Thursday, July 31, 2008

The First Stars

How the First Stars Were Born

A new supercomputer simulation offers the most detailed view yet of how the first stars evolved after the Big Bang. The model follows the simpler physics that ruled the early universe to see how cold clumps of gas eventually grew into giant star embryos.

"Until you put that physics in the code, you can't evaluate how the first protostars formed," said Lars Hernquist, an astrophysicist at Harvard University whose early-stars model is detailed in this week's issue of the journal Science.

Mysterious "dark matter" provided the first gravitational impetus for hydrogen and helium gas to start clumping together, Hernquist said. The gas began releasing energy as it condensed, forming molecules from atoms, which further cooled the clump and allowed for even greater condensing.

Unlike previous models, the latest simulation takes this cooling process of "complex radiative transfer" into account, said Nagoya University astrophysicist Naoki Yoshida, who headed up the modeling project.

Eventually gravity could not condense the gas cloud any further, because the densely-packed gas exerted a pressure against further collapse. That equilibrium point marked the beginning of an embryonic star, called a protostar by astronomers.

Simulation runs show that the first protostar likely started with just 1 percent the mass of our sun, but would have swelled to more than 100 solar masses in 10,000 years.

"No simulation has ever gotten to the point of identifying this important stage in the birth of a star," Hernquist noted.

The first protostars reached such massive size because they consisted of mainly simple elements such as hydrogen and helium. That bloated existence means the stars which eventually form from such protostars could create heavier elements such as oxygen, carbon, nitrogen and iron in their fiery furnaces.

CfA Press Release
Simulating the first stars by Centauri Dreams.

Wednesday, July 23, 2008

The True Colour of Blackholes

Schematic display of the polarisation observation. The red star-like object in the upper left is one of the quasars observed. The light is thought to originate from the accretion disk with a strong contamination from messy dust clouds, as shown by the drawing on the upper-right panel. When we put in a polarisation filter, these clouds are suppressed from view, giving us the true colour of the accretion disk, as shown in the two lower panels.

Image: M. Kishimoto with cloud image by M. Schartmann

The central regions of active galaxies are thought to be powered by supermassive black holes accreting gas from their surrounds. An important ingredient of the so-called "standard model" of Active Galactic Nuclei or AGN is a massive accretion disk which is believed to be the source of most of the radiation from the AGN. Until recently, the presence of such accretion disks was only theoretically assumed. An international team of astronomers, led by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, found a clever way to get around observational problems caused by the dust environment of the nucleus. They could eliminate the influence of dust contamination by observing polarised emission directly from the central region of the AGN. Thus they could show that the spectrum of the central source is as blue as expected from theory, verifying a long-standing prediction about the intensely luminous radiation emitted by these accretion disks. The results are published in this week's issue of the journal "Nature".

Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding "accretion disk".

Such black holes and their accretion disks are thought to be in a messy environment - surrounded by many clouds of dust. This has confused astronomers who tried to study the spectrum of the black hole vicinity - the strong emission from these clouds badly contaminates their precious spectrum. "Astronomers were puzzled by the fact that the most extensively studied models of these disks couldn't quite be reconciled with some of the observations, in particular, with the fact that these disks did not appear as blue as they should be", explains Makoto Kishimoto from MPIfR. However, an international team of astronomers, led by Kishimoto, found a clever way to get around this. Since the disk light is scattered in the vicinity of the disk and thus appears polarised, they could use the polarised light to separate the disk from the surrounding dust clouds.

For their observations in the infrared the researchers used polarising filters at some of the largest telescopes on Earth - one of the 8.2m VLT telescopes at the Paranal observatory of ESO in Chile as well as the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii. This enabled them to get rid of emission from hot dust outside the accretion disk, and they could demonstrate that the disk spectrum is as blue as predicted.

Dr. Robert Antonucci of the University of California at Santa Barbara, a fellow investigator, says: "Our understanding of the physical processes in the disk is still rather poor, but now at least we are confident of the overall picture." The disk behaviour found in the paper is expected to originate in the outermost region of the disk, where important questions are yet to be answered: how and where the disk ends and how material is being supplied to the disk. "In the near future, our new method may pioneer the way to address these questions", says Makoto Kishimoto.

