January 15, 2000
The surprising findings, presented at the 195th Meeting of the American Astronomical Society, show that a close encounter with a neighboring star can severely disrupt the evolution and appearance of thin disks, which are the nurseries of planetary systems. Similar fly-bys of our solar system long ago may have reshuffled the comets that now populate our Oort cloud and Kuiper belt.
Discovered in 1983, the dust disk around the nearby star Beta Pictoris -- long suspected to harbor a planetary system -- has puzzled astronomers because it contains more dust grains than any other comparable system. Also, the dust spreads over a huge 65-billion-mile-diameter area. Yet, one side of the disk is 20 percent longer and thinner than the other side.
In these latest findings, Hubble astronomers carefully studied the appearance of the disk using 10 years of archival data from the Hubble Space Telescope and from ground-based telescopes in Hawaii and Chile. Hidden within the densest part of the disk are clumps of dust that are present only on the long, thin side of the disk. Because the disk is tilted edge-on to our line-of-sight, the astronomers inferred that the clumps might represent rings if the disk was viewed face-on. They hypothesized that these rings must be highly elliptical if they appear only on one side of the disk, and this could arise if another massive object, like a passing star, recently disturbed the entire system.
To test their ideas the researchers asked theorist John Larwood of Queen Mary and Westfield College (London, United Kingdom) to create a computer simulation of a quiescent disk made of one million test particles orbiting a virtual star. The simulation explored what would happen if another star zipped by it in a near-collision trajectory. In the simulation, the gravity of the passing star rearranged the orbit of each particle, setting up an elliptical ring system 100,000 years after the almost catastrophic event. The model also reproduced the 20 percent asymmetry in the disk, which has mystified astronomers since the Beta Pictoris disk was first seen 16 years ago.
The astronomers are continuing their detective work, searching for the intruder star among 186 suspects near Beta Pictoris. Their simulations predict it might be only a fraction of the mass of our Sun (a class called an M-dwarf star). The present results will be published in a future issue of Astrophysical Journal Letters.
The Hubble research team, led by Paul Kalas (Space Telescope Science Institute, Baltimore, Md.), consists of John Larwood (Queen Mary and Westfield College, London, United Kingdom), Bradford Smith (University of Hawaii, Institute for Astronomy, Honolulu, Hawaii), and Alfred Schultz (Space Telescope Science Institute).
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
PHOTO CAPTION:
Image credit: NASA and Paul Kalas (Space Telescope Science Institute)
[Right]
A still frame from a computer simulation, which shows a circumstellar
dust disk highly perturbed by the gravitational pull of a bypassing
star. The gray solid area represents the initial shape and size of
the undisturbed disk. In the simulation, the gravity of the passing
star rearranges the orbit of each particle, setting up an elliptical
ring system that may have survived for the last 100,000 years since
the impact occurred.
Simulation courtesy: John Larwood (Queen Mary and Westfield College, London, United Kingdom)
Space Telescope Science Institute, Baltimore, MD
January 8, 1998
These conclusions are based on unprecedented detail in Hubble images taken with the Wide Field Planetary Camera 2, which reveal the dim outermost reaches of the disk, which are 7 billion miles from the central star.
Though other Hubble teams have seen a warp in the inner edge of the disk and attributed it to the gravitational tug of unseen planets, the new WFPC2 images show that the warp in the outer edge of the disk is too great to be easily explained by the effects of planets, say researchers.
These results are being presented at the 191st meeting of the American Astronomical Society in Washington, DC, by team leader Al Schultz of Computer Sciences Corporation (CSC) at the Space Telescope Science Institute in Baltimore, MD, and Fred Bruhweiler of The Catholic University of America (CUA) in Washington, DC. (Other team members include Cherie Miskey, Brendan Smith, Jeff Silvis and Michael DiSanti of CUA, Helen Hart of CSC/STScI, Glenn Schneider of Steward Observatory, in Tucson, and Kent Reinhard of Doane College in Nebraska).
According to Bruhweiler, "The distortions we are seeing may have been caused by the passing of a nearby star within the past few 100 million years since the disk was formed. The culprit could easily be a thousand light-years away by now. We probably will never know who did it."
On the other hand, team lead Al Schultz favors the idea that the warp could be caused by a small faint brown dwarf star which may be circling Beta Pictoris at large distances. He suggests a search for faint stellar companions might find such a star.
Schultz and Bruhweiler agree their findings do not necessarily rule out the presence of one or more planets circling Beta Pictoris at closer distances.
Previous Hubble observations, as well as new images using the Space Telescope Imaging Spectrograph aboard Hubble, show warping or bending of the disk close to Beta Pictoris at distances within 750 million miles (80 astronomical units) of the star.
In 1996, Chris Burrows of the Space Telescope Science Institute originally proposed this warping could be due to a massive planet orbiting Beta Pictoris at angles out of the plane of the disk. This was proposed to explain the warp in the disk on opposite sides of the star, like a twist in an airplane propeller.
"The new Hubble pictures show many new indications that this disk may be the outer reaches of a solar system around Beta Pictoris," said Bruhweiler, "It could be what our solar system looked like four billion years ago."
This huge dust disk seems to have an analogue in our own solar system and appears to be similar to the primordial disk dating from the time of the formation of the solar system. The comets of our solar system have their origins in a similar disk or cloud still surrounding the Sun.
