Supersized stellar blackhole prompts model rewrite

Supersized stellar blackhole

Boffins go back to the drawing board

Researchers have located the most massive stellar black hole ever discovered, just three million light-years away in a nearby galaxy. The stellar remnant is in a binary system known as M33, orbiting a huge companion star. The researchers say the find is "intriguing", because of what it suggests about stellar evolution.

A stellar black hole is what is left after the death-by-collapsing-core of a massive star. The star that formed this one must have been huge. The scientists used the Chandra X-Ray observatory and the Gemini telescope on Mauna Kea in Hawaii to measure the mass of the black hole, and discovered the remnant still has 15.7 times the mass contained in our own modest, yellow sun. Its companion star is also a humdinger - checking in at roughly 70 solar masses, it is the largest known companion star to a black hole. Eventually it too will go supernova, leaving a binary system containing only black holes.

"This discovery raises all sorts of questions about how such a big black hole could have been formed," said Jerome Orosz of San Diego State University, lead author of a paper appearing in the 18 October issue of Nature. Conventional models of black hole formation suggest that the star would have been much larger even than its 70-solar-mass companion. It would have been so big that its radius would have been larger than the current separation between the two bodies, NASA's boffins explain. This means the two stars must have drawn closer together while sharing a common outer atmosphere. But if this were the case, according to conventional models, the black hole shouldn't have retained such a large mass.

Still, it did, so the models are being re-thought. The researchers say the star must have lost mass roughly 10 times more slowly than they expected before it exploded. The discovery could help explain an incredibly bright supernova, observed in 2006. The progenitor of this supernova is thought to have been about 150 solar masses when it exploded, which would make more sense if more massive stars lose their mass more slowly.The system is also interesting because it is an eclipsing black hole. This unusual property is what allowed researchers to make "unusually accurate" estimates of the mass of both the black hole and its companion.

Acknowledgements: Report by Lucy Sheriff through Yahoo news

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Neutron stars warp space-time

Astronomers have pioneered a technique for determining the properties of ultradense objects.

Provided by the Goddard Space Flight Center

August 28, 2007

Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein's predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultradense objects. Neutron stars contain the densest observable matter in the universe. They cram more than a sun's worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer."This is fundamental physics," says Sudip Bhattacharyya of NASA's Goddard Space Flight Center in Greenbelt, Md. and the University of Maryland, College Park. "There could be exotic kinds of particles or states of matter, such as quark matter, in the centers of neutron stars, but it's impossible to create them in the lab. The only way to find out is to understand neutron stars. "To address this mystery, scientists must accurately and precisely measure the diameters and masses of neutron stars.

In two concurrent studies, one with the European Space Agency's XMM-Newton X-ray Observatory and the other with the Japanese/NASA Suzaku X-ray observatory, astronomers have taken a big step forward.Using XMM-Newton, Bhattacharyya and his NASA Goddard colleague Tod Strohmayer observed a binary system known as Serpens X-1, which contains a neutron star and a stellar companion. They studied a spectral line from hot iron atoms that are whirling around in a disk just beyond the neutron star's surface at 40 percent the speed of light.Previous X-ray observatories detected iron lines around neutron stars, but they lacked the sensitivity to measure the shapes of the lines in detail. Thanks to XMM-Newton's large mirrors, Bhattacharyya and Strohmayer found that the iron line is broadened asymmetrically by the gas's extreme velocity, which smears and distorts the line because of the Doppler Effect and beaming effects predicted by Einstein's special theory of relativity. The warping of space-time by the neutron star's powerful gravity, an effect of Einstein's general theory of relativity, shifts the neutron star's iron line to longer wavelengths. "We've seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well.

