer wavelengths. The greater the shift in wavelength toward the redder regions of the spectrum, the more distant the object is.

“The galaxy we have discovered is extremely faint, and verifying its distance has been an extraordinarily challenging adventure,” said Dr. Kneib. “Without the magnification of 25 afforded by the foreground cluster, this early object could simply not have been identified or studied in any detail at all with the present telescopes available. Even with aid of the cosmic lens, the discovery has only been possible by pushing our current observatories to the limits of their capabilities!”

Using the combination of the high resolution of Hubble and the large magnification of the cosmic lens, the astronomers estimate that this object, although very small ? only 2,000 light-years across ? is forming stars extremely actively. However, two intriguing properties of the new source are the apparent lack of the typically bright hydrogen emission line and its intense ultraviolet light which is much stronger than that seen in star-forming galaxies closer by.

“The properties of this distant source are very exciting because, if verified by further study, they could represent the hallmark of a truly young stellar system that ended the Dark Ages,” added Dr. Richard Ellis, Steele Professor of Astronomy at Caltech, and a co-author in the article.

The team is encouraged by the success of their technique and plans to continue the search for more examples by looking through other cosmic lenses in the sky. Hubble’s exceptional resolution makes it ideally suited for such searches.

“Estimating the abundance and characteristic properties of sources at early times is particularly important in understanding how the universe reionized itself, thus ending the Dark Ages,” said Mike Santos, a former Caltech graduate student, now a postdoctoral researcher at the Institute of Astronomy, Cambridge, UK. “The cosmic lens has given us a first glimpse into this important epoch. We are now eager to learn more by finding further examples, although it will no doubt be challenging.”

“We are looking at the first evidence of our ancestors on the evolutionary tree of the entire universe,” said Dr. Frederic Chaffee, director of the W. M. Keck Observatory, home to the twin 10-meter Keck telescopes that confirmed the discovery. “Telescopes are virtual time machines, allowing our astronomers to look back to the early history of the cosmos, and these marvelous observations are of the earliest time yet.”

The Caltech team reporting on the discovery consists of Drs. Jean-Paul Kneib, Richard S. Ellis, Michael R. Santos and Johan Richard. Drs. Kneib and Richard also serve the Observatoire Midi-Pyrenees of Toulouse, France. Dr. Santos also represents the Institute of Astronomy, Cambridge, UK.

Public funding for this project was provided in part by NASA GSRP grant NGT5-50339 and NASA STScI grant HST-GO-09452.01-A.

The W. M. Keck Observatory is managed by the California Association for Research in Astronomy (CARA), a scientific partnership among the California Institute of Technology (Caltech), the University of California, and the National Aeronautics and Space Administration (NASA). The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

Original Source: Hubble News Release

Image credit: SDSS
The most distant known quasars show that some supermassive black holes formed when the universe was merely 6 percent of its current age, or about 700 million years after the big bang.

How black holes of several billion solar masses formed so rapidly in the very early universe is one mystery raised by astronomers with the Sloan Digital Sky Survey (SDSS). They have discovered 13 of the oldest, most distant quasars yet found.

“We hope to at least double that number in the next three years,” said Xiaohui Fan of the University of Arizona?s Steward Observatory in Tucson.

Fan led the SDSS team that discovered the distant quasars, which are compact but luminous objects thought to be powered by supermassive black holes. The most distant quasar, in the constellation Ursa Major, is roughly 13 billion light years away.

The most ancient quasars raise other tantalizing questions about the early universe. Fan talked about it today (Feb. 13) at the American Association for the Advancement of Science annual meeting in Seattle.

The infant universe was hydrogen and helium.

“But we see a lot of other elements around those early quasars,” Fan said. “We see evidence of carbon, nitrogen, iron and other elements, and it?s not clear how these elements got there. There is as much iron, proportionate to the population of those early systems, as there is in mature galaxies nearby.”

Astronomers estimate the current age of the universe at 13.7 billion years. Quasars in the early universe looked as mature as nearby galaxies that, like the Milky Way, formed a couple of billion years after the big bang.

Also, radio astronomers collaborating with SDSS researchers detected carbon monoxide, a key component of molecular clouds, near the ancient quasars.

All this evidence suggests that the first mature galaxies formed right along with the ancient supermassive black holes in the very early universe.

Although cosmologists aren?t panicked, they need to refine theory to clarify what?s going on.

Fan and his colleagues believe the oldest quasars can be used to probe the end of the Cosmic Dark Ages and the beginning of the Cosmic Renaissance.

In so-called Cosmic Dark Ages, the universe was a cold, opaque place without stars. Then came a critical phase where the universe went through a rapid transition. The first galaxies and quasars formed in the Cosmic Renaissance, heating the universe so it became the place we see today.

Fan and his colleagues believe some of their oldest known quasars may span the critical transition.

“Our observations suggest that what we may be seeing during this transition is atomic hydrogen becoming completely ionized. This ionization process was one of the important processes going on during the first one billion years.”

Current observations have just begun to reveal when and how this ionization process occurred. Data from distant quasars combined with other evidence, such as from the cosmic microwave background, which is relict radiation from the big bang, will begin to test theory of how the first galaxies appeared in the universe, Fan said.

It may take the large-aperture space telescope, NASA’s 6.5-meter James Webb Space Telescope, to really explore what happened between the Cosmic Dark Ages and the Cosmic Renaissance, Fan said.

Optical/infrared ground-based telescopes cannot detect objects red-shifted much beyond 6.5, Fan noted. Water vapor in Earth?s atmosphere absorbs longer infrared wavelengths, so it will take a space-based telescope, probably with an aperture larger than that of the NASA Spitzer Telescope now orbiting Earth, to study objects at redshift 7, 8, or 10 in detail, Fan said.

(So-called redshift is a phenomenon proportional to the velocity of a a celestial object speeding away from Earth. The lines in its spectrum shift toward longer, red wavelengths. Astronomers now believe that the most distant objects recede from Earth at the highest velocities, so the farther away an object is, the greater its redishift.)

Original Source: UA News Release