50 years of Brown Dwarfs
October 17, 2012
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Exactly 50 years ago Shiv Kumar has theoretically predicted the existence of Brown Dwarfs, which are the link between stars and planets. It took another 30 years until these exotic objects were actually detected by observations. The origin of these mysterious objects is still not fully understood. All this is reason enough for renowned experts to meet on October 21.-24. in an international conference at Ringberg Castle nearby the Tegernsee. Present will be Shiv Kumar as well as the discoverers of the first Brown Dwarfs, Ben Oppenheimer, Rafael Rebolo and Gibor Basri.
Brown Dwarfs are often called failed stars because they are too cool too sustain enough nuclear fusion to shine as the sun or other stars. On the other hand, they share many properties with giant planets, such as relatively cool atmospheres in which clouds can form. The exploration of Brown Dwarfs is, therefore, a key to understand both the formation and evolution of planets as well as those of low-mass stars.
Brown dwarfs are cool
The existence of substellar objects, which do not produce enough internal energy to shine steadily for a long time, was predicted by Shiv Kumar in 1962. The term "Brown Dwarf" was proposed in 1975 by Jill Tarter, a researcher now at the SETI Institute. However, the actual color of Brown Dwarfs is rather red or magenta. Therefore, Brown Dwarfs are not only very faint, but also radiate mainly in infrared light. It required enormous technical advancements particularly in the field of infrared detectors, to allow their discovery in the mid 90s.
One of the first Brown Dwarfs discovered, Teide 1, appeared in 1994 as an unusual red object in the camera of Rafael Rebolo of the Instituto de Astrofísica de Canarias and has been confirmed by Gibor Basri as a young Brown Dwarf. An even cooler object was found in the same year by Ben Oppenheimer and Tadashi Nakajima with the Hubble Space Telescope. They were able to even detect methane in the atmosphere of this companion of the star Gl229.
The clouds that can form in the cool atmospheres of Brown Dwarfs, can consist of e.g. iron instead of water as on earth, as Christiane Helling and Mark Marley show in their model calculations. Last year, a group of astronomers around Mike Cushing has discovered the first so-called Y-Dwarfs with the WISE-satellite. With temperatures below 300 degrees, they are the coldest, free floating celestial objects detected so far.
Origin is a mystery
Due to their low mass, a star-like formation by the gravitational collapse of gas and dust clouds is not easy to explain. Nevertheless, such a scenario seems possible to some researchers. One of many alternative formation scenarios is the ejection of "stellar embryos" out of their birth place before they can grow up to real stars.
"Some observations actually indicate a star-like formation. For example, the discovery of Brown Dwarfs that have been formed in isolation or very wide Brown Dwarf binaries – both cases which do not hint at strong dynamical interactions. Furthermore, young Brown Dwarfs were found to be surrounded by disks and to drive jets and outflows – similar to young stars", explain Viki Joergens and Thomas Henning from the Max Planck Institute for Astronomy in Heidelberg (MPIA). Their team detected this years for the first time such disks at submillimeter wavelengths with the Herschel Space Telescope and also found jets with the ESOs VLT Observatory. Such disks have been also seen in the millimeter regime with ESOs ALMA Observatory by a team including Leonardi Testi.
The conference organized by Viki Joergens and Thomas Henning from MPIA entitled "50 Years of Brown Dwarfs" will provide a lively exchange between observers and theorists, and will bring together many of the world's most renowned experts working in that field.
Brown Dwarfs have a mass of less than 75 Jupiter masses (Jupiter is the largest planet in our solar system). This means that their mass is less than one tenth of a solar mass. With a surface temperature of less than 300 to 2500°C, they are much cooler than the sun which has a surface temperature of 5500°C.
The size of Brown Dwarfs is determined by quantum mechanical effects and is about one Jupiter radius, when they have passed their "adolescence". Despite their name they are not really brown, but rather red or magenta.
How brown dwarfs form is still one of the main open questions in the theory of star formation. A key role to answer this question play brown dwarfs as members of binary and multiple systems. Steadily improving instrumental performance led to the discovery of companions around brown dwarfs down to planetary masses, to size (radii) and dynamical mass determinations, and to statistically significant samples of very low-mass binaries. These detailed empirical characterizations of brown dwarfs enable us to test and calibrate increasingly sophisticated models of internal structure, atmosphere, and formation of substellar objects.
There is evidence that even among the coldest Brown Dwarfs, called T-and Y-Dwarfs, binary systems were found. Their discovery might be published during this conference.