"The rings surrounding the giant planets in our own solar system are held in place by the gravitational force of moons orbiting nearby," explains Brad Smith of the University of Hawaii. "The narrow width of the HR 4796A ring implies that it also is held in place by unseen bodies, most probably planets or protoplanets."
Smith, Glenn Schneider (University of Arizona) and their colleagues reported the discovery today at the 193rd Meeting of the American Astronomical Society in Austin, Texas.
HR 4796A is about 70% larger than the Sun and probably less than 10 million years old. "Considering our own solar system to be middle aged, HR 4796A would be a mere infant," says Smith. "It is especially interesting this stellar ring is so young, indicating planetary bodies have already formed in less than 10 million years."
Unlike the extensive disks of dust seen around other young stars, the HR 4796A dust ring, 6.5 billion miles from the star, is tightly confined within a relatively narrow zone less than 17 Astronomical Units wide. (An Astronomical Unit is the distance from the Earth to the Sun.) For comparison, the ring width is approximately equal to the distance separating the orbits of Mars and Uranus in our own solar system.
All dust rings, whether around stars or planets, can only stay intact by some mechanism confining the dust. Otherwise, disruptive forces, such as those produced by particle collisions, light pressure, etc., would cause the ring to spread both inward and outward until it finally lost its identity.
The star HR 4796A first drew attention in 1991 when Michael Jura (UCLA) reported an unusual amount of dust surrounding the star, based on observations made by IRAS (Infrared Astronomical Satellite) in 1984. Last year, David Koerner (Univ. Pennsylvania) and others announced ground-based thermal infrared data indicating that the dust around HR 4796A is in the form of a disk, with relatively clear inner region that lies within 6 billion miles from the star. "However, the thermal infrared data lacked the resolution to precisely define the geometry of the disk," says Smith.
Just visible to the naked eye, HR 4796A is located 220 light-years away in the southern constellation of Centaurus. The ring is about a thousand times fainter than the star. "The angular extent of the HR 4796A disk is equal to that of a dime seen at a distance of more than 4 miles," says Schneider. To image it HST scientists had to use a coronagraphic camera on Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS). A coronagraph blocks out the glare of a star's light (just as the Moon blocks the Sun's light during a solar eclipse) so that much fainter surrounding material can be seen.
"The ring's color is reddish-grey," says Schneider, "which shows that it is not made up of interstellar dust particles." Although the origin of the dust remains unknown, a good possibility is that it is composed of debris from collisions between planetesimals (comet-like bodies) that are also confined within the ring.
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The near-infrared light reflecting off the dust ring is about 1,000 times fainter than the illuminating central star.
Astronomers used a coronagraphic camera on Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), specifically designed to enable observations of very faint and low surface brightness objects in the close proximity to bright stars. Even with the coronagraph, the glare from HR 4796A overwhelms the much-fainter ring at distances less than about 4 billion miles (inside the blacked-out circle, centered on the star).
Hubble's crisp view was able to resolve the ring, seen at lower resolution at longer wavelengths, in ground-based thermal infrared images, as a disk with some degree of central clearing. The ring has an angular radius of 1.05 arc seconds, equivalent to the apparent size of a dime seen more than 4 miles away.
Unlike the extensive disks of dust seen around other young stars, the HR 4796A dust ring, 6.5 billion miles from the star, is tightly confined within a relatively narrow zone less than 17 Astronomical Units wide. An Astronomical Unit is the distance from the Earth to the Sun). For comparison, the ring width is approximately equal to the distance separating the orbits of Mars and Uranus in our own Solar System. All dust rings, whether around stars or planets, can only stay intact by some mechanism confining the dust, likely the gravitational tug of unseen planets.
The image was taken on March 15, 1998, centered at a near infrared wavelength of 1.1 microns. The false-color corresponds to the ring's brightness (yellow is bright, purple is faint). The ring, which is undoubtedly circular, appears elliptical since it is inclined to our line-of-sight. Thus, the apparent spacing of the ring-particles, which act as reflectors of starlight, is greatest at the ansae of the projected ellipse giving rise to the brightening in these regions.
This image is being released today at the 193rd Meeting of the American Astronomical Society in Austin, Texas.
Credit: Brad Smith (University of Hawaii), Glenn Schneider (University of Arizona), and NASA.