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Journal

No. 50 - November 2006

Brown Dwarfs

Introduction

This article is based on research that was carried out in preparation for a talk to the Society in March 2006. The talk was inspired by the increased rate of detection of these objects. They are no longer theoretical and no longer rare or exotic.

The first brown dwarf was detected in 1995 although astrophysicists had predicted their existence for decades. Apparently there was no reason to rule out the existence of objects which have masses in between Jupiter sized bodies and the lowest mass objects which are capable of long term nuclear fusion within their cores.

Some people have argued that brown dwarves are misnamed. Firstly, they are not brown. They are envisaged as dim (in visible light) red objects with darker bands similar to the bands of Jupiter. Secondly, is a dwarf a meaningful term for an object that can be sixty times the mass of Jupiter? So far no one has thought of a better alternative name than 'sub-stellar object'.

Originally brown dwarves were called black dwarves. Now this term is reserved for the ultimate fate of white dwarves and neutron stars. As it will take a trillion years for these objects to cool to the state of blackness (no emission of visible light) none exist in the universe at the present time.

By all accounts Jill Tarter, who is the chief scientist of the SETI Institute, came up with the name 'Brown Dwarf'. She called, in a doctoral thesis in the 1970s, 'Sub Stellar Objects' 'Brown Dwarfs'.

The parameters of a brown dwarf

Brown dwarfs are defined relative to stars of solar mass, red dwarfs and Jupiter sized planets.

Solar Mass Stars

Stars of between 1.2 and 0.7 solar masses have similar spectra. They remain on the main sequence fusing hydrogen in a stable manner for ten billion years. These stars, such as our sun, when they run out of hydrogen, are massive enough to fuse carbon. They will become red giants and cast off planetary nebula. The remnant hot core will slowly cool as a white dwarf.

Red Dwarfs

Stars of between 0.7 and 0.08 solar masses evolve differently. They take a much longer time to reach the fusion stage (the main sequence) to condense from a mass of dust and gas. They spend millions of years condensing (the smaller the mass the longer it takes). The energy that is acquired from gravitational contraction is emitted in the infra red. During this phase red dwarves are indistinguishable from brown dwarves.

Once their cores are hot and dense enough fusion starts and proceeds at a leisurely pace. Fusion may continue for the lifetime of the universe. Their indefinite lives mean that red dwarves are now relatively plentiful in old globular clusters and in our galaxy. Even if the universe lasted long enough red dwarves could not become red giants. They do not have the mass to create the conditions in heir core that would allow the fusion of carbon.

Red dwarfs have surface temperatures of around 3500 K and K and M class spectra. The lower mass limit for a red dwarf is, by definition, 0.08 solar mass as that was thought to be the minimum mass that is required to produce the core temperature and pressure that will support nuclear fusion.

Brown Dwarves

It is now realised that high mass brown dwarves, approaching the 0.08 limit, can support fusion for about ten million years. It is not hydrogen that is burnt but heavy hydrogen (deuterium) and lithium.

The lower mass limit of a brown dwarf (and the notional boundary between these objects and a planet) is defined as 20 Jupiter masses (20 Jm). However, some authorities state that the boundary should be a spectrograph that indicates an object has a surface temperature of 1000 K.

Two new spectrographic classes have been created for brown dwarves. The first is called L and refers to the presence of lithium lines. Lithium is quickly and completely destroyed in a star. It will be present in the spectrum of a brown dwarf that has never undergone any fusion. To illustrate that nothing is clear cut lithium can be present in pre main sequence stars.

Brown dwarves can have a surface temperature of between 1500 K and 1000 K. Methane and water molecules can exist within this range. Their spectral lines characterise spectral class T.

The evolution of a high mass brown dwarf can be summarised as follows:

Although this will be the standard evolutionary history of a brown dwarf those in binary or other complex systems may gain mass and become stars. A brown dwarf may also gain mass by collision or impact.

Orion Nebula, visible light
Figure 1 - the Trapezium cluster in visible light (WFPC2).
STScI PRC00-19. NASA, C.R. O'Dell and S.K. Wong (Rice University).

Detection of Brown Dwarves

Infra red telescopes have detected brown dwarves within the neighbourhood of the solar system. More probably remain to be detected. At the moment, out to 17 light years, there are 60 stellar and sub stellar objects. These comprise 3 high mass stars, 7 solar mass stars, 45 red dwarfs, 5 white dwarfs and 7 brown dwarfs. These proportions may be galactic proportions.

The first brown dwarf to be detected is called Gliese 229B. It is 18 light years away. It rotates around Gliese 229A (a sun like star). Gliese 229B is 1/400,000 as bright as 229A in visible light. It is a T class brown dwarf and is thought to have a mass between 20 and 55 Jm.

A brown dwarf has been found with a 5 Jm planet.

A brown dwarf called LP 944-20 is 16.3 light years away in the constellation Fornax. It is thought to be about 60 Jm. On 15 December 1999 it emitted a small solar sized X ray flare which lasted for 2 hours. This shows that a brown dwarf may have stellar characteristics. Although fusion may have stopped or never started the interiors of a brown dwarf may be hot enough to generate turbulent and highly magnetised gases.

The Hubble telescope has photographed a star forming area of the Orion Nebula in visible light (figure 1). The same region in infra red light (figure 2) reveals a swarm of brown dwarfs. Perhaps the image suggests that brown dwarfs are very common objects in the galaxy.

Orion Nebula, infrared light
Figure 2 - the Trapezium cluster in infra red (NICMOS).
STScI PRC00-19. NASA, K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory, University of Arizona, Tuscon, Ariz.)

Brown Dwarfs and Life

The discovery that brown dwarfs are common must change the chances of life existing elsewhere in the galaxy. The chances must dramatically increase as they are more common than solar mass stars. Although small compared with our sun they can have planetary sized satellites and satellite systems. There is nothing to rule out an Earth type planet orbiting a brown dwarf which could b an abode for life - provided it was close enough. A brown dwarf can emit sufficient energy (albeit mostly within the infrared) to support life. As brown dwarves are more common than solar mass stars the life that evolves in their systems may be characteristic of our galaxy and the Universe as a whole. The most common form of eyes may only detect light in the infra red!

Further information:

Most of the information for this article was taken from websites on the Internet. Try 'Googling' Brown Dwarf / Brown Dwarfs / Brown Dwarves. Some sites that are recommended are:

Des Loughney, June 2006


Contents

Cover page

Presidential news

The March 29 eclipse from Libya

Let there be rock - The story of the Hambleton meteorite

Let there be rock - addendum

Brown dwarfs

About the ASE Journal


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