Sunday, July 06, 2008

Binary Pulsar System

The double pulsar PSR J0737-3039A/B consists of a binary system made up of two pulsars in a 2.4-hour orbit. Each pulsar emits radio waves along its magnetic poles that illuminate Earth-based radio-telescopes like rotating lighthouse beacons as they spin; one every 23 milliseconds and the other every 2.8 seconds.

The fortunate almost-perfect alignment of our line of sight with the orbital plane of the system gives rise to an eclipse of the 23-ms pulsar, once per orbit, as it moves behind its 2.8-s pulsar companion. The eclipse is created by the magnetosphere of the 2.8-s pulsar, a region in which a dense cloud of plasma is trapped by the magnetic field of the pulsar.

These eclipses allow us to infer the orientation the 2.8-s pulsar since changes in the geometry would affect the way that light emitted by the other pulsar is transmitted to us during the eclipse.

According to classical Newtonian physics, the spin axis about which a star rotates should remain fixed with respect to the background stars as it orbits another star. Einstein's general relativity predicts, however, that the spin axis should slowly precess, like the gentle wobble of a tilted spinning top.
(Credit: Daniel Cantin, DarwinDimensions. McGill University)

[+/-] Click here to expand

Researchers at McGill University's Department of Physics -- along with colleagues from several countries -- have confirmed a long-held prediction of Albert Einstein's theory of general relativity, via observations of a binary-pulsar star system.
Their results were published July 3 in the journal Science.

Pulsars are small, ultradense stellar objects left behind after massive stars die and explode as supernovae. They typically have a mass greater than that of our Sun, but compressed to the size of a city like Montreal. They spin at staggering speeds, generate huge gravity fields and emit powerful beams of radio waves along their magnetic poles.

These illuminate Earth-based radio-telescopes like rotating lighthouse beacons as the pulsar spins. More than 1,700 pulsars have been discovered in our galaxy, but PSR J0737-3039A/B, discovered in 2003, is the only known double-pulsar system; that is, two pulsars locked into close orbit around one another. The two pulsars are so close to each other, in fact, that the entire binary could fit within our Sun. PSR J0737-3039A/B lies about 1,700 light years from Earth.

This new test of Einstein's theory was led by McGill astrophysics PhD candidate René Breton and Dr. Victoria Kaspi, leader of the McGill University Pulsar Group.

"A binary pulsar creates ideal conditions for testing general relativity's predictions because the larger and the closer the masses are to one another, the more important relativistic effects are," Breton explained.

"Binary pulsars are the best place to test general relativity in a strong gravitational field," agreed Kaspi, McGill's Lorne Trottier Chair in Astrophysics and Cosmology and Canada Research Chair in Observational Astrophysics. ""Einstein's theory predicted that, in such a field, an object's spin axis should slowly change direction as the pulsar orbits around its companion. Imagine a spinning top when its slightly non-vertical: the spin axis slowly changes direction, an elegant motion called 'precession.'"

The researchers discovered that one of the two pulsars is indeed precessing -- just as Einstein's 1915 theory predicts. If Einstein had been wrong, the pulsar wouldn't be precessing, or would precess in some other way.

Pulsars are too small and too distant to to allow us to directly observe their orientation, the researchers explained. However, they soon realized they could make such measurements using the eclipses visible when one of the twin pulsars passes in front of its companion. When this occurs, the magnetosphere of the first pulsar partly absorbs the radio "light" being emitted from the other, which allows the researchers to determine its spatial orientation. After four years of observations, they determined that its spin axis precesses just as Einstein predicted.

Even though spin precession has been observed in Earth's solar system, differences between general relativity and alternative theories of gravity might only shake out in extremely powerful gravity fields such as those near pulsars, Breton said.

"However, so far, Einstein's theory has passed all the tests that have been conducted, including ours. We can say that if anyone wants to propose an alternative theory of gravity in the future, it must agree with the results that we have obtained here."

