Binary Star Formation

Observational evidence strongly suggests that double stars are the rule rather than the exception in our Galaxy. Recent studies of molecular clouds, using sensitive infrared and millimetre wave detectors (because the visual absorption can exceed 1000 magnitudes), have shown that many of the objects found in these clouds are double or multiple.

Stars are born in dense clouds which consist almost totally of molecular hydrogen along with a small admixture of dust. At the temperature typical of these clouds, about 10 K, the hydrogen cannot be detected. Most clouds also contain traces of carbon monoxide which produces very bright spectral lines at wavelengths of 1.3 and 2.6 mm and it is these which allow astronomers to trace the distribution of hydrogen. To date about 120 other molecules have been found, ranging from water and ammonia to more complex organic structures such as methanol and ethanol.

Molecular clouds come in a range of sizes and composition. The small cloud complex Chamaeleon III, for instance is about 10 pc in diameter, has a maximum visual extinction of a few magnitudes and a temperature of about 10 K. There are a few stars, none of which are massive and no star clusters. The largest complexes in Orion, however, are perhaps 50 pc across, with 100 magnitudes of visual extinction and a gas temperature of 20 K. These are populated by thousands of stars in dense clusters, including massive OB stars. Star formation occurs most frequently in the more massive clouds. Other well-known regions of star formation are known simply by the constellation in which they appear: Taurus - Auriga, Ophiuchus, Lupus, and Perseus, for example.

How then do binary stars form from the nascent interstellar material? Recent simulations on powerful computers can explain not only many of the observed properties of binary stars but also the existence of large numbers of brown dwarfs. These are objects which, in terms of their mass, lie between the massive Jupiterlike planets and the faintest of stars - the red dwarfs. The mass of brown dwarfs (about 0.07 times that of the Sun or alternatively 70 Jupiter masses) is not sufficient for the nuclear reactions in the core to start but they are warm enough to be seen in sensitive infrared detectors.

Bate et al.1 have recently published the results of collapsing a simulated interstellar cloud in the computer and following its evolution. They begin with a cloud of 50 solar masses and about a light year in diameter and the process starts with the formation of cores which then collapse gravitationally, some being more massive than others. The dense cores are usually surrounded by a dusty disk which is left behind as they contract more and more rapidly. These disks are thought to be the major source for the formation of brown dwarfs. Many interactions occur within the cloud before the stars have reached their full size and as a result the less massive fragments are ejected from the cluster by a slingshot mechanism. The most massive cores are attracted to each other and form close binaries and multiple systems which then undergo further evolution.

When the calculation was stopped (it took 100,000 CPU hours!) the result was the formation of 23 stars and 18 brown dwarfs, so Bate and colleagues conclude that brown dwarfs should be as common as stars. The number of known brown dwarfs is very small but that is largely due to the fact that they are so difficult to detect. Another prediction of this programme is that brown dwarf binaries do form but they need to be very close in order to survive and the few binary brown dwarfs found so far fit this criterion. It was previously thought that the production of close and wide binaries was a result of different processes but this current theory has the advantage of producing many of the observed properties of multiple stars and brown dwarfs.

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