It shows that the way neutron stars accrete matter is not very different from that of black holes, and it gives us a new tool to probe Einstein's theory," says Strohmayer. A group led by Edward Cackett and Jon Miller of the University of Michigan, which includes Bhattacharyya and Strohmayer, used Suzaku's superb spectral capabilities to survey three neutron-star binaries:Serpens X-1, GX 349+2, and 4U 1820-30. This team observed a nearly identical iron line in Serpens X-1, confirming the XMM-Newton result. It detected similarly skewed iron lines in the other two systems as well."We're seeing the gas whipping around just outside the neutron star's surface," says Cackett. "And since the inner part of the disk obviously can't orbit any closer than the neutron star's surface, these measurements give us a maximum size of the neutron star's diameter. The neutron stars can be no larger than 18 to 20.5 miles across, results that agree with other types of measurements. Now that we've seen this relativistic iron line around three neutron stars, we have established a new technique", adds Miller. "It's very difficult to measure the mass and diameter of a neutron star, so we need several techniques to work together to achieve that goal. "Knowing a neutron star's size and mass allows physicists to describe the "stiffness," or "equation of state," of matter packed inside these incredibly dense objects. Besides using these iron lines to test Einstein's general theory of relativity, astronomers can probe conditions in the inner part of a neutron star's accretion disk.The XMM-Newton paper appeared in the August 1 Astrophysical Journal Letters. The Suzaku paper has been submitted for publication in the same journal.

Acknowledgements: Astronomy newsletter

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Goldilocks planet found?

NASA's Spitzer Space Telescope has uncovered a developing exoplanet with earthlike conditions in a star system 424 light-years away.

October 4, 2007

Scientists have discovered a huge belt of warm dust — enough to build a Mars-size planet or larger — swirling around a distant star that is just slightly more massive than our Sun. The dust belt, which they suspect is clumping together into planets, is located in the middle of the system's terrestrial habitable zone. This is the region around a star where liquid water could exist on any rocky planets that might form. Earth is located in the middle of our sun's terrestrial habitable zone. At approximately 10 million years old, the star is also at just the right age for forming rocky planets. "The timing for this system to be building an Earth is very good," says Dr. Carey Lisse, of the Johns Hopkins University Applied Physics Laboratory. "If the system was too young, its planet-forming disk would be full of gas, and it would be making gas-giant planets like Jupiter instead. If the system was too old, then dust aggregation or clumping would have already occurred and all the system's rocky planets would have already formed."

According to Lisse, the conditions for forming an Earth-like planet are more than just being in the right place at the right time and around the right star — it's also about the right mix of dusty materials. Using Spitzer's infrared spectrometer instrument, he determined that the material in HD 113866 is more processed than the snowball-like stuff that makes up infant solar systems and comets, which are considered cosmic "refrigerators" because they contain pristine ingredients from the early solar system. However, it is also not as processed as the stuff found in mature planets and the largest asteroids. This means the dust belt must be in a transitional phase, when rocky planets are just beginning to form.How do scientists know the material is more processed than that of comets? From missions like NASA's Deep Impact — in which an 820-pound impactor spacecraft collided with comet Tempel 1 — scientists know that early star systems contain a lot of fragile organic material. That material includes polycyclic aromatic hydrocarbons (carbon-based molecules found on charred barbeque grills and automobile exhaust on Earth), water ice, and carbonates (chalk). Lisse says that HD 113766 does not contain any water ice, carbonates or fragile organic materials. From meteorite studies on Earth, scientists also have a good idea of what makes up asteroids — the more processed rocky leftovers of planet formation. These studies tell us that metals began separating from rocks in Earth's early days, when the planet's body was completely molten. During this time, almost all the heavy metals fell to Earth's center in a process called "differentiation".

Lisse says that, unlike planets and asteroids, the metals in HD 113766 have not totally separated from the rocky material, suggesting that rocky planets have not yet formed."The material mix in this belt is most reminiscent of the stuff found in lava flows on Earth. I thought of Mauna Kea material when I first saw the dust composition in this system — it contains raw rock and is abundant in iron sulfides, which are similar to fool's gold," says Lisse, referring to a well-known Hawaiian volcano. "It is fantastic to think we are able to detect the process of terrestrial planet formation. Stay tuned — I expect lots more fireworks as the planet in HD 113766 grows," he adds.

Acknowledgements: JHUAPL, Astronomy newsletter

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