David Aguilar
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Christine Pulliam
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CfA Press Release
 Release No.: 01-07

For Release: 9:20 am (PDT) June 7, 2001

Astronomers Detect Dust Disks Around Very Young Brown Dwarfs in the Orion Nebula

Pasadena, CA--An international team of astronomers is announcing today that they have found dusty disks surrounding numerous very faint objects believed to be recently formed brown dwarfs in the Orion Nebula. The results are being reported to the American Astronomical Society Meeting in Pasadena California by Mr. August A. Muench of the University of Florida in Gainesville Florida, USA, Drs. Joao F. Alves of the European Southern Observatory in Garching, Germany, Elizabeth A. Lada of the University of Florida, and Charles J. Lada of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA. This finding is of special interest because it sheds light on the origin and nature of substellar objects, known as brown dwarfs. In particular, these results suggest that brown dwarfs share a common origin with stars. This further indicates that brown dwarfs are more similar in nature to stars than to planets and, like stars, have the potential to form with accompanying systems of planets. Finally, the presence of dusty protoplanetary disks around the faintest objects in the Orion Nebula cluster confirms both the membership of these faint stars in the cluster and their nature as bona fide substellar objects, making this the largest population of brown dwarf objects yet known.

Over the past 5 years several groups of astronomers have identified a group of very faint substellar objects within our galaxy. These gaseous objects have masses so low that they will never shine like normal stars because they cannot achieve central temperatures high enough for sustained thermal nuclear reactions to occur in their cores. Such objects must have masses less than about 7/100th that of our Sun and have been variously called "Brown Dwarfs", "Failed Stars" or "Super Planets". Indeed, since they do not have sustained energy generation by thermal nuclear reactions they have many properties more similar to the giant gas planets in our own solar system such as Jupiter than to stars like the Sun. For example, even though their masses range between 10- 70 times that of Jupiter, the largest and most massive planet in our solar system, the sizes of brown dwarfs are comparable to that of Jupiter, about 10 times smaller to the sun.

Among the most fundamental issues raised by the existence of brown dwarfs is the question of their origin and genetic relationship to planets and stars. Are brown dwarfs giant planets or small, failed stars, or, something else altogether different? The critical test needed to resolve this question is to find whether brown dwarfs form by a process similar to that which produces stars or that which produces planets. Stars are thought to form when gravity causes a cold, dusty and rarefied cloud of gas to contract. Such clouds are inevitably rotating so the gas naturally collapses into a rotating disk before it falls onto the forming star. These disks are called circumstellar or protoplanetary disks. Circumstellar disks have been found around virtually all young stars and are thought to be sites of planet formation. Gravity helps planets form too, but this occurs by condensation and agglomeration of material contained in the circumstellar disk around a young star. Thus, stars form with a disk around them while planets form within disks around young stars. The planets in our own solar system were formed in such a circumstellar disk around a young sun more than 5 billion years ago.

To date, the most important observations bearing on the question of brown dwarf origin have been: 1) the observed lack of brown dwarf companions to normal stars (something astronomers have called the brown dwarf desert) and 2) the existence of free floating brown dwarfs in the galaxy. Both facts would appear to implicate a stellar (non-planetary like) origin for these objects. Alternatively, one could explain these two observations if most brown dwarfs initially formed as companions to stars (within disks) but were ejected from the systems by encounters with other stars. So the issue of brown dwarf origin is still very much in doubt.

To resolve this mystery an international team of astronomers obtained sensitive near-infrared observations of young brown dwarf candidates in the Trapezium cluster. They used a state of the art near-infrared camera on the European Southern Observatory 3.5-m New Technology Telescope (NTT) on La Silla, a mountain in the Chilean Andes. The Trapezium cluster is a group of young stars within the Orion Nebula that together with the Orion Nebula appears, to the unaided eye, as the faint central "star" in the sword of the hunter in the constellation Orion. This cluster is at a distance of 1200 light years and contains nearly 1000 stars, younger than one million years. The stars in this cluster are in their infancy when compared to our middle aged Sun which is four and a half billion years old. If middle age for a human is 40 years old these newborn stars would only be 3 days old! Among the hundreds of normal stars in the Trapezium cluster astronomers have previously identified a population of objects so faint that they have been considered as prime candidates for very young brown dwarfs.

The sensitive near-infrared observations obtained with the NTT allowed the astronomers to examine the near-infrared light of more than 100 brown dwarf candidates in the cluster. A large fraction (more than half) of which were found to have excess near-infrared light compared to that a normal young brown dwarf should emit. This extra light originates from glowing, hot dust within protoplanetary disks surrounding these objects. The presence of such hot, dusty disks around these objects indicates their extreme youth and confirms both their membership in the young cluster and their nature as bona fide substellar objects. Thus, the Trapezium cluster contains the largest population (~100) of brown dwarfs yet known. Indeed, only about 80 freely floating brown dwarfs have so far been positively identified in all interstellar space outside this cluster. "Brown dwarfs are considerably easier to detect and study when they are young, because they are ten times larger and thousands of times brighter during their early youth than during their middle age," says Elizabeth Lada who is a professor of astronomy at the University of Florida. "Even at their brightest however, most brown dwarfs are still 100 or more times fainter than our sun, explaining why astronomers find such objects so difficult to detect," says August Muench, team member and a graduate student at the University of Florida.

"The high incidence of disks around both young stars and brown dwarfs in this cluster strongly suggests that both stars and brown dwarfs trace their origin to a common physical process and that brown dwarfs are more similar in nature to stars than to planets," says Charles Lada, who is a senior astrophysicist on the staff of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. Moreover, as is the case for stars, the disks that surround brown dwarfs may be capable of forming systems of planets. According to Joao Alves, an astronomer on the staff of the European Southern Observatory, "It is entirely possible that the Galaxy contains numerous planetary systems that orbit cold and dark, failed stars. Whether these disks can indeed form planetary systems, however, remains to be determined." Elizabeth Lada adds "Even if brown dwarfs do have planetary systems, their planets would not have a stable climate and thus would be inhospitable to life as we know it. This is because brown dwarfs do not generate their own energy for any substantial period of time but instead fade rapidly as they age." The identification of disks around the brown dwarf candidates in the Trapezium cluster rests entirely on the near-infrared colors of these objects. Additional confirmation of the presence of such dust disks can be obtained with sensitive infrared observations made at longer wavelengths. Such observations are possible with the largest telescopes on the ground but may be best done from space with the upcoming SIRTF infrared satellite mission of NASA.

Note to Editors:

The research reported here was supported in part by the National Science Foundation.


David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics

Images and diagrams available from: http://www.cfa.harvard.edu/news/archive/pr0107image.html

(This press release is being issued jointly by the European Southern Obsevatory, Harvard-Smithsonian Center for Astrophysics, and the University of Florida.)

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