Posted on by Gemma Lavender

Astronomers Complete the Puzzle of Black Hole Description

The optical image on the left, from the Digitized Sky Survey, shows Cygnus X-1 outlined in a red box located near large active regions of star formation in the Milky Way that spans 700 light-years across. An artist’s illustration on the right depicts what astronomers believe is happening within the Cygnus X-1 system with the black hole pulling material from a massive, blue companion star. This material forms a disk (shown in red and orange) that rotates around the black hole before falling into it or being directed away in the form of powerful jets. Credit: X-ray: NASA/CXC; Optical: Digitized Sky Survey.

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Light may not be able to escape a black hole, but now enough information has escaped one black hole’s clutches that astronomers have, for the first time, been able to provide a complete description of it. A team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and San Diego State University have made the most accurate measurements ever of X-ray binary system Cygnus X-1, allowing them to unravel the longstanding mysteries of its black hole and to retrace its history since its birth around six million years ago.

Cygnus X-1, which consists of a black hole that is drawing material from its massive blue companion star, was found to be emitting powerful X-rays nearly half a century ago. Since its discovery in 1964, this galactic X-ray source has been intensely scrutinized with astronomers attempting to gain information about its mass and spin. But without an accurate measurement of its distance from the Earth, which has been estimated to be between 5,800 and 7,800 light-years, we could only imagine what secrets Cygnus X-1 was harboring.

Astronomer Mark Reid of CfA led his team to garner the most accurate measurement of the distance to Cygnus X-1 with the help of the National Science Foundation’s Very Long Baseline Array (VLBA), a continent-wide radio-telescope system. The team locked down a direct trigonometric measurement of 6,070 light-years.

“Because no other information can escape a black hole, knowing its mass, spin and electrical charge gives a complete description of it,” says Reid who is a co-author of three papers on Cygnus X-1, published in the Astrophysical Journal Letters (available here, here, and here). “The charge of this black hole is nearly zero, so measuring its mass and spin make our description complete.”

Using their new precise distance measurement along with the Chandra X-ray Observatory, the Rossi X-ray Timing Explorer, the Advanced Satellite for Cosmology and Astrophysics and visible-light observations made over more than two decades, the team pieced together the “No Hair” theorem – the complete description that Reid speaks of – by revealing a hefty mass of nearly 15 solar masses and a turbo spin speed of 800 revolutions per second. “We now know that Cygnus X-1 is one of the most massive stellar black holes in the Milky Way,” says Jerry Orosz of San Diego State University, also an author of the paper with Reid and Lijun Gou of the CfA. “It’s spinning as fast as any black hole we’ve ever seen.”

As an added bonus, observations using the VLBA back in 2009 and 2010 had also measured Cygnus X-1’s movement through the galaxy leading scientists to the conclusion that it is much too slow to have been produced by the explosion of a supernova and without evidence of a large “kick” at birth, astronomers believe that it may have resulted from the dark collapse of a progenitor star with a mass greater than about 100 times the mass of the Sun that got lost in a vigorous stellar wind. “There are suggestions that this black hole could have formed without a supernova explosion and our results support those suggestions,” says Reid.

It seems that with these measurements, Professor Stephen Hawking has well and truly had to eat his own words after placing a bet with fellow astrophysicist Kip Thorne, a professor of theoretical physics at the California Institute of Technology, that Cygnus X-1 did not contain a black hole.

“For forty years, Cygnus X-1 has been the iconic example of a black hole. However, despite Hawking’s concession, I have never been completely convinced that it really does contain a black hole – until now,” says Thorne. “The data and modeling in these three papers at last provide a completely definitive description of this binary system.”

Sources: CfA

Posted on by Gemma Lavender

“Baby” Planet Caught in the Act of Forming

The left image shows the transitional disk around the star LkCa 15. All of the light at this wavelength is emitted by cold dust in the disk. The hole in the centre indicates an inner gap with a radius of around 55 times the distance from the Earth to the Sun. The right image is an expanded view of the central part of the cleared region, illustrating a composite of two reconstructed images (blue: 2.1 micrometres, from November 2010; red: 3.7 micrometres) for LkCa 15. The location of the central star is also marked. Image: Kraus & Ireland 2011.

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Astronomers have taken a step closer to finding out how planetary systems form with the discovery of the ‘youngest’ planet ever found. LkCa 15 b is so young, it is still in the act of forming. This is the first direct image of a planet in the process of forming, and data indicates the planet is still being pieced together by gas and dust falling into its clutches from a cooler envelope that surrounds it.

The hot protoplanet orbits a star which possesses a mass comparable to our Sun, and is the youngest planetary system ever to be identified, with LkCa 15 aged at 2 million years, “We really have the age of the star and not the planet,” said Michael Ireland, a lecturer in astrophotonics at the Australian Astronomical Observatory. “The age of the star was determined by a great many people studying the gravitational contraction of both LkCa 15 and all of the other stars in the Taurus star forming region, which formed at nearly the same time.”

The observations were made by astronomers from the University of Hawaii and the Australian Astronomical Observatory using the keen eyesight of the twin 10-metre Keck telescopes located on the summit of Hawaii’s dormant Mauna Kea volcano.

