RSSArchive for March, 2011

Starquakes reveal stars’ inner secrets

Oscillations in red giant stars

Oscillations in starlight reveal information about the internal structure of stars, in much the same way that seismologists use earthquakes to probe the Earth's interior.

  • Turbulence in stars’ interiors cause continuous ‘starquakes’
  • Long-term monitoring of starlight picks up the quakes
  • Provides a window into the internal life of stars

AUSTRALIAN ASTROPHYSICISTS from the University of Sydney are behind a major breakthrough in the study of stars known as red giants, finding a way to peer deep into their cores to discover which ones are in early infancy, which are fresh-faced teenagers, and which are facing their dying days.

The discovery, published in the latest edition of the journal Nature and made possible by observations using NASA’s powerful Kepler space telescope, is shedding new light on the evolution of stars, including our own Sun.

“Red giants are evolved stars that have exhausted the supply of hydrogen in their cores that powers nuclear fusion, and instead burn hydrogen in a surrounding shell,” said Professor Tim Bedding, the paper’s lead author. Then, “towards the end of their lives, red giants begin burning the helium in their cores.”

The Kepler space telescope has enabled Professor Bedding and colleagues to continuously study starlight from hundreds of red giants at an unprecedented level of precision for nearly a year, giving a window into the stars’ cores.

“The changes in brightness at a star’s surface is a result of turbulent motions inside that cause continuous star-quakes, creating sound waves that travel down through the interior and back to the surface,” Professor Bedding said.

Size comparison of the Sun and red giant

Red giant stars are the focus of University of Sydney research in 'asteroseismology', which aims to probe the internal life of stars.

“Under the right conditions, these waves interact with other waves trapped inside the star’s helium core,” he adds. “It is these ‘mixed’ oscillation modes that are the key to understanding a star’s particular life stage.

“By carefully measuring very subtle features of the oscillations in a star’s brightness we can see that some stars have run out of hydrogen in the centre and are now burning helium, and therefore at a later stage of life.”

Astronomer Travis Metcalfe of the US National Centre for Atmospheric Research, in a companion piece in the same Nature issue which highlights the discovery’s significance, compares red giants to Hollywood stars, whose age is not always obvious from the surface.

“During certain phases in a star’s life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside,” said Dr Metcalfe.

Starquakes

Professor Bedding and his colleagues work in an emerging field called asteroseismology. “In the same way that geologists use earthquakes to explore Earth’s interior, we use star quakes to explore the internal structure of stars,” he explained.

Professor Bedding said: “We are very excited about the results. We had some idea from theoretical models that these subtle oscillation patterns would be there, but this confirms our models. It allows us to tell red giants apart, and we will be able to compare the fraction of stars that are at the different stages of evolution in a way that we couldn’t before.”

Daniel Huber, a PhD student working with Professor Bedding, added: “This shows how wonderful the Kepler satellite really is. The main aim of the telescope was to find Earth-sized planets that could be habitable, but it has also provided us with a great opportunity to improve our understanding of stars.”

Adapted from information issued by the University of Sydney. Images courtesy ESO / NASA.

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Saved from black hole doom

Artist's impression of Cygnus X-1

An artist's impression of the Cygnus X-1 black hole system. Gas from a nearby supergiant star spirals down into the black hole but a small fraction is diverted by magnetic fields into jets that shoot back into space.

  • Matter falling towards a black hole is usually doomed
  • But strong magnetic fields could form an ‘escape tunnel’

EUROPE’S GAMMA-RAY SPACE TELESCOPE, Integral, has spotted extremely hot matter just a millisecond before it plunges into the oblivion of a black hole.

But is that matter really doomed? Integral’s unique observations suggest that some of it may be making a great escape.

No one would want to be so close to a black hole. Just a few hundred kilometres away from its deadly surface, space is a maelstrom of particles and radiation. Vast storms of particles are falling to their doom at close to the speed of light, raising the temperature to millions of degrees.

Ordinarily, it takes only a millisecond for the particles to cross this final distance…but hope may be at hand for a small fraction of them.

