 | ||
| |
 |
| ||
| ||
|
| |
|
| ||
Supervisor: Dr R F Jameson.
Brown dwarfs were until recently hypothetical astronomical objects of mass 20 to 80 times the mass of Jupiter. They were proposed to fill the mass gap between planets and the lowest mass stars. A number of brown dwarfs have now been identified and many more can confidently be expected to be discovered in the near future. This opens up a new area in astrophysics.
Brown dwarfs are best thought of as failed stars. A star such as the Sun generates energy from thermonuclear reactions in the ultra-hot and ultra-dense core. When a star has a mass below 0.08 the mass of the Sun, its central temperature is not hot enough to convert hydrogen into helium by the proton-proton cycle of nuclear reactions. Objects below this mass are termed brown dwarfs. Until recently the only objects known with masses below the hydrogen burning limit were the planets of our solar system. Our largest planet, Jupiter has a mass of 0.001 the mass of the Sun. One Jupiter mass (MJ) is equal to 2 ×1027 kg, and is used as a mass unit when describing brown dwarfs.
Thus brown dwarfs were hypothesised to fill the region between the lowest mass stars and the planets, a factor of 80 in mass. Although the boundary between brown dwarfs and stars seems fairly clear cut, the division between planets and brown dwarfs remains as yet uncertain. Can planets form which are heavier than brown dwarfs ? How do we distinguish massive planets from low-mass brown dwarfs ? In this paper we classify a brown dwarf as an object that forms the same ways as a star but with insufficient mass to burn hydrogen. Planets, on the other hand, are formed from the debris of stellar formation by the growth of an icy or rocky core before the accumulation of gas. Our definitions do not provide an obvious observational test for distinguishing a planet from a brown dwarf, but they do relate to the true nature of the objects.
Both brown dwarfs and extrasolar planets have now been shown to definitely exist. Studying brown dwarfs will pose two major challenges. The first is understanding their cool outer layers, or atmospheres. The behaviour of their atmospheres is dominated by molecules such as H2O, CH4 and also solid dust particles. The second is a full understanding of their interiors. The interior densities and pressures are such as to range from an electron degenerate gas in the centre to a near perfect gas in their atmospheres. This encompasses a combination of pressure and temperature that is much more complicated than that found in normal stars and also beyond the range that can be studied in the laboratory.
Brown dwarfs are important in another way. How many are there ? A census of all stars within 10 parsecs of the Sun indicates that 70% of our known nearest neighbours are low luminosity, low mass stars - M dwarfs. And there are many more still waiting to be found. If we have only found one third of the nearest M dwarfs, then it is not surprising that it has taken so long to find a brown dwarf in the solar neighbourhood. There is ample evidence from studying the dynamics of galaxies, that galaxies contain large amounts of hidden or `dark' matter. Indeed this problem of dark matter extends beyond galaxies to clusters of galaxies and the Universe as a whole. It may be that as much as 98% of the mass of the Universe is unaccounted for, and may not even be in the form of normal matter. Brown dwarfs have no nuclear furnace to sustain them and become very faint as they age, so they are a potential reservoir of dark matter. They could make an important contribution at least to the dark matter in galaxies.
 |
Page maintained by Keith Sohl (kbs@star.le.ac.uk) |  |