Astronomers theorize that when the solar system formed some 4.6 billion years ago, all the gas and dust quickly settled into a flat disk. This disk is thought to have been quite large, some 200,000 times the distance between the Earth and the Sun, and extended a significant distance to the nearest star. Over time, the orbits of dust and comets in the extreme outreaches of this disk, were transformed by the faint tugs of gravity from passing nearby stars. The successive tugs of these passing stars altered the shape of the disk until the outermost comets of this disk formed an almost spherical, or possibly even football shaped, halo around the Sun (called the Oort's Cloud after the Dutch astronomer who hypothesized its existence). The inner region of the solar system's originally flat disk has been perturbed very little by the gravitational tug of nearby stars and still seems to have remained essentially flat. This inner region is called the Kuiper Belt.
"What is so surprising is that it appears that we are looking at the disk of Beta Pictoris at an angle that is almost exactly edge-on. The probability of this happening is very small," said team member Helen Hart. The images show a sharp, bright, straight ridge extending over the entire length of the disk, as well as the increased thickness or "flaring" of the disk close in toward the star.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
A photo and caption are available on the World Wide Web
PHOTO CAPTION:
The "false color" images show gradations in the brightness of the disk, caused simply by the fact that the disk shines by reflected starlight, and so the farther the dust is from the central star, the fainter it is. In both views the bright glare of the central star is blocked by a black strip that divides the disk into left (east) and right (west) components. Because the disk is tilted nearly edge-on to Earth the images show a sharp, bright, straight ridge extending over the entire length of the disk.(in our solar system this feature is seen as zodiacal light, where sunlight is reflected by a concentration of dust in the ecliptic plane).
The orbits of the planets of our solar system are added for scale.
[TOP] - This Wide Field Planetary Camera 2 image shows the full extent of the disk, which spans 140 billion miles (1500 astronomical units) edge-to-edge. An unusual flaring at the top of the right side of the disk (the Southwest side of the disk) shows that dust has been pulled above the dense plane of the disk beyond what is observed in the left side. A gravitational perturbation by an unseen substellar- mass companion farther from the star than planets would be, or a tug from a bypassing star might cause this flaring. The image was taken on June 22, 1995.
Credit: Al Schultz (CSC/STScI, and NASA)
[BOTTOM] - An unprecedented detailed close-up view of the inner region of the disk taken with the Space Telescope Imaging Spectrograph shows a warp in the disk. Though this warp was first seen by Hubble in 1995, the new images go closer to the star than ever before to about 1.4 billion miles (15 astronomical units) -- a radius smaller than that of Uranus' orbit. These new details support the presence of one or more planets orbiting the star. The image was taken in September 1997.
Credit: Sally Heap (GSFC/NASA)
European Southern Observatory
June 11, 1997
This new and very detailed image of the famous circumstellar disk around the southern star Beta Pictoris was obtained with the ESO ADONIS adaptive optics system at the 3.6-m telescope and the Observatoire de Grenoble coronograph. It shows (in false colours) the scattered light at wavelength 1.25 micron (J band) and is one of the best images of this interesting feature obtained so far.
It has a direct bearing on the current search for extra-solar planetary systems, one of the most challenging astronomical activities. While spectroscopic, astrometric and photometric studies may only provide indirect evidence for planets around other stars, coronographic images like this one in principle enable astronomers to detect dusty disks directly. This is very important for our understanding of the physics of planetary formation and evolution.
The disk around Beta Pictoris is probably connected with a planetary system. In particular, various independent observations have led to the conclusion that comets are present around this star, and variability of its intensity has been tentatively attributed to the occultation (partial eclipse) by an orbiting planet.
The new image was obtained by combining coronography (in which the light from the star is greatly weakened by covering its image with a small disk at the focus of the telescope) with the technique of high-angular resolution by means of adaptive optics which nearly eliminate the adverse effect of the turbulence in the terrestrial atmosphere. This results in a very sharp image with a high spatial resolution (0.12 arcsec) and a high dynamical range (105) which allows to follow the disk to a very small distance from the star, in this case only 24 AU (3.6 109 km), i.e. a distance where planets could be present. In the Solar System, this corresponds to a distance from the Sun to about halfway between Uranus and Neptune.
Although no planets are directly seen in this image, their gravitational effects on the dust in the circumstellar disk may be detected. For instance, it is evident that there is a warp (bending of the main plane) in the inner part of the disk, a feature that has earlier been seen on images obtained with the Hubble Space Telescope. However, while the HST picture was obtained in visual light, this image is at a longer wavelength, in near-infrared. Moreover, in view of the extensive image processing procedures involved in the preparation of this image, it is very useful to obtain this corroborating information from two entirely independent instruments.
The warped feature may be explained by the presence of a planet moving along an inclined orbit (3o) within 20 AU (3 109 km) of the star.
The image was obtained by Jean-Luc Beuzit, Anne-Marie Lagrange (Observatoire de Grenoble, France) and David Mouillet (Observatoire de Paris-Meudon, France). In addition, the interpretation and discussion about the implications of planetary perturbations of the disk involved J. Larwood and J. Papaloizou. This study is described extensively in a forthcoming scientific paper in the journal Monthly Notices of the Royal Astronomical Society (UK).
Technical information: The J-band image covers an area of 13.1 x 13.1 arcsec2 at an angular resolution of about 0.12 arcsec. North is up and East is left. It was obtained on Jan 6, 1996, with ADONIS and the Observatoire de Grenoble coronograph, attached to the ESO 3.6-m telescope at La Silla, Chile.
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