Breton, Kaspi and colleagues in Canada, the United Kingdom, the U.S., France and Italy studied the twin-pulsar using the 100-metre Robert C. Byrd Green Bank Radio Telescope at the National Radio Astronomy Observatory in Green Bank, WV.

"I think that if Einstein were alive today, he would have been absolutely delighted with these results," said Dr. Michael Kramer, Associate Director of the Jodrell Bank Centre for Astrophysics at Manchester University. "Not only because it confirms his theory, but also because of the novel way the confirmation came about."


Wednesday, July 02, 2008

Supernova Remnant

A new star, likely the brightest supernova in recorded human history, lit up planet Earth's sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, found in the southerly constellation of Lupus, still puts on a cosmic light show across the electromagnetic spectrum. In fact, this composite view includes X-ray data in blue from the Chandra Observatory, optical data in yellowish hues, and radio image data in red.

Now known as the SN 1006 supernova remnant, the debris cloud appears to be about 60 light-years across and is understood to represent the remains of a white dwarf star. Part of a binary star system, the compact white dwarf gradually captured material from its companion star. The buildup in mass finally triggered a thermonuclear explosion that destroyed the dwarf star.

Because the distance to the supernova remnant is about 7,000 light-years, that explosion actually happened 7,000 years before the light reached Earth in 1006. Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays.

For Zoomable Image of SN1006 from Hubble Click here

This image, taken by NASA's Hubble Space Telescope, is a very thin section of a supernova remnant caused by a stellar explosion that occurred more than 1,000 years ago.
[+/-] Click here to expand

On or around May 1, 1006 A.D., observers from Africa to Europe to the Far East witnessed and recorded the arrival of light from what is now called SN 1006, a tremendous supernova explosion caused by the final death throes of a white dwarf star nearly 7,000 light-years away. The supernova was probably the brightest star ever seen by humans, and surpassed Venus as the brightest object in the night time sky, only to be surpassed by the moon. It was visible even during the day for weeks, and remained visible to the naked eye for at least two and a half years before fading away.

It wasn't until the mid-1960s that radio astronomers first detected a nearly circular ring of material at the recorded position of the supernova. The ring was almost 30 arcminutes across, the same angular diameter as the full moon. The size of the remnant implied that the blast wave from the supernova had expanded at nearly 20 million miles per hour over the nearly 1,000 years since the explosion occurred.

In 1976, the first detection of exceedingly faint optical emission of the supernova remnant was reported, but only for a filament located on the northwest edge of the radio ring. A tiny portion of this filament is revealed in detail by the Hubble observation. The twisting ribbon of light seen by Hubble corresponds to locations where the expanding blast wave from the supernova is now sweeping into very tenuous surrounding gas.

The hydrogen gas heated by this fast shock wave emits radiation in visible light. Hence, the optical emission provides astronomers with a detailed "snapshot" of the actual position and geometry of the shock front at any given time. Bright edges within the ribbon correspond to places where the shock wave is seen exactly edge on to our line of sight.

Today we know that SN 1006 has a diameter of nearly 60 light-years, and it is still expanding at roughly 6 million miles per hour. Even at this tremendous speed, however, it takes observations typically separated by years to see significant outward motion of the shock wave against the grid of background stars. In the Hubble image as displayed, the supernova would have occurred far off the lower right corner of the image, and the motion would be toward the upper left.

SN 1006 resides within our Milky Way Galaxy. Located more than 14 degrees off the plane of the galaxy's disk, there is relatively little confusion with other foreground and background objects in the field when trying to study this object. In the Hubble image, many background galaxies (orange extended objects) far off in the distant universe can be seen dotting the image. Most of the white dots are foreground or background stars in our Milky Way galaxy.

This image is a composite of hydrogen-light observations taken with Hubble's Advanced Camera for Surveys in February 2006 and Wide Field Planetary Camera 2 observations in blue, yellow-green, and near-infrared light taken in April 2008. The supernova remnant, visible only in the hydrogen-light filter was assigned a red hue in the Heritage colour image.


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