Thanks to the new Integral observations, astronomers now know that this chaotic region is threaded by magnetic fields.

This is the first time that magnetic fields have been identified so close to a black hole. Most importantly, Integral shows they are highly structured magnetic fields that are forming an escape tunnel for some of the doomed particles.

The great escape

Philippe Laurent, CEA Saclay, France, and colleagues made the discovery by studying the nearby black hole, Cygnus X-1, which is ripping a companion star to pieces and feeding on its gas.

Integral, artist’s impression

Artist’s impression of the Integral gamma-ray observatory

Their evidence points to the magnetic field being strong enough to tear away particles from the black hole’s gravitational clutches and funnel them outwards, creating jets of matter that shoot into space.

The particles in these jets are being drawn into spiral trajectories as they climb the magnetic field to freedom and this is affecting a property of their gamma-ray light known as polarisation.

A gamma ray, like ordinary light, is a kind of wave and the orientation of the wave is known as its polarisation. When a fast particle spirals in a magnetic field it produces a kind of light, known as synchrotron emission, which displays a characteristic pattern of polarisation.

It is this polarisation that the team have found in the gamma rays, and it was a difficult observation to make.

“We had to use almost every observation Integral has ever made of Cygnus X-1 to make this detection,” says Laurent.

Ongoing debate

Amassed over seven years, these repeated observations of the black hole now total over five million seconds of observing time, the equivalent of taking a single image with an exposure time of more than two months. Laurent’s team added them all together to create just such an exposure.

“We still do not know exactly how the infalling matter is turned into the jets. There is a big debate among theoreticians; these observations will help them decide,” says Laurent.

Jets around black holes have been seen before by radio telescopes but such observations cannot see the black hole in sufficient detail to know exactly how close to the black hole the jets originate. That makes these new observations invaluable.

Adapted from information issued by ESA.

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Ghostly green eye

Planetary nebula NGC 6826

NGC 6826 is a planetary nebula, the dying stages of a star's life.

THIS AMAZING IMAGE shows the ghostly “eye-like” planetary nebula NGC 6826, located 2,200 light-years from Earth.

Despite their name, planetary nebulae have nothing to do with planets. They got their name because, through early telescopes, they looked more like planets than stars.

In fact, a planetary nebula is a complex cloud of gas produced in the dying stages of certain stars’ lives.

A star’s life ends when the fuel available to its thermonuclear engine runs out. When the star is about to expire, it becomes unstable and ejects its outer layers, forming a planetary nebula and leaving behind a tiny, but very hot, stellar remnant, known as white dwarf.

Schematic of NGC 6826

Anatomy of planetary nebula NGC 6826

At NGC 6826’s centre, the white dwarf is driving a fast “wind” of gas into older gas material, forming a hot interior bubble that pushes the older gas ahead of it to form a bright rim. The faint green of the eye is believed to be gas that made up almost half of the star’s mass for most of its life.

The red blobs at the edges are called FLIERs, or Fast Low-Ionisation Emission Regions. They’re thought to be dense regions of gas either flung off by the star, or floating in space and caught up in the outflowing rush of the stellar wind.

Stellar evolution theory predicts that our Sun will experience a similar fate to NGC 6826 in about five billion years (out of an estimated overall lifespan of some ten billion years).

And by the way, what does the NGC in its name stand for? The New General Catalogue is a huge list of more than 7,800 “deep space” objects compiled in 1880s by the Danish-Irish astronomer J.L.E. Dreyer.

Downloadable wallpaper image: 1280 x 1280

Adapted from information issued by Bruce Balick (University of Washington), Jason Alexander (University of Washington), Arsen Hajian (U.S. Naval Observatory), Yervant Terzian (Cornell University), Mario Perinotto (University of Florence, Italy), Patrizio Patriarchi (Arcetri Observatory, Italy) and NASA/ESA.

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NASA’s new target – asteroids!

NASA HAS ANNOUNCED its intention to pursue manned missions to an asteroid and possibly one of the moons of Mars.

