Eclipsing brown dwarfs with Kelvan Stassun

Brown dwarfs have masses between large gas giant planets like Jupiter and the lowest-mass stars. They are not massive enough to shine as stars do, via thermonuclear fusion reactions in their interiors. That’s why the study of dim brown dwarfs is relatively recent. Astronomers studied a pair of brown dwarfs in the direction of the Orion Nebula to learn fundamental properties of these objects.

Astronomer Keivan Stassun of Vanderbilt University calls brown dwarfs failed stars. They started out forming as stars do, but they weren’t born with enough mass to ignite and shine as stars. Stassun and colleagues spent 12 years observing two brown dwarfs orbiting each other.

Keivan Stassun: Specifically, as they orbit one another, they periodically block one another, as seen by us on Earth. And when we do find such a system, we’re able to directly and accurately measure all of the fundamental physical properties of the two objects that make up that system – in this case, two brown dwarfs.

Those fundamental properties of these failed stars include their mass, diameter, emitted light, and surface temperature, which weren’t precisely known for any brown dwarf until now.

Keivan Stassun: What we found, to our surprise, with this system is that the lighter one, the wimpier one, is actually hotter at its surface than the heavier one. And we don’t entirely understand what that means right now.

Stassun said one possibility is that these two brown dwarfs formed separately, then became bound by gravity later in their lives.

With the detailed measurements of this brown dwarf pair, the path is open now to identifying other objects in our galaxy and determining just how many brown dwarfs there really are.

The binary dwarf system studied by Stassun lies in the Orion Nebula. The brown dwarf pair cannot be seen directly because the pair is too close together to be resolved with present-day telescopes. But by observing their light, astronomers can tell that the orbital plane of this system is tilted edge-on to Earth. As they watch, astronomers can see that the two stars are seen eclipsing each other, as seen from our earthly vantage point.

In this system, the dwarfs complete an orbit approximately once every ten days. So many observations are possible. Eclipsing binary stars like these allow precise measurements of each companion star’s mass, diameter, surface temperature, and intrinsic brightness. What’s exciting and new here is that these are not stars, but brown dwarfs, which are much less understood than stars.

Our thanks to:
Keivan Stassun
Vanderbilt University

October 1, 2006

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