A new study has caught a newborn star similar to the sun in a fiery outburst. X-ray observations of the flare-up, which are the first of their kind, are providing important new information about the early evolution of the sun and the process of planet formation. The study, which was conducted by a team of astronomers headed by Joel Kastner of the Rochester Institute of Technology that included David Weintraub from Vanderbilt, is reported in the July 22 issue of the journal Nature.
Last January, Jay McNeil, an amateur astronomer in west Kentucky, discovered a new cloud of dust and gas in the Orion region. Previously, the cloud, now named the McNeil nebula, was not visible from earth. But a new star inside the dark cloud had flared up in brightness, lighting up the surrounding nebula. Looking back at the images taken of this part of the sky revealed that a young star about the size of the sun had burst into visibility last November.
Despite the fact that hundreds of telescopes scan the sky nightly, the discovery of a new star is an extremely rare event, having occurred only twice in the last century. What made this star even more special was that the fact that it appears to be an extremely young star – far less than a million years old – that is about the same mass as the sun. Astronomers know of fewer than a dozen of these stars, which they call FU-Orionis-type. Although this is the third FU-Orionis that has been caught in the act of flaring, it is the first that has occurred in modern times when its behavior could be monitored not only in visible light, but also in radio, infrared and X-ray wavelengths.
"In FU-Orionis stars, these outbursts are very brief," says Weintraub, associate professor of astronomy. "They brighten by as much as 100 thousand times in a few months and then fade away over a number of months."
Knowing that time was short, Kastner and Weintraub submitted an emergency request for viewing time on the orbiting Chandra X-ray observatory. Because X-rays are generated by extremely violent events, they provide a critical window for observing extreme stellar flare-ups of this sort. The astronomers were granted two viewing times in early and late March.
Using Chandra, the astronomers discovered that the star, which has been officially named V1647 Ori, was a very bright X-ray source in early March, but its X-ray brightness had decreased substantially by the end of the month before the star disappeared from view behind the sun. (At the same time, the new star was fading in visible and infrared wavelengths.)
In addition, the astronomers learned that Ted Simon from the Institute for Astronomy in Hawaii had taken some serendipitous X-ray images of the same area in 2002 for another purpose. These showed no X-rays coming from the V1647 Ori's location at the time, supporting the idea that the recent X-ray production was directly associated with the star's flare-up.
Kastner and Weintraub propose a novel mechanism to explain their observations. Many stars, including the sun, produce X-rays by a mechanism that depends on the star's rotation rate and convection depth. But the astronomers calculate that the temperature of the gas that is producing the X-rays at V1647 ORI is substantially higher than can be explained by this traditional mechanism.
Observations of V1647 Ori indicate that it possesses a "protoplanetary" disk – a thin disk extending out from a star's equator that contains dust and gas left over from the star's formation and from which planets form. Kastner and Weintraub argue that the flare was touched off by a sudden avalanche of disk material falling onto the surface of the star and that this was the source of the intense X-rays as well as the other forms of radiation.
If their hypothesis is correct, X-ray observations may help discriminate between young stars that possess protoplanetary disks and those that don't, Weintraub says.
There is a disagreement among astronomers about whether FU-Orionis stars undergo outbursts of this sort only once, several times or dozens of times before they settle into maturity. Other astronomers who have looked further back in the astronomical records for V1647 Ori have found that it also flared up in 1965, which provides added support for the multiple outburst theories.
Other participants in the study were Michael Richmond at Rochester Institute of Technology, Nicolas Grosso and H. Ozawa at the Laboratoire d'Astrophysique de Grenoble, A. Frank at the University of Rochester, Kenji Hamaguchi at NASA's Goddard Space Flight Center and Arne Henden at the U.S. Naval Observatory.
Kastner and Weintraub have been awarded time to conduct additional observing time on Chandra so they can measure the X-ray activity of the new star beginning in October when it comes visible once again.
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