Before it can do so, the space agency needs a new generation of manned spacecraft that can accomplish long-duration missions much further away from the Earth than the Moon is.

This video comes from Lockheed-Martin, manufacturers of Orion, slated to be the USA’s next government-owned manned spacecraft. Orion will be capable of carrying out the asteroid and Martian moon missions.

Adapted from information issued by Lockheed-Martin.

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Space umbrella to protect new telescope

LIKE A BEACH UMBRELLA protects people from the Sun’s heat and ultraviolet radiation, the James Webb Space Telescope’s sunshield will protect the telescope and the sensitive infrared instruments from the Sun’s heat and light.

“Each of the five layers of the shield is less than half the thickness of a piece of paper. The five work together to create an effective SPF (or Sun Protection Factor) of 1,000,000,” said John Durning, Deputy Project Manager for the James Webb Space Telescope Project, at NASA’s Goddard Space Flight Centre.

The large sunshield is 20 metres by 12 metres and made of a material called Kapton that can be folded like a blanket. Kapton is a film developed by DuPont which can remain stable and strong over the wide range of temperatures, from 36K to 650 Kelvin (K) (-237 to 377°C), the sunshield will experience during its launch and deployment.

Once on orbit, the sunshield creates a 330 K (117°C to -212°C) temperature differential between the hottest and coldest layers. Using multiple separated layers allows most of a layer’s heat to radiate to space before it reaches the next one, forming a substantial temperature drop from one layer to the next.

Artist's concept of JWST

Artist's concept of the James Webb Space Telescope, showing the five-layer sunshield.

The James Webb Space Telescope will observe primarily the infrared light from faint and very distant objects. But all objects, including telescopes, also emit infrared light in the form of heat energy. To avoid swamping the very faint astronomical signals with radiation from the telescope and the telescope from seeing its own thermal signature, the telescope and its instruments must be very cold, at an operating temperature of under -223°C.

The observatory will be pointed so that the Sun, Earth and Moon are always on one side, and the sunshield will act like a beach umbrella, keeping the Optical Telescope Element and the Integrated Science Instrument Module on the telescope’s topside cool by keeping them in the shade and protecting them from the heat of the Sun and warm spacecraft electronics.

The Webb telescope will orbit 1,513,000 km from Earth at the L2 Lagrange point and will be the first deployable optical telescope in space. It will undergo a complex post-launch sequence of deployments including the sunshield, before it becomes fully operational.

For more information on the sunshield, visit http://www.jwst.nasa.gov/sunshield.html

For more information on the James Webb Space Telescope, visit http://jwst.nasa.gov/

Adapted from information issued by Rob Gutro, NASA’s Goddard Space Flight Centre.

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Really cool stars

Artist’s impression of CFBDSIR 1458+10

Artist’s impression of the pair of brown dwarfs named CFBDSIR 1458+10. Observations suggest this is the coolest pair of brown dwarfs found so far. The colder of the two (background) could have a temperature similar to that of a cup of freshly made tea.

  • Brown dwarfs are halfway between big planets and small stars
  • They don’t shine, but give off only a small amount of heat
  • Newly found brown dwarf seems to be the coldest yet discovered

ASTRONOMERS HAVE FOUND a new candidate for the coldest known star—a ‘brown dwarf’ in a binary star system that has about the same temperature as a freshly made cup of tea.

That’s hot in human terms, but extraordinarily cold for the surface of a star.

This object is cool enough to begin crossing the blurred line dividing small, cold stars from big, hot planets.

Brown dwarfs are essentially failed stars—they don’t have enough mass for gravity to trigger the nuclear reactions that make stars shine.

The newly discovered brown dwarf, identified as CFBDSIR 1458+10B, is the dimmer member of a binary brown dwarf system located just 75 light-years from Earth.

The powerful X-shooter spectrograph on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) was used to show that the object was very cool by brown dwarf standards.

“We were very excited to see that this object had such a low temperature, but we couldn’t have guessed that it would turn out to be a double system and have an even more interesting, even colder [star],” said Philippe Delorme of the Institut de planétologie et d’astrophysique de Grenoble (CNRS/Université Joseph Fourier), a co-author of the paper.

