FIRST DETECTION OF BROWN DWARFS

For decades researchers have speculated about the existence of brown dwarfs - celestial objects which probably constitute a link between stars with lower masses and giant planets, such as Jupiter, whose mass is approximately one thousandth of the mass of the Sun. There is no reason to assume that these substellar objects cannot form randomly in space through a process similar to that of the stars; i.e. as a result of gravitational collapse and fragmentation of dust and gas clouds. However, despite many searches carried out, their existence had not yet been unequivocably proved.

A brown dwarf is a self-gravitating gaseous object composed mainly of hydrogen and helium, whose mass is too small to induce stable hydrogen fusion in its interior. All the theoretical surveys conducted agree that the limiting mass which separates stars from brown dwarfs is about 7 or 8% of the mass of the Sun. Incapable of generating nuclear energy, the gravitational contraction of a brown dwarf takes place unavoidably until the pressure of the degenerated electrons in its interior interrupts the whole process. The nearby star cluster of the Pleiades, a group of stars which formed about a hundred million years ago at a distance of approximately 400 light years (3780 billion kilometers) from the Sun, is considered to be one of the most suitable astronomical sources for the detection, and the subsequent study of brown dwarfs. At such early ages, these objects should be undergoing gravitational contraction, radiating much more energy than in later stages of their evolution. More massive brown dwarfs in the Pleiades should be detectable in sufficiently deep surveys.

After only 0.3% of the cluster's area had been explored using IAC80 telescope at Teide Observatory, a faint object was detected, whose extremely red colour possibly indicated a very low surface temperature. Firstly, its motion in space was confirmed to coincide with that of the stars of the cluster and, later, a precise photometric characterization was achieved. Several high resolution spectra between 600 and 900 nm were obtained with the WHT. These spectra confirmed the discovery of one of the coldest quasi-stellar objects known in the Universe. The spectral lines of neutral potassium between 767 and 770 nm indicated that it was an object with high surface gravity, as was expected for a brown dwarf, and the presence of prominent bands of titanium oxide and, especially, vanadium oxide at 750 nm allowed to derive its spectral classification and an estimate of its effective surface temperature, which turned out to be some 2350 K. The spectrum allowed to infere a velocity measurement of this object in regard to the Sun, which happened to be very similar to that of the stars in the cluster. All the entire set of observations suggested that it was a member of the cluster and, therefore, that its age was the same as the cluster's: 100 million years approximately, with a margin of error below 30%. It was the first time that the age of a celestial object of this nature had been so accurately determined, overcoming one of the most important restrictions preventing the true substellar nature of brown dwarf candidates to be classified. From the cluster's distance it was possible to determine that the luminosity of   Teide 1 (this is how the discoverers decided to call the object) was one thousandth of the solar luminosity. The comparison of its principal features (luminosity, temperature and age) with all the evolutionary models available in the scientific literature led to the conclusion that Teide 1 had to be a brown dwarf.

In 1996 the International Time Project "Observational Properties of Brown Dwarfs" detected new brown dwarfs in the Pleiades cluster. Several have masses similar to Teide 1 (55 Jupiter masses approximately) or higher, but various present slightly lower masses. They were all first detected using the INT. Subsequent confirmation involved spectra from the WHT and infrared photometry from UKIRT and WHT. The Keck telescope was then used to detect the element lithium in the spectra of brown dwarfs. Lithium is an important test for brown dwarfs because it is destroyed by nuclear reactions in stars of low mass but not in brown dwarfs.