How Lithium Strips Brown Dwarfs of Their Red Disguise

By Ken Croswell

Published in New Scientist (December 18/25, 1992, page 15)

If you want to know if a star is a brown dwarf, look to see if it has lithium in its spectrum, say astronomers in Italy and Spain. Brown dwarfs are dim stars that are not massive enough to burn hydrogen. They could make up much of the invisible "dark matter" known to exist in the universe.

Astronomers have identified several faint red stars that may be brown dwarfs. But these stars might instead be red dwarfs, stars which look the same but are nonetheless burning hydrogen.

In theory, astronomers can distinguish between a brown and a red dwarf by estimating its mass: brown dwarfs have less than 8 percent of the Sun's mass, whereas red dwarfs have more. But this test rarely works, because a star's mass is impossible to measure unless the star is orbiting a companion.

Now there may be a better way, say Antonio Magazz¨ of the Astrophysical Observatory in Catania, Italy, and Eduardo MartÝn and Rafael Rebolo of the Institute of Astrophysics in the Canary Islands. Many brown dwarfs should contain large amounts of lithium, which create a dip in their spectra. Lithium absorbs red light at a wavelength of 6,707 angstroms.

Lithium, with atomic number three, is one of the lightest elements. It forms in interstellar space when heavy atoms are split in collisions with high-speed particles called cosmic rays. Because stars form from clouds of interstellar gas, they inherit this lithium when they are born.

But lithium is a fragile element which is soon destroyed by a star's heat. If the lithium is exposed to a temperature above 2.4 million Kelvin, it is converted into helium: a proton hits the lithium, splitting it into two helium nuclei. Although the surface of a low-mass star is cool, surface material is constantly dragged down into the star's core where it is subjected to high temperatures.

Magazz¨ and his colleagues say that the interior of a red dwarf is hot enough to destroy all the star's lithium. Even brown dwarfs with masses between 6 and 8 percent that of the Sun are hot enough to destroy their lithium. Only brown dwarfs with less than about 6 percent of the Sun's mass remain cool enough to preserve their lithium. So any faint red star with a large amount of lithium must be a brown dwarf.

The astronomers searched for lithium in several faint red stars by using the 2.5-metre Isaac Newton Telescope and the 4.2-metre William Herschel Telescope, both in the Canary Islands. But they did not find the feature due to lithium in the spectrum of any of the stars.

Some of the stars were known to be red dwarfs, such as Luyten 726-8, a double star 8.4 light-years away in the constellation of Cetus. The two stars of Luyten 726-8 have 11 and 12 percent of the Sun's mass, so it is not surprising that neither has lithium. They provided a test of the method.

Magazz¨'s team also investigated Wolf 424, a double star 14 light-years away in the constellation of Virgo. In 1989, Wulff Heintz of Swarthmore College in Pennsylvania claimed that the two stars of Wolf 424 are brown dwarfs, because they have 5 and 6 percent the mass of the Sun (Astronomy and Astrophysics, vol 217, p 145). But last year other astronomers disputed Heintz's claim. They said that the stars have more mass, making them red dwarfs, not brown dwarfs (Astronomy and Astrophysics, vol 254, p 116).

If Heintz had been right, the fainter star should have had lithium, because a brown dwarf with 5 percent of the Sun's mass preserves its entire supply of the element. The failure to detect lithium means that the star is either a red dwarf or a brown dwarf that is more massive than Heintz claimed.

Magazz¨ and his colleagues plan to search for lithium in other brown dwarf candidates. They will report the first results of their work next year in Astrophysical Journal Letters.

Ken Croswell is an astronomer in Berkeley, California, and author of The Alchemy of the Heavens and Planet Quest.

"An engaging account of the continuing discovery of our Galaxy."--New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"An excellent introduction to a branch of astronomy that may eventually help to establish the presence or absence of life elsewhere in the universe...Fascinating and worthwhile."--New York Times Book Review. See all reviews of Planet Quest here.