Keeping its cool

CFBDSIR 1458+10 is the name of the binary system. The individual stars are known as CFBDSIR 1458+10A and CFBDSIR 1458+10B, with the latter the fainter and cooler of the two. They seem to be orbiting each other at a separation of about three times the distance between the Earth and the Sun with a period of about thirty years.

Brown dwarf binary CFBDSIR 1458+10

Actual image of the brown dwarf binary CFBDSIR 1458+10, obtained using the Laser Guide Star (LGS) Adaptive Optics system on the Keck II Telescope in Hawaii. Adaptive optics cancels out much of Earth’s atmospheric interference, improving the image sharpness by a factor of 10.

The dimmer of the two dwarfs has now been found to have a temperature of about 100 degrees Celsius — the boiling point of water, and not much different from the temperature inside a sauna.

By comparison the temperature of the surface of the Sun is about 5,500 degrees Celsius.

“At such temperatures we expect the brown dwarf to have properties that are different from previously known brown dwarfs and much closer to those of giant exoplanets—it could even have water clouds in its atmosphere,” said Michael Liu of the University of Hawaii’s Institute for Astronomy, who is lead author of the paper describing this new work.

“In fact, once we start taking images of gas-giant planets around Sun-like stars in the near future, I expect that many of them will look like CFBDSIR 1458+10B.”

Three telescopes needed

Unravelling the secrets of this unique object involved exploiting the power of three different telescopes. CFBDSIR 1458+10 was first found to be a binary using the Laser Guide Star (LGS) Adaptive Optics system on the Keck II Telescope in Hawaii.

Liu and his colleagues then employed the Canada–France–Hawaii Telescope, also in Hawaii, to determine the distance to the brown dwarf duo using an infrared camera. Finally the ESO VLT was used to study the object’s infrared spectrum and measure its temperature.

The hunt for cool objects is a very active astronomical hot topic. The Spitzer Space Telescope has recently identified two other very faint objects as other possible contenders for the coolest known brown dwarfs, although their temperatures have not been measured so precisely.

Future observations will better determine how these objects compare to CFBDSIR 1458+10B.

Liu and his colleagues are planning to observe CFBDSIR 1458+10B again to better determine its properties and to begin mapping the binary’s orbit, which, after about a decade of monitoring, should allow astronomers to determine the binary’s mass.

Adapted from information issued by ESO / Michael Liu (University of Hawaii) / L. Calçada.

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Diamond stars in a sea of colour

R136

R136 is a group of young, hot, massive stars in the 30 Doradus Nebula in the Large Magellanic Cloud, a galaxy close to our Milky Way.

THIS MASSIVE, YOUNG STELLAR grouping, called R136, is only a few million years old and resides in the 30 Doradus Nebula, a turbulent star-birth region in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way.

Many of the diamond-like icy blue stars are among the most massive stars known. Several of them are over 100 times more massive than our Sun. These hefty stars are destined to pop off, like a string of firecrackers, as supernovae in a few million years.

The image, made from exposures in ultraviolet, visible, and red light by Hubble’s Wide Field Camera 3, spans about 100 light-years.

Despite being in another galaxy, the nebula is close enough to Earth that Hubble can resolve individual stars, giving astronomers important information about the stars’ birth and evolution. There is no known star-forming region in our galaxy as large or as prolific as 30 Doradus.

The brilliant stars are carving deep cavities in the surrounding material by unleashing a torrent of ultraviolet light, and hurricane-force stellar winds (streams of charged particles), which are etching away the enveloping hydrogen gas cloud in which the stars were born.

The image reveals a fantasy landscape of pillars, ridges, and valleys, as well as a dark region in the centre. The brilliant stars can also help create a successive generation of offspring—when the winds hit dense walls of gas, they create shockwaves, which compress the gas and potentially triggers a new wave of star birth.

The cluster is a rare example of the many super star clusters that formed in the distant, early universe, when star birth and galaxy interactions were more frequent. Previous Hubble observations have shown astronomers that super star clusters in faraway galaxies are common.

