Observations show that brown dwarfs form like stars
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: June 12, 2001
An international team of astronomers has announced the discovery of dusty disks surrounding numerous very faint objects that are believed to be recently formed brown dwarfs in the Orion Nebula.
This finding is based on detailed observations with SOFI, a specialised infrared-sensitive instrument at the ESO 3.5-m New Technology Telescope at the La Silla Observatory. It is of special interest because it sheds light on the origin and nature of substellar objects, known as "brown dwarfs".
Moreover, 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.
Faint substellar objects in the Milky Way
Such objects weigh less than about 7% of our Sun and have been variously called "brown dwarfs", "Failed Stars" or "Super Planets". Indeed, since they have no sustained energy generation by thermal nuclear reactions, many of their properties are more similar to those of 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 still comparable to that of Jupiter, approximately 140,000 km, or roughly 10 times smaller than the Sun.
Are brown dwarfs giant planets or failed stars?
The critical test needed to resolve this very basic question is to learn whether brown dwarfs form by a process similar to what produces stars or rather to one 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. They have been found around virtually all young stars and are considered 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 the young Sun about 4.6 billion years ago.
To date, the most important observations bearing on the question of Brown Dwarf origin have been: the observed lack of Brown Dwarf companions to normal stars (something astronomers have called the "Brown Dwarf desert"), and the existence of free-floating brown dwarfs in the Milky Way galaxy.
Both facts would appear to imply a stellar, rather than a planet-like origin for these objects. However, one might also explain these observations if most brown dwarfs initially formed as companions to stars (within circumstellar disks), but were later ejected from the systems, e.g., because of gravitational effects during encounters with other stars. So the issue of Brown Dwarf origin is still unsettled.
NTT observations of substellar objects in the Orion Nebula
The Trapezium Cluster is a group of young stars that appears to the unaided eye as a faint central 'star' in the Orion Nebula. This cluster is located at a distance of about 1200 light-years and contains nearly 1000 stars, most of which are younger than 1 million years. The stars in this cluster are in their infancy when compared to our middle-aged Sun that is about 4.6 billion years old (reduced to a human timescale, they would be just 3 days old, compared to the Sun's 40 years). 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 observations obtained with the NTT benefitted from superb atmospheric conditions (e.g., a seeing of 0.5 arcsec) and allowed the astronomers to examine the near-infrared light of more than 100 of the Brown Dwarf candidates in the cluster. More than half of them were found to have excess near-infrared light, compared to that a normal young Brown Dwarf should emit. The only plausible explanation is that this extra light originates from glowing, hot dust within protoplanetary disks surrounding these objects. It was the same method, albeit at longer infrared wavelengths, that first led to the discovery of dust disks around several normal stars, some of which have later been studied in much detail, e.g., that at the southern star Beta Pictoris.
In fact, and strongly supporting this explanation, twenty-one of the Brown Dwarf candidates detected via the NTT observations are also identified with optical "proplyds", the famous dusty disks first imaged in 1994 by the Hubble Space Telescope (HST) at optical wavelengths.
Indeed, only about 80 freely floating brown dwarfs have so far been positively identified 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 from the University of Florida and a member of the team.
Her colleague August Muench explains that "even at their brightest, however, most brown dwarfs are still 100 or more times intrinsically fainter than our Sun, explaining why astronomers have great difficulties in detecting such objects".
A common origin of normal stars and brown dwarfs
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 from ESO, "it is entirely possible that the Milky Way Galaxy contains numerous planetary systems that orbit cold and dark, failed stars. Whether these disks can indeed form planetary systems, however, still remains to be determined".
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 next steps
The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.