Newfound Star Sparks Brown-Dwarf Debate

[pmbryant]

From Sky and Telescope magazine:

January 25, 2005 | In principle, weighing an astronomical object is easy: just track its orbit around another similar body and apply basic physics to derive its mass. In practice, however, astronomers can use this approach only rarely, and only a handful of brown dwarfs — substellar gas balls that can't sustain nuclear fusion in their cores — have been weighed this way even coarsely. Consequently, to decide whether a pinprick of light like 2M 1207b comes from a "planet" (with less than 1.3 percent of the Sun's mass, or 13 Jupiters), a brown dwarf (13 to 75 Jupiters), or a star (more than 75 Jupiters), one usually must rely on evolutionary models that relate an object's luminosity to its age and mass.

Now, though, an international research team has determined an orbit — and a precise mass — for the youngest brown dwarf yet. There's just one problem: that object, AB Doradus C, isn't a brown dwarf after all. It has an ostensibly stellar mass of 90 Jupiters — up to twice what evolutionary models predict for the 50-million-year-old object given its distance and near-infrared magnitude.

...

Spotting and "weighing" so dim a companion is a noteworthy achievement, astronomers agree. But Close and his colleagues may get more press for the inferences they draw than from their technical prowess. Because AB Dor C's dynamical mass significantly exceeds the predictions of most widely used evolutionary models, Close implies, those models are now suspect. "Some young objects that people are calling brown dwarfs are really low-mass stars," he says, and "things that people are calling free-floating planets, in almost every case, are likely low-mass brown dwarfs."

That doesn't sit well with Isabelle Baraffe (Astronomical Research Center of Lyon, France), one of the reigning model's architects. "We never claimed that our models . . . can be blindly applied to very young objects," she says, and earlier studies already have highlighted similar discrepancies. What's more, she doubts that Close's team really can constrain the primary star's mass and age as precisely as its report in last Thursday's issue of Nature suggests — and AB Dor C's inferred mass depends sensitively on both these quantities. Unfortunately, independent tests may be few and far between until NASA's SIM PlanetQuest (formerly the Space Interferometry Mission) finds and characterizes numerous binaries in young open clusters.

Link: http://skyandtelescope.com/news/article_1449_1.asp

[pmbryant]

1. Here's a longer article in the NYT

This is actually a very important result. Big implications for our understanding (or lack thereof) of how stars, brown dwarfs (objects too small to be stars, but too large to be planets), and planets form and evolve.

http://www.nytimes.com/2005/01/25/science/space/25star.html?oref=login

Tiny Star's Unexpected Weight Raises Big Astronomical Questions

By DENNIS OVERBYE

The smallest star ever to be reliably weighed has tipped the scales at more than twice its expected mass, astronomers say.

As a result, much of what they thought they knew about the low-mass bits of cosmic litter, which hover on the edge of stardom, may be in doubt. The work, released last week, suggests that astronomers may have systematically underestimated the masses of these objects and thus misidentified the smallest members of their realm.

"This discovery will force astronomers to rethink what masses of the smallest objects produced in nature really are," said Dr. Laird M. Close of the University of Arizona, leader of an international team of astronomers, which reported the result in the journal Nature.

Dr. Alan Boss, a theorist at the Carnegie Institution in Washington, called the work "a most interesting astronomical advance" that underscored the need for astronomers to anchor their theories with measurements.

...

Astronomers usually deduce the masses of stars from their luminosities, and on that basis the star, AB Doradus C, should be about 50 times as massive as Jupiter, making it a brown dwarf, a failed star.

But on the basis of its orbit, they were able to determine that AB Doradus C was actually 93 times as massive as Jupiter, putting it in the range of a small star.

...

For a star, mass is everything. How massive it is determines how dense and hot its core is and what kind, if any, of thermonuclear reactions can go on there. According to theory, an object that is less than about 75 times as massive as Jupiter will not have enough oomph to ignite and start burning hydrogen into helium, and so will remain a brown dwarf. The Sun, by comparison, is about 1,000 times as massive as Jupiter.

...

The issue of how small a star can be is one that astronomers are eager to explore, but as Dr. Close admitted, his new result suggests that such objects are fainter and harder to study than astronomers had hoped. So the answer will be hard to resolve.