The Brown Dwarf That Wasn't

Massive companion. AB Doradus C, the pinkish dot in the lower left of this binary system, may be twice as massive as current theoretical models would predict.

In the animal kingdom, weight reveals a lot about an animal--how much energy it consumes, for instance. The same is true of stars, which burn through their fuel at different rates depending on their bulk. Now a study suggests that astrophysicists--who face a tougher challenge than most wildlife biologists--have been misgauging the mass of some of the smallest stars, which may actually be heftier than originally thought.

The mass of stellar objects determines their future, but weighing them isn't easy, especially when they're solitary. The best astrophysicists can currently do is use theoretical models based on observable properties like color and brightness. But while such models have been shown to accurately measure the mass of mature stars, some scientists have questioned their applicability for younger, smaller objects like brown dwarfs and free-floating giant planets.

Now, Laird Close, an astronomer at the University of Arizona, and his collaborators believe they have succeeded in directly and accurately weighing a young, low-mass object for the first time. The object is part of a binary system 48 light-years away in the constellation Doradus the Goldfish. The team combined high-resolution radio observations of the wobbling parent star with images of its faint companion taken with a new, sensitive infrared camera at the Very Large Telescope (VLT) in Chile. The data allowed the researchers to use straightforward celestial mechanics rather than theoretical predictions to measure the weight of the companion object, known as AB Doradus C.

The results, reported this week in Nature, indicate that AB Doradus C is massive enough to sustain hydrogen burning in its core and should thus be classified as a normal star. Based on its age and brightness, current theoretical models would have predicted it to be shrimpier and a brown dwarf. As a result, Close and his collaborators suggest that the current models underestimate the mass of young, lightweight objects and need to be revised. The same would likely be true for even less massive objects, which have been classified as free-floating giant planets in the past. If these are also more massive than the theory would suggest, they may actually be brown dwarfs.

Theoretical astrophysicist Adam Burrows, also of the University of Arizona, agrees that there appears to be a discrepancy between theory and observation. "This is interesting observational evidence that suggests the modelers have a lot of work ahead of them," he says. Ultimately, this work could lead to a better understanding of the formation of both stars and planets.

Related websites
Background information on brown dwarfs
More on the NACO/SDI camera used in the research

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