Unraveling Brown Dwarf Origins

摘要

Brown dwarfs are not massive enough to be stars, nor are they planets in the traditional sense. They are often found in isolation but may also be companions to stars. They share some characteristics with stars and others with planets, and occupy the mass range in between: from about 75 to about 10 Jupiter masses. Although there is the potential for an identity crisis (1), brown dwarfs may hold clues to the formation of stars as well as planets. The identification of hundreds of brown dwarfs over the past 5 years in the solar neighborhood and in young star clusters (see the figure) has led to widespread interest in their origin. [For a discussion of brown dwarf chemistry, see (2).] How can free-floating objects with inferred masses as low as 10 Jupiter masses form? One possibility is that brown dwarfs form as a result of the turbulent fragmentation and contraction of molecular cloud cores―that is, in the same way as stars. In 1976, Low and Lynden-Bell (3) calculated that the Jeans mass (the minimum mass for a cloud fragment to overcome pressure and collapse under gravity) could be as small as about 7 Jupiter masses, comparable to the masses of some planets detected around nearby Sun-like stars and below the threshold of 13 Jupiter masses for deuterium burning. This deuterium-burning limit is taken by some astronomers as the boundary between brown dwarfs and planets. Simulations suggest that fragmentation of a collapsing cloud might produce self-gravitating objects with initial masses as low as 1 Jupiter mass, provided that magnetic field tension effects are important in the cloud environment (4).