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. Exploding Star Within A Star Surprises Astronomers

White-dwarf star RS Ophiuchi explodes within the atmosphere of its red-giant companion in this artist's impression. Image credit: David A. Hardy/PPARC.
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  • by Phil Berardelli
    SpaceDaily US Editor
    Washington DC (SPX) Apr 10, 2006
    Last Feb. 12, amateur astronomers around the world began reporting something that teams of their professional counterparts had been eagerly awaiting: A faint star in the constellation Ophiuchus suddenly became clearly visible to the naked eye in the night sky. With that news, an international team trained dozens of instruments both ground-based and orbiting on RS Ophiuchi, a star known to have exhibited five such brightenings over the past 108 years.

    "We were able to get observations with telescopes around the world and in space within just a few days of the explosion," astrophysicist Tim O'Brien, with the Jodrell Bank Observatory in Cheshire, England, told SpaceDaily.com.

    "This means we have seen the very earliest phases of its evolution as it brightened in X-rays and radio waves, allowing us to probe conditions close to the central binary system," O'Brien said. "Our frequent follow-up observations have also led to the discovery of an entirely unexpected component in the X-ray emission, (which is) extremely bright and rapidly varying."

    RS Oph, as astronomers call it, is indeed a binary system comprising a white-dwarf star - the super-dense core of a former star but reduced to Earth's size after expelling its outer layers - and a much, much larger red giant. Located about 5,000 light-years away, the two stars orbit so closely together that the white dwarf's intense gravity continually strips away hydrogen-rich gas from the red-giant companion.

    Eventually - about every couple of decades - the dwarf accumulates enough gas to trigger a tremendous thermonuclear explosion, which causes the brightening. That explosion temporarily increases RS Oph's energy output to about 100,000 times the Sun's, ejecting gas into space at speeds of several thousand kilometers per second - or nearly 2 percent of the speed of light.

    Astronomers already knew this much about the binary RS Oph from previous observations. From X-ray and radio data collected during the last outburst in 1985, they also knew the star was likely to become extremely bright in these wavelengths, and they suspected this might be due to the speeding expelled material colliding with the pre-existing stellar wind.

    The problem, 21 years ago, was the observations did not begin quickly enough to examine important early stages of the explosion, so astronomers could not advance their understanding beyond basic suppositions.

    Based on the readings so far, it seems the white dwarf has a rather voracious appetite. It strips off enormous amounts of gas from the red giant, creating as gas cloud that envelops both stars.

    As a result, when the white dwarf periodically explodes, the event actually occurs inside its companion's extended atmosphere. Eventually, the ejected gas slams into that atmosphere at very high speed, producing the secondary brightening and the X-ray and radio emissions.

    "We realized from the few X-ray measurements taken late in the 1985 outburst that this was an important part of the spectrum in which to observe RS Oph," said astrophysicist Michael Bode of Liverpool John Moores University in Birkenhead, England. Bode led the observing team during the 1985 outburst, and he now heads a group using NASA's Swift spacecraft that is observing the explosion.

    "The expectation was that shocks would be set up both in the ejected material and in the red giant's wind, with temperatures initially of up to around 100 million degrees Celsius - nearly 10 times that in the core of the Sun," said Bode, who presenting the current findings on the event at last week's Royal Astronomical Society meeting. "We have not been disappointed," he added.

    Swift's first observation, taken only three days after RS Oph's latest outburst began, revealed a very bright X-ray source. Over the initial few weeks, it became even brighter and then began to fade. Spectral readings suggested the gas was cooling down - although it remained tens of millions of degrees hot.

    "About a month after the outburst, the X-ray brightness of RS Oph increased very dramatically," said Julian Osborne of the University of Leicester. "This was presumably because the hot white dwarf, which is still burning nuclear fuel, then became visible through the red giant's wind."

    Osborne said the new X-ray emissions also were extremely variable, to the point where they pulsed about every 35 seconds. Although the data are preliminary, he said, "one possibility for the variability is that this is due to instability in the nuclear burning rate on the white dwarf."

    Another team - using the U.K. MERLIN radiotelescope array and beginning their observations only four days after RS Oph exploded - recorded radio emissions that were "much brighter than expected," said Stewart Eyres of the University of Central Lancashire. "Since then it has brightened, faded and then brightened again. With radio telescopes in Europe, North America and Asia now monitoring the event very closely, this is our best chance yet of understanding what is truly going on."

    Teams studying the event in optical and infrared wavelengths have reported similar surprises. "The observations we have already obtained from the ground, from the U.K. Infrared Telescope on the summit of Mauna Kea in Hawai'i, far surpass the data we had during the 1985 eruption," said Nye Evans of Keele University in England, who heads the infrared follow-up team.

    The shocked red giant wind and the material ejected in the explosion give rise to emission … in the infrared, via coronal lines," Evans said. "These will be crucial in determining the abundances of the elements in the material ejected in the explosion and in confirming the temperature of the hot gas."

    One other critical element in the studies involves classifying the RS Oph explosion. It is possible, astronomers said, that it might be a precursor to a supernova.

    "RS Oph is a member of a class of binary star system that has been proposed as a progenitor of a Type Ia supernova," astrophysicist Sumner Starrfield of Arizona State University in Tempe, told SpaceDaily.com.

    "These are the supernovae where it is thought that the explosion occurs because a white dwarf grows too large in mass and the interior pressure can no longer provide enough support for the star," Starrfield explained. "They are the same type of supernova used to study the evolution of the universe."

    Such a conclusion remains speculative, he added. "We do not know if RS Oph will become a supernova, and we don't know the systems that actually become a Type Ia supernova. So, anything we learn about RS Oph can confirm or deny the theories."

    Starrfield also said astronomers have been very surprised to see how bright the event has been in the X-ray spectrum, and how long it has remained bright. "It has been bright for nearly two months and previously we thought it would have started to decline after a month or even less.

    This indicates to me that the mass of the white dwarf is probably less than we had believed, and that the mass may not be growing. However, we are still in the middle of the outburst and with any transient object of this kind we expect to be surprised again."

    Among those surprises, he said, could be the composition of the material ejected in the explosion - although data collected via the Chandra spacecraft has helped to determine the nature of the material remaining on the white dwarf. Therefore it might be possible to determine the composition of the ejected gases as they interact with the red giant atmosphere.

    "In terms of stellar evolution, our current observations are enabling us to explore the evolution of an analogue to a supernova remnant, but evolving over months, rather than millennia," Bode told SpaceDaily.com.

    "As an added bonus," he said, "we will be mapping out the distribution of material in the wind of the red giant star as the forward shock set up by the ejecta is traversing it. Before too long, this shock should in fact run off the end of the wind, which was re-established between the 1985 outburst and Feb. 12 this year, and we are all set to look for evidence of this ‘shock break out' over the coming weeks."

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