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Thursday, September 23, 2010

Engulfing companion star leads to stellar youthful look

Astronomers can usually distinguish young from old stars in a number of ways. Joel Kastner of the Rochester Institute of Technology (RIT) led a team of cosmologists from RIT, UCLA, Berkelely, Observatoire Astronomique de Strasbourg and Laboratoire d’Astrophysique de Grenoble in uncovering a case of misleading stellar advertising.

The star in question, BP Piscium (BP Psc), has at least a couple of the hallmarks of youth. It is orbited by a disk of gas and dust that could one day coalesce into planets and moons, and it is ejecting powerful jets of material from its poles. Both of these attributes normally belong only to very young stars.

On the other hand, there were a few anomalies that made the astronomers say, ‘hmmm’. For one thing, BP Psc is giving off orders of magnitude fewer X-rays than should be expected. Rather than up to a few thousand per day, the Chandra X-ray Observatory only counted 18. That’s normal for an older star, not a young one. Among the other oddities were the facts that BP Psc is not located among other young stars, and that there isn’t enough lithium on its surface.

After much observation, the astronomers have a working hypothesis to explain BP Psc’s youthful appearance. They propose that BP Psc is actually a billion-year old star in its ‘red giant stage’, the stage at which stars of a certain size run out of fuel and expand tremendously. The sun is expected to undergo this expansion in about 5 billion years, at which point it will extend up to the current orbit of the Earth. In the course of expanding, BP Psc engulfed any planets it might have had, plus at least one companion star. That swallowed star resulted in both the disk of material surrounding BP Psc and the several light year long jets spouting from its poles.

One interesting possible result would be the formation of young planets around an old star. It had been thought that planet formation could only occur during the early stages of stellar evolution, when the disk of gas and dust surrounding the baby star starts to coalesce. However, Grandpa BP Psc also has such an orbiting disk and could potentially have a second family of planets to replace any that were engulfed.

Left: The composite image shows X-ray and optical data for BP Piscium (BP Psc). Chandra X-ray Observatory data are colored in purple, and optical data from the 3-meter Shane telescope at Lick Observatory are shown in orange, green and blue.

Right: An artists’s impression of a close-up view. For clarity a narrow jet is shown, but the actual jet is probably much wider, extending across the inner regions of the disk.

Credit: X-ray: NASA/CXC/RIT/J. Kastner et al. Optical: UCO/Lick/STScI/M. Perrin et al. Illustration: CXC/M. Weiss

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