For decades, astronomers have been aware that many young stars that pepper the Universe are shrouded by clouds of gas and dust. And since this realization they have enlisted the help of powerful infrared space observatories such as NASA’s Spitzer Space Telescope to peer into dusty cosmic regions that are hidden from optical telescopes.

Until now scientists had not been lucky enough to capture observations of new planets forming around these young stars, but thanks to the trickery of adaptive optics combined with ‘aperture mask interferometry’ that allows astronomers to resolve discs of dust around stars without the hindrance of dazzling starlight, imaging LkCa 15 b became possible. “It’s like we have an array of small mirrors,” said Adam Kraus of the University of Hawaii’s Institute for Astronomy. “We can manipulate the light and cancel out distortions.”

The location of LkCa 15 can be found using the above chart. Image: Adam Kraus/IAU/Sky & Telescope.

The astronomers have made the clever technique operable since 2008, which allowed them to search for gaps between stars and their protoplanetary dust discs where they figured planets are most likely to be lurking. In 2009 they were rewarded for their efforts as LkCa 15 b presented itself hugging its star, still bright from the energy of its formation. “LkCa 15 was only our second target and we immediately knew we were seeing something new,” said Kraus. “We could see a faint point source near the star, so thinking it might be a Jupiter-like planet we went back a year later to get more data.”

This hot, young world provides a view of the hellish birth of nascent planets.

“The protoplanet is heated up by its gravitational contraction energy,” said Ireland. “Gravitational potential energy is enough to make a truck’s brakes really hot when it goes down a mountain too fast. The potential energy of an entire planet being dropped onto itself is enough to make it glow red hot for millions of years. The planet is more than 1000 degrees Celsius – measuring its temperature more accurately is one of our goals next year. The dust and gas is mostly heated by the radiation field of the star and planet, and in equilibrium, reaches a temperature of less than 100 kelvins [-170 degrees Celsius].”

However, as the young planet pulls in more gas and dust onto itself, the astronomers can only guess as to how big this distant world could get. “The large outer disc around LkCa 15 still has about 55 Jupiter masses of material left in it,” said Ireland. “It is very difficult to estimate just how much of this material could end up on LkCa 15 b. If the orbit is nearly circular, and there is only one planet, then I believe that only a very small fraction of this matter could end up as part of LkCa 15 b. If I had to guess, I’d say around 10 times the mass of Jupiter for a final mass, with a little orbital migration to a closer orbit. However, we’ll get a better idea on this over the coming years with new theoretical models and after we see more of the orbit of the planet.”

The team’s paper can be found here.

An artist's impression of LkCa 15 b orbiting its star. Image: Karen L. Teramura, UH IfA.
Posted on by Gemma Lavender

Puzzling Comet Composition Solved?

How Are Comets Formed?
The Deep Impact spacecraft successfully flew past Comet Hartley 2 in November 2010 and is an example of the type of comet that the UCLA scientists describe in their research. Image: UPI/NASA/JPL-Caltech/UMD.

For years comets have mystified scientists with their compositions that appear to have formed in both warm and cold environments, rather than in one location of a uniform temperature. But new research shows that the reason some comets feature patches of differing surface composition is not because they are made from material that formed in different parts of the Solar System, but because some parts of their surface absorb heat at varying rates. This leads to localized heat sinks and cold traps, according to a new model constructed by David Jewitt and Aurelie Guilbert-Lepoutre from the University of California, Los Angeles (UCLA). Their model shows that the chemical composition of a comet can evolve in the ten million year period during which a comet is classed as a Centaur, migrating from the Kuiper Belt to the inner Solar System.

“The Centaurs are objects which have escaped from the Kuiper belt and are drifting amongst the giant planets,” says Jewitt. “Their lifetimes in these orbits are limited to about 10 million years because they are gravitationally perturbed by the planets to other orbits. At least half are ejected from the Solar System to the interstellar medium. Some are kicked inside the orbit of Jupiter, where the ice begins to sublimate and we call them comets.”

The key is variances in the surface – thermal conductivity, reflectivity (albedo), obliquity (tilt) and even topography such as craters or hilly terrain. This leads to the creation of ‘thermal shadows’.

“Just as it is cooler in the shadow of a building than standing in the full Sun, the region beneath a bright spot or a boulder on the surface of a comet will remain cooler than the surroundings,” says Jewitt. The higher the albedo, the more sunlight is reflected away, keeping that particular patch of the comet 20 to 30 degrees Celsius cooler than its surroundings. The thermal shadows can be maintained “We have calculated the way the cool spot extends down into the interior of the comet, and examined how deep and how long-lived this cool shadow region can be for objects moving on a variety of different orbits.”

Being colder, the thermal shadows attract volatile materials such as water-ice and carbon dioxide from elsewhere on the comet, enhancing the composition there. Consequently the composition of the comet becomes strongly non-uniform, as does the activity on the comet, manifest in jets of the kind seen, for example, by the Deep impact spacecraft on the Comet Hartley 2 in November 2010.

The paper can be found on the astro-ph archive and can be read here.