The LMC is located 170,000 light-years away and is a member of the Local Group of Galaxies, which also includes the Milky Way.

The Hubble observations were taken October 20-27, 2009. The blue colour is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen.

Full-size image suitable for screen wallpaper (1280 x 1280 pixels)

Adapted from information issued by NASA, ESA, and F. Paresce (INAF-IASF, Bologna, Italy), R. O’Connell (University of Virginia, Charlottesville), and the Wide Field Camera 3 Science Oversight Committee.

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Toddler stars tear up the nursery

NGC 6729

Baby stars (hidden behind thick clouds of dust) are ejecting gas at speeds as high as one million kilometres per hour, tearing up the clouds within which they were born.

THE DRAMATIC EFFECT newborn stars have on the gas and dust from which they formed is shown in a new image from the European Southern Observatory’s (ESO) Very Large Telescope

Although the stars themselves are not visible, material they have ejected is colliding with the surrounding gas and dust clouds and creating a surreal landscape of glowing arcs, blobs and streaks.

The star-forming region NGC 6729 is part of one of the closest stellar nurseries to the Earth and hence one of the best studied.

Stars form deep within thick gas clouds, which means the earliest stages of their development cannot be seen with visible-light telescopes because of obscuration by dust.

In this image, there are very young stars hidden behind the gas and dust at the upper left of the picture. Although they can’t be seen, the havoc that they have wreaked on their surroundings is clearly visible.

High-speed jets of gas shooting out from the baby stars at velocities as high as one million kilometres per hour are slamming into the surrounding gas and creating shock waves. These shocks cause the gas to shine and form the strangely coloured glowing arcs and blobs known as Herbig–Haro objects.

This enhanced-colour picture was created from images taken using the FORS1 instrument on ESO’s Very Large Telescope. Images were taken through two different filters that isolate the light coming from glowing hydrogen (shown as orange) and glowing ionised sulphur (shown as blue).

Adapted from information issued by ESO.

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Cosmic tails betray a close encounter

SMC and LMC

The Small (left) and Large (right) Magellanic Cloud galaxies orbit together around our Milky Way galaxy. A large stream of gas, not visible in this image, stretches between the two galaxies, the result of a close encounter around a billion years ago.

OUR NEAREST GALACTIC NEIGHBOURS became entangled in a cosmic dance over the past few billion years, with a dramatic close encounter around 1.2 billion years ago, say astronomers.

International Centre for Radio Astronomy Research (ICRAR) astronomers Jonathan Diaz and Dr Kenji Bekki have used computer modelling to study the movement of the Large and Small Magellanic Clouds around the Milky Way and the structure of the gas that surrounds them.

The Large Magellanic Cloud and the Small Magellanic Cloud are the two closest reasonable size galaxies to our own Milky Way. Southern Hemisphere stargazers can easily see them in the night sky from dark locations.

“An enormous stream of hydrogen gas trails behind the Magellanic Clouds as they orbit the Milky Way,” says ICRAR student Jonathan Diaz. ICRAR is a joint venture between Curtin University and The University of Western Australia, located in Perth.

Animation of the Magellanic Stream

Simulation of the orbits of the Large and Small Magellanic Cloud galaxies (red and green lines) around the Milky Way. A close approach around a billion years ago was responsible for forming a huge cloud of gas around the galaxies.

“Previous explanations for the oversized tail had it being stripped away from the Magellanic Clouds during a close approach of the Milky Way around 2 billion years ago.”

However, recent observations made by the Hubble Space Telescope have cast doubt on whether that close approach actually occurred. The new data from Hubble shows that the Magellanic Clouds are moving differently than originally thought.

“We have found a solution to the question raised by the Hubble data,” explains Diaz. “We’ve shown that its possible for the gas stream to form through a violent interaction between the two small galaxies around 1.2 billion years ago, without the need for a strong interaction with the much larger Milky Way.”

