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CfA Press Release
 

Release No.: 98-11

Click here for media coverage of this story.

(Note: A coordinated press advisory was released simultaneouslyfrom NASA Headquarters. This advisory represents the interests ofthe National Science Foundation, Harvard Smithsonian Center forAstrophysics, University of Florida, and the National Optical AstronomyObservatories / Cerro Tololo Inter-American Observatory.)

ASTRONOMERS DISCOVER POSSIBLE PLANET-FORMING DISK

Two teams of astronomers searching for signs of other solar systemswith telescopes in Chile and Hawaii have independently discovered adisk around a nearby star that may be forming--or may have alreadyformed--planets.

The disk of dust, about three times the size of Pluto's orbit aroundthe Sun, surrounds a star roughly 220 light-years from Earth, which,according to some theories, has the right age to be forming planetsnow. Like the majority of stars in our galaxy, this object is amember of a binary system, suggesting that the presence of a companionstar does not necessarily disrupt a disk before it has had enough timeto form planets.

The joint discovery was made by one team comprised of RayJayawardhana, Lee Hartmann, and Giovanni Fazio of theHarvard-Smithsonian Center for Astrophysics in Cambridge,Massachusetts, and Scott Fisher, Charles Telesco, and Robert Pina ofthe University of Florida in Gainesville, which used the NationalScience Foundation's 4-meter Blanco Telescope at the Cerro TololoInter-American Observatory in La Serena, Chile; and by a second teamcomprised of David Koerner, Michael Ressler, and Michael Werner ofCalTech/Jet Propulsion Laboratory and Dana Backman of Franklin &Marshall College, which used the 10-meter Keck II Telescope on MaunaKea, Hawaii.

The newly-discovered disk surrounds a star known to astronomers asHR 4796A in the southern constellation Centaurus. HR 4796A is roughly 20times more luminous than the Sun and a few times more massive. It isseparated from its faint companion, dubbed HR 4796B, by about 500Astronomical Units. (An Astronomical Unit, or AU, is the distancebetween the Sun and the Earth, roughly 150,000,000 kilometers or93,000,000 miles.) The disk itself appears to be roughly 250 AUacross, and is seen nearly edge-on in images taken at wavelengths inthe mid-infrared part of the spectrum. "What's most exciting is thatwe are looking at a disk just at the time it is forming planets or hasrecently done so," says Jayawardhana, who found the disk while makingobservations for his PhD thesis.

This disk may be a close cousin of the disk around the star BetaPictoris that astronomers have known about for 14 years. BetaPictoris is estimated to be about 200 million years old whereas theHR 4796 pair is only about 10 million years in age. "Unlike the case ofBeta Pictoris, we know the age of this star pretty well, and it seemsperfect for planets to be forming in its disk," Jayawardhana explains.

"From previous work by Michael Jura at UCLA and colleagues, it wasknown that the primary star was surrounded by a dust cloud with a holein it," according to Hartmann. "Our images show that the cloud isindeed a disk, so that the hole could be cleared out by the gravity ofone or more inner planets."

Hartmann, author of a soon-to-be-released textbook on starformation, adds "This discovery could also tell us about how binarycompanions affect disks. Perhaps this disk is truncated on the outsideat a radius of about 125 AU because of the companion star's gravity."A true test of this idea will require better measurements of thecompanion's orbit.

The HR 4796A disk was first seen in images taken at 20 micrometers, awavelength in the mid-infrared part of the spectrum. (20 micrometersis about 40 times the wavelength of visible light.) The images wereobtained with a state-of-the-art mid-infrared array camera, known asOSCIR, built at the University of Florida.

"The new generation of mid-infrared detectors is what made thisdiscovery possible," says Telesco, who built the camera. Advanceddetector chips, like the one used in OSCIR, are a recent innovationthat has made it possible for astronomers to make observations in thisrelatively unexplored part of the spectrum. "These sensitive cameras,when used on the world's best telescopes, will lead to an explosion ofnew results as exciting as the HR 4796A disk," Telesco predicts.

Support for operating OSCIR at observatories around the world isprovided by the National Science Foundation (NSF) and NASA.The Cerro Tololo Inter-American Observatory (CTIO)is part of the National Optical Astronomy Observatories (NOAO),operated by the Association of Universities for Research in Astronomy,AURA, Inc., under cooperative agreement with the National ScienceFoundation.

Hugh Van Horn, Director of NSF's Division of Astronomical Sciences,comments that "Numerous discoveries of extrasolar planetary systemshave shown that we have much to learn about the process of planetformation. The exciting new observations by the Harvard/Floridateam working at CTIO -- and by their colleagues at the KeckObservatories -- will certainly help to advance our understandingof the planet-forming process. The new work bythese two teams provides a significant contribution to 'originsresearch' supported cooperatively by NSF and NASA and illustratesthe strength of the U.S. system of national and independentobservatories for astronomical research."

The research of both teams was supported in large part by the NASAOrigins Program, with additional support to the CfA/Florida team fromNSF, NOAO, and the Smithsonian Institution.

CONTACT INFORMATION:

Harvard-Smithsonian Center for Astrophysics
Ray Jayawardhana, rjayawardhana@cfa.harvard.edu, 617-493 5571
Lee Hartmann, lhartmann@cfa.harvard.edu, 617-495 7487

University of Florida, Gainesville
Charles Telesco, telesco@astro.ufl.edu, 352-392 4455

National Optical Astronomy Observatories
Bruce Bohannan, bruce@noao.edu, 520-318-8157

Caltech
Michael Werner, mww@ipac.caltech.edu, 818-354-0146

University of Pennsylvania, Philadelphia
David Koerner, davidk@upenn5.hep.upenn.edu, 215-573-5630

NASA Headquarters
Don Savage, dsavage@hq.nasa.gov, 202-358-1547

Jet Propulsion Laboratory
Jane Platt, Jane.Platt@jpl.nasa.gov, 818-354-5011

W. M. Keck Observatory
Andy Perala, aperala@keck.hawaii.edu, 808-885-7887

NOTES TO EDITORS:

Representatives of both groups will describe the discovery andits interpretation at a press conference at 11:00 am EDT, Tuesday,April 21, at NASA Headquarters, Washington, DC.

Image Caption:
A false color image of the disk around the star HR 4796A.The position of the star A and its companion B are indicated bycrosses. The disk is seen at the mid-infrared wavelength of 19.2micrometers. The emission arises from small solid particles,resembling dust, that are heated by star A's visible and ultravioletlight. The elongated shape of the emission indicates that the disk isseen nearly edge-on. In addition, the disk appears to lie the orbitalplane of the binary star system, since the emission is nearly parallelto the imaginary line connecting A and B. The dust may be in theprocess of clumping together in the early stages of planet formation.Image credit: University of Florida/CfA/NOAO

A false-color image of the HR 4796A disk will be available on theWorld Wide Web at:

http://www.astro.ufl.edu/news/

 
 
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