CXC: BP Psc: Chandra Finds Evidence for Stellar Cannibalism

Find out the latest thinking about our universe.
Post Reply
User avatar
bystander
Apathetic Retiree
Posts: 21571
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

CXC: BP Psc: Chandra Finds Evidence for Stellar Cannibalism

Post by bystander » Tue Sep 14, 2010 9:38 pm

BP Psc: Chandra Finds Evidence for Stellar Cannibalism
Chandra X-ray Observatory | 14 Sept 2010
The composite image on the left shows X-ray and optical data for BP Piscium (BP Psc), a more evolved version of our Sun about 1,000 light years from Earth. 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.

BP Psc is surrounded by a dusty and gaseous disk and has a pair of jets several light years long blasting out of the system. A close-up view is shown by the artist's impression on the right. For clarity a narrow jet is shown, but the actual jet is probably much wider, extending across the inner regions of the disk. Because of the dusty disk, the star's surface is obscured in optical and near-infrared light. Therefore, the Chandra observation is the first detection of this star in any wavelength.

The disk and the jets, seen distinctly in the optical data, provide evidence for a recent and catastrophic interaction in which BP Psc consumed a nearby star or giant planet. This happened when BP Psc ran out of nuclear fuel and expanded into its "red giant" phase.

Jets and a disk are often characteristics of very young stars, so astronomers thought BP Psc might be one as well. However, the new Chandra results argue against this interpretation, because the X-ray source is fainter than expected for a young star. Another argument previously used against the possible youth of BP Psc was that it is not located near any star-forming cloud and there are no other known young stars in its immediate vicinity. The Chandra image supports this absence of a cluster of young stars, since multiwavelength studies show that most of the X-ray sources in the composite image are likely to be rapidly growing supermassive black holes in the centers of distant galaxies.

Credit: X-ray (NASA/CXC/RIT/J.Kastner et al), Optical (UCO/Lick/STScI/M.Perrin et al); Illustration: NASA/CXC/M.Weiss
Chandra Finds Evidence for Stellar Cannibalism
Chandra X-ray Observatory | 14 Sept 2010
Evidence that a star has recently engulfed a companion star or a giant planet has been found using NASA's Chandra X-ray Observatory. The likely existence of such a "cannibal" star provides new insight into how stars and the planets around them may interact as they age.

The star in question, known as BP Piscium (BP Psc), appears to be a more evolved version of our Sun, but with a dusty and gaseous disk surrounding it. A pair of jets several light years long blasting out of the system in opposite directions has also been seen in optical data. While the disk and jets are characteristics of a very young star, several clues -- including the new results from Chandra -- suggest that BP Psc is not what it originally appeared to be.

Instead, astronomers have suggested that BP Psc is an old star in its so-called red giant phase. And, rather than being hallmarks of its youth, the disk and jets are, in fact, remnants of a recent and catastrophic interaction whereby a nearby star or giant planet was consumed by BP Psc.
...
Several pieces of information have led astronomers to rethink how old BP Psc might be. First, BP Psc is not located near any star-forming cloud, and there are no other known young stars in its immediate vicinity. Secondly, in common with most elderly stars, its atmosphere contains only a small amount of lithium. Thirdly, its surface gravity appears to be too weak for a young star and instead matches up with one of an old red giant.

Chandra adds to this story. Young, low-mass stars are brighter than most other stars in X-rays, and so X-ray observations can be used as a sign of how old a star may be. Chandra does detect X-rays from BP Psc, but at a rate that is too low to be from a young star. Instead, the X-ray emission rate measured for BP Psc is consistent with that of rapidly rotating giant stars.

The spectrum of the X-ray emission -- that is how the amount of X-rays changes with wavelength -- is consistent with flares occurring on the surface of the star, or with interactions between the star and the disk surrounding it. The magnetic activity of the star itself might be generated by a dynamo caused by its rapid rotation. This rapid rotation can be caused by the engulfment process.
Enigmatic Star Devours Companion; Possibly Pregnant with Second-generation Planets
Rochester Institute of Technology | 14 Sept 2010
RIT astronomer wraps up 15-year study of star in Pisces constellation

An astronomer may have caught a cannibalistic star in the act of devouring a companion and making a second generation of exoplanets from the resulting orbiting disk.

Using data from NASA’s Chandra X-ray Observatory, Joel Kastner, professor at Rochester Institute of Technology, has found evidence that a variable star in the constellation of Pisces, BP Piscium, is not the young star it appears to be, but is more likely a one billion-year-old red giant that has gobbled up a star or planet in its vicinity.

The star’s extreme properties have puzzled astronomers since Kastner and Ben Zuckerman, professor at the University of California, Los Angeles, first looked at BP Psc 15 years ago. Conflicting characteristics have defied the star’s classification as either young or old.

Kastner attributes the star’s potentially deceptive youthful appearance to two things: an orbiting disk that resembles the sort that forms planets around young stars and prominent jets extending from the poles of the star that eject material at high velocity. A typical young star accretes material from its orbiting disk as debris from the disk falls onto the star. In turn, the star incorporates about 90 percent of the material and recycles the remainder through its jets.

Crucial details left Kastner and his colleagues doubtful about the youth of the star, however. For one thing, the star is isolated, whereas most young stars form in clusters.
...
Second, this enigmatic star in the Pisces constellation lacks the large abundance of lithium on its surface that is typical of young stars. Older stars lose their lithium in nuclear reactions when mixing and churning folds the gases into the center of the star. According to Kastner, other key spectral features involving the star’s radius and surface gravity also point to the star’s advanced age.
...
If BP Psc were a young star, it would emit X-rays in the hundreds, even up to a few thousand, in a day’s observing time with Chandra, Kastner notes. Instead, it is a weak X-ray source.
...
The rate of X-rays coming from the star are in keeping with a class of rapidly rotating old stars having similar temperature to BP Psc, Kastner says. This class is thought to be the result of one star swallowing another close companion star. These giant stars’ rates of X-ray production and their rapid spinning suggest they have engaged in such stellar cannibalism.
...
The enigmatic star is likely about a billion years old and just entering the red giant stage in its life cycle in which it swells to digest its star or planet companion.
Chandra X-ray Detection of the Enigmatic Field Star BP Psc - JH Kastner et al

Post Reply