Pedagoguery

What is a planet? As the debate over Pluto's status last year demonstrated, the boundaries are rather fuzzy. The lower boundary, between planets and minor bodies like asteroids and Kuiper belt objects was drawn during that debate, but the upper end still has some uncertainty. At the boundary between planet and star lies brown dwarfs.

Brown dwarfs are bodies, typically massing between 12 and 75 times the mass of Jupiter, that are more massive than typical planets, but are not quite big enough to allow hydrogen fusion. They are thought to form in much the same way as stars, but something arrests their growth early on, preventing them from gaining enough mass to initiate hydrogen fusion. The typical birth of a brown dwarf is believed to go like this: First a region within a giant cloud undergoes gravitational collapse. The center of this region serves as the embryo, and surrounding gas and dust collects into an accretion disk, gradually allowing the embryo to gain more mass. So far, it is not any different from the process by which a star is formed, but after about 100,000 years, a brown dwarf for some reasons stops growing. After anywhere from 1 to 10 million years, the brown dwarf has gotten large enough to fuse deuterium, which will provide an energy source for around 100 million years. After the deuterium is gone, however, the brown dwarf will gradually cool, becoming more and more planet-like as it does so.

Can a brown dwarf form like a planet, accreting gas and dust within a disk surrounding a protostar? The current belief is that that process of accretion is too slow to allow a planet to get larger than 10 to 15 times Jupiter's size before the central star blows away the available gas and dust in the disk. Observation seems to back this up, so a stellar origin is favored for brown dwarfs.

Stars form in large molecular clouds. Particularly dense portions of the cloud, called cloud cores, are what collapse to form stars. The smallest core that can collapse depends on the temperature and composition of the cloud and is termed the Jeans mass. The typical Jeans mass is about one solar mass. However, the entire core does not necessarily form a single star. Typically, the densest regions inside the core will break up and form multiple embryos, which themselves can be as small as one Jupiter mass. These embryos then go on to sweep up most of the mass surrounding them. If a Jeans mass core collapsed into 10 embryos, each would be expected to end up with about one tenth of a solar mass. Brown dwarfs, however, are about one tenth of that mass. What prevents them from growing larger?

There are two competing theories about brown dwarf formation. One is called the ejection scenario, and the other is the turbulence scenario. In the ejection scenario, the embryos within the collapsing core gravitationally interact, with smaller embryos getting ejected from the core before they can grow too large. The ejected embryos then become brown dwarfs.

The alternative says that turbulent motion within the molecular cloud can induce cores to collapse that would otherwise be too small to collapse on their own. Therefore, with less mass available to accrete, the resulting object stays small.

Is there any way of distinguishing between the scenarios? In fact, there is. The primary difference is that in the turbulence scenario, brown dwarfs and stars are virtually indistinguishable. Only the overall mass is different. In the ejection scenario, however, ejected embryos will lose some of their star-like characteristics in the course of their ejection. Thus, if the turbulence scenario is true, we should see proto brown dwarfs with small accretion disks as well as binary and multiple brown dwarf systems in about the same abundance as regular stars. In the ejection scenario, however, much of the disk of the embryo will be stripped away in the course of the ejection. In addition, binary systems should be much less common, since the ejection event would tend to send the embryos flying out in different directions.

Observations of young brown dwarfs indicate that most of them do in fact have accretion disks, which seems to favor the turbulence scenario. However, the case is not closed. We are currently, limited in where and what we can observe. It is possible that we live in an area dominated by turbulent star formation, and that the ejection scenario dominates elsewhere. Further observations, particularly with the Spitzer Space Telescope, should be able to shed greater light on the issue.

Next issue: Intermediate mass black holes.