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AAO Press Releases

9 July 1997

Cool Customer May Hold Key to Dark Universe

At the Annual Conference of the Astronomical Society of Australia, it was announced today that Australian and French astronomers have detected a `brown dwarf' 40 light years away in the constellation of Corvus ("the Crow"). Brown dwarfs are star-like objects too small to burn nuclear fuel like the Sun, and a proposed source of the Universe's dark matter. The recent discovery is a true light-weight, about 60 times as massive as the planet Jupiter. It has only 1/16th the mass of the Sun, itself a very modest star, and is at least 20,000 times fainter. This feeble not-quite-star has surface temperatures as low as 1700°, compared to the Sun's 5,500°.

Astronomers have known for almost 20 years that most of the matter in the universe is invisible. Unfortunately, they haven't been able to work out what form that invisible matter takes. One of the prime contenders has been a class of objects which are not quite stars, but also not quite planets. Known as "brown dwarfs" they have been predicted to exist for many years, but actually seen only very recently.

Normal stars (like our Sun) are bright becuase they burn nuclear fuel. The Sun's planets by comparison do not,  and are consequently  very dim. Jupiter is the largest planet  in the Solar System, weighing in at about 1000th the mass of the Sun. And only reason we can see it at all  in our night sky, is because it reflects light from the Sun. Its intrinsic brightness is billions of times smaller than the Sun, which means that giant planets outside our Solar System are, to all intents and purposes, undetectable to current technology. Brown dwarfs are objects which are larger than giant planets, but small enough that they cannot burn like stars. This means that although brighter than planets, they are still so faint that after almost 20 years of searching, astronomers only found the first confirmed brown dwarf a year ago.

The difficulty in finding brown dwarfs is their extreme faintness - which means that even with the largest telescopes they can only be found within about 50 light-years of the  Sun. In astronomical terms, that's right in the Solar "backyard" , since the Milky Way galaxy as a whole is almost 80000 light-years across. With such a small fraction of our Galaxy available for astronomers to search, it is necessary to examine almost the entire sky in order to find just a few brown dwarfs. Moreover, you can't use traditional astronomical imaging techniques, which see visual light.

The human eye (along with most astronomical instrumention) has evolved to detect light in the visual wavelength range, where the Sun emits most of its light. However, objects which are smaller and fainter than the Sun are also considerably cooler. This has the effect of shifting the wavelength at which the they emit most of their light towards  the red end of the spectrum. For brown dwarfs this shift is so extreme that they emit mostly in infrared light, and almost nothing in visual . So if you want to search for them, you need to do it in the infrared.

And that is just that which Australian and European astronomers have been doing. Using a dedicated telescope on the isolated mountain of Cerro La Silla in Chile, members of the European DEep Near Infrared Southern Sky Survey team (or DENIS) are imaging the entire southern sky at infrared wavelengths. This means they are uniquely placed to find large numbers of brown dwarfs. In order to exploit this data, Dr Chris Tinney of the Anglo-Australian Observatory has joined up with DENIS team members Dr Thierry Forveille and Xavier Delfosse of the Observatoire de Grenoble, France. Together, they have been using the 3.9m Anglo-Australian Telescope to study the first brown dwarf candidates from the DENIS all-sky data.

One object in the constellation of Corvus ("the Crow") has turned out to be a real light weight. Observations made with the 3.9m Anglo Australian Telescope have found the element lithium in this "not quite star". This fragile element is destroyed in the hot centers of stars, but is preserved in the cooler centers of brown dwarfs. In fact, the presence of lithium indicates that the Corvus brown dwarf has a mass of only 1/16th that of the Sun. Only two other old brown dwarfs like it are known, and both of those are much less massive. This means that the Corvus brown dwarf is vitally important to astronomers, because it defines the `benchmark' for the transition from star to brown dwarf.

Even more excitingly, this discovery has been made using only a tiny fraction of the DENIS survey. This powerful collaboration of the light-gathering power of the 3.9m Anglo-Australian Telescope and its innovative suite of instruments, with the unique  European DENIS survey, can be expected to find many more of these enigmatic objects in the next few years.

Photograph available in colour and black-and-white in GIF Format.

Photograph available in colour and black-and-white in Postscript Format.

Photograph available in colour and black-and-white in Encapsulated Postscript format.

Figure caption : The image shows the region of the sky around the newly discovered brown dwarf in the constellation of Corvus (arrowed). The overlaid line shows an AAT spectrum with a characteristic `dip' due to the presence of lithium. This proves the object is a brown dwarf., since lithium is not seen in hotter stars.

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Roger Bell
01 Jan 1998