Physics – Optics
Scientific paper
Jun 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres...14.&link_type=abstract
ESO Press Release, 06/2001
Physics
Optics
Scientific paper
Dusty Disks Detected around Very Young Substellar Objects in the Orion Nebula
Summary
An international team of astronomers [2] is announcing today the discovery of dusty disks surrounding numerous very faint objects that are believed to be recently formed Brown Dwarfs in the Orion Nebula [3].
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" .
In particular, these results suggest that Brown Dwarfs share a common origin with stars and 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.
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 .
These important results are being reported today to the American Astronomical Society Meeting in Pasadena (California, USA).
PR Photo 22a/01 : Infrared picture of the Orion Nebula (NTT + SOFI). PR Photo 22b/01 : "Finding Chart" for Very Young Brown Dwarfs in the Orion Nebula. PR Photo 22c/01 : Animated GIF presentation of PR Photos 22a+b/01. Faint substellar objects in the Milky Way
Over the past 5 years, several groups of astronomers have identified a type of very faint, substellar objects within our Milky Way galaxy. These gaseous objects have very low masses and 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 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?
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 perhaps something completely different?
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
ESO PR Photo 22a/01
ESO PR Photo 22a/01
[Preview - JPEG: 400 x 434 pix - 192k] [Normal - JPEG: 800 x 877 pix - 496k] [Full Resolution - JPEG: 1772 x 1943 pix - 1.2Mb
Caption : PR Photo 22a/01 shows a colour composite of near-infrared images of the central regions of the Orion Nebula, obtained on March 14, 2000, with the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT) at La Silla. Three exposures were made through J- (wavelength 1.25 µm here colour-coded as "blue"), H- (1.65 µm; "green") and Ks-filters (2.16 µm; "red"), respectively. The central group of bright stars is the famous "Trapezium" . The total effective exposure time was 86.4 seconds per band. The sky field measures about 4.9 x 4.9 arcmin 2 (1024 x 1024 pix 2 ). North is up and East is left.
ESO PR Photo 22b/01
ESO PR Photo 22b/01
[Preview - JPEG: 400 x 439 pix - 35k] [Normal - JPEG: 800 x 877 pix - 90k]
Caption : PR Photo 22b/01 contains the corresponding "finding chart" with the positions of the very young Brown Dwarfs in the Orion Nebula that were studied during the present investigation. The starlike symbols represent the brightest stars in PR Photo 22a/01 and are plotted for reference. In this chart, very young Brown Dwarfs are represented by a double open circle (if a dusty disk was detected) or with a single open circle (if no dusty disk was detected). The scale is exactly as in PR Photo 22a/01 .
ESO PR Photo 22c/01
ESO PR Photo 22c/01
[Animated GIF: 482 x 465 pix - 248k]
Caption : PR Photo 22c/01 is an animated GIF-composite of PR Photo 22a/01 and PR Photo 22b/01 for easy comparison.
To resolve this mystery, an international team of astronomers [2] has obtained sensitive near-infrared observations of young Brown Dwarf candidates in the Trapezium cluster , at the centre of the Orion Nebula. For this, they used the state-of-the-art near-infrared SOFI instrument on the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile).
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 d
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