“Past models have assumed that the Magellanic Clouds have been cosmic companions since birth, but our work demonstrates a recent and quite dramatic coupling between the Clouds.”

“Our model shows the Magellanic Clouds have been drifting around the Milky Way for many billions of years, but have only just recently found each other,” says Dr Kenji Bekki, supervisor of the project.

“Were going to conduct further simulations and refine our model but this result shows us we still have more to learn about our galaxy and its neighbourhood.”

The research will be published in the Monthly Notices of the Royal Astronomical Society.

Adapted from information issued by ICRAR. Images courtesy Jonathan Diaz (ICRAR) / Eckhard-Slawik / Serge Brunier.

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Countdown to Mercury

Artist's impression of MESSENGER spacecraft in orbit at Mercury

Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury.

  • Mercury is the smallest, hottest and densest planet in the Solar System
  • MESSENGER spacecraft has travelled 7.6 billion kilometres to reach it
  • Will study the planet’s surface, thin atmosphere and geologic history

NASA’S MESSENGER SPACECRAFT is scheduled to slide into orbit around the closest planet to the Sun on March 18 (Sydney time zone). The mission is an effort to study Mercury’s geologic history, magnetic field, surface composition and other mysteries.

The findings are expected to broaden our understanding of rocky planets, more and more of which are being discovered in other Solar Systems.

At 11:45am on March 18, Sydney time (8:45pm US EDT on March 17) the MESSENGER spacecraft will execute a 15-minute braking manoeuvre that will place it into orbit around Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet.

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

MESSENGER image of Mercury

It might look like the Moon, but Mercury is very different. It is the smallest, hottest and densest planet in the Solar System.

Mercury is an extreme among the rocky planets in our Solar System—it is the smallest, the densest (after correcting for self-compression) and the one with the oldest surface and largest daily variations in surface temperature and the least explored.

Understanding this “end member” among the terrestrial planets is crucial to developing a better understanding of how the planets in our Solar System formed and evolved.

“Now that so many new planets are being discovered around stars in other Solar Systems, we need to know the effects of space weathering on rocky surfaces so we can accurately interpret telescopic and other remote sensing data we obtain from other rocky or dusty worlds,” says Ann Sprague, a research scientist at the University of Arizona’s Lunar and Planetary Laboratory.

The heat is on

When MESSENGER streaked into the early morning sky over Cape Canaveral on August 3, 2004, very little was known about Mercury. No spacecraft had approached the planet since the Mariner 10 space probe performed three fly-by manoeuvres over the course of 1974 and 1975, imaging the planet’s surface. However, Mariner 10 sent back photos of only one side of the planet, leaving the other shrouded in mystery.

One of the mysteries scientists are hoping to solve with the MESSENGER mission surrounds Mercury’s magnetic field. At a diameter only slightly larger than that of the moon (about 4,800 kilometres), Mercury should have solidified to the core. However, the presence of a magnetic field suggests the planet’s innards are partially molten.

Artist's impression of a rupes (cliff) on Mercury

Artist's impression of a rupes (cliff) on Mercury. These giant cliffs are believed to have formed when Mercury¹s interior cooled and the entire planet shrank slightly as a result.

During its long journey toward Mercury, MESSENGER passed the planet several times, filling in the imaging gaps left by Mariner 10.

Now, the entire planet with the exception of about five percent has been observed. MESSENGER will focus its cameras on getting the best possible images of the remaining portions, mostly in the polar regions.

One of the great challenges MESSENGER will face is the intense heat due to Mercury’s proximity to the Sun. At the planet’s equator, surface temperatures become hot enough to melt lead. The heat reflected from the planet’s surface is so intense that the spacecraft’s instruments need to be shielded against the glare.

Follow the live webcast of MESSENGER’s arrival from 10:55am Sydney time on Friday, March 18 (7:55pm US EDT on March 17): MESSENGER arrival webcast

Adapted from information issued by the University of Arizona. Images courtesy NASA / JHU APL / CIW. Rupes artwork: Michael Carroll/Alien Volcanoes by Lopes and Carroll, The Johns Hopkins University Press, 2008.

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