Disks around failed stars: A question of age
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: August 5, 2002
From two of these, mid-infrared radiation is detected - for the first time ever from such objects with a ground-based telescope. While the younger Brown Dwarf, aged a few million years, is found to be surrounded by a dusty disk, no warm dust is present around the older ones.
The new observations support the following formation hypothesis for Brown Dwarfs: they are born in the same way as "real" stars, by contraction in interstellar clouds of gas and dust. During the later stages of this process, the infalling material is transferred onto the star via a gas and dust disk. This disk - in which planets may possibly form - then disperses with time.
Brown Dwarfs are faint and cool objects
They do not burn hydrogen to helium as "real" stars do, but continue to emit faint light as they slowly contract and cool during millions of years. They end their inglorious life with a whimper and finally fade into eternal insignificance.
Although Brown Dwarfs were theoretically predicted already in 1963, astronomers had to wait until 1995 for the first one to be discovered. This was mainly due to their extreme faintness as compared to normal stars - even the most nearby Brown Dwarfs shine so faintly that they can only be observed with relatively large telescopes. As they are rather cool objects, they emit mostly in the infrared spectral region; hence they are best observed with astronomical instruments that operate at those wavelengths.
With improved techniques, however, more and more Brown Dwarfs have been found and the count has now reached several hundred. Many of these are located in the well-known Orion Nebula. Others move through interstellar space, like the lonely KELU-1 first discovered in 1997 at the ESO La Silla Observatory by Chilean astronomers. With a distance of only 33 light-years from the Sun, it was one of the closest Brown Dwarfs known at that time.
Formation of Brown Dwarfs
Recent observations at ESO have shown that the Brown Dwarfs in the Orion Nebula most likely have formed as stars do, i.e. by contraction in a cloud of dust and gas. The clue to this was the observation of an excess of near-infrared radiation from many of these objects, interpreted as the presence of dusty disks around them. The astronomers then argued that if the young Brown Dwarfs possess such disks exactly like real stars do, then they must also form in the same way.
Infrared observations of Brown Dwarfs
Pioneering observations in this wavelength interval of some Brown Dwarfs were made in mid-1995 by the ESA Infrared Space Observatory. However, the ISO instruments provided comparatively low image sharpness and these observations were hampered by confusion with the radiation from other objects in the same sky field. And the ISO mission was over before Brown Dwarf objects were discovered in larger numbers.
Astronomers have therefore long wanted to observe Brown Dwarfs with large ground-based telescopes in the mid-infrared spectral region. But these objects are faint and few suitable instruments that work at these wavelengths are available at the world's large astronomical telescopes. Long exposures are necessary to record the faint emissions and until now, it had not been possible to perform such highly demanding observations of Brown Dwarfs.
TIMMI2 observes Brown Dwarfs
They pointed the telescope towards a total of eight Brown Dwarf objects and recorded the emission at three different mid-infrared wavelengths (5, 9.8 and 11.9 micron). "We were delighted", says team leader Daniel Apai, "to detect radiation from two of these with TIMMI2. These are the first observations of their kind with a ground-based instrument. And although we could only establish upper limits for the radiation from the five other objects, these results are highly significant for our attempts to understand the formation and evolution of Brown Dwarfs."
One of the objects, known as Cha HA 2 and located in the southern constellation Chamaeleon, had earlier been observed with ISO. It is a bona-fide Brown Dwarf object and an image obtained with the Hubble Space Telescope indicates that it may possibly be double. It is a relatively young Brown Dwarf and is a member of the very young Cha I star-forming region - the age has been estimated at 2 - 4.5 million years.
The other Brown Dwarf from which TIMMI2 has now detected mid-infrared radiation is one of the closest of its type. Designated LP 944-20, it is located in the southern constellation Fornax (The Oven) at a distance of only ~15 light-years. It is much older than Cha HA 2, though, probably 500 - 650 million years. In this case, the age was determined by measuring the strengths of spectral lines of the element Lithium; the older the object, the less is the content of Lithium.
The observations show that the radiation from LP 944-20 comes from the cool star itself - it does not possess a surrounding disk as does the much younger Cha HA 2.
Evolution of Brown Dwarfs
Nobody knows yet whether planets form in those disks around young Brown Dwarfs (as this was the case in the disk around the young Sun and other stars), but it might happen. Only future observations with much more sensitive instruments will be able to cast more light on this intriguing question.
Further into the future, the Atacama Large Millimeter Array (ALMA) will provide excellent opportunities for in-depth investigations of Brown Dwarfs. With unequalled sensitivity and very good image sharpness, ALMA will be able to image disks around the nearest Brown Dwarfs and possibly, to detect signs of (forming) planets in them.
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