If, as we have often speculated in these pages, there is a brown dwarf closer to us than the Centauri stars, it may well be the WISE mission that finds it. The Wide-field Infrared Survey Explorer is a 40 cm telescope cooled below 17 K (-430 Fahrenheit) that will image the entire sky in four infrared wavelengths. If we’re looking for nearby brown dwarfs, an all-sky survey like this is the way to go, because such stars should be distributed uniformly in the space around us.
According to information Amanda Mainzer (JPL) presented yesterday at the American Astronomical Society meeting in Long Beach (CA), brown dwarfs are now thought to make up two-thirds of the stars in our stellar neighborhood, most of them as yet undetected. One of them might well be closer than the 4.3 light years that separate us from Alpha Centauri. And WISE should be up to the challenge of finding it, being able to detect cool brown dwarfs (down to 200 K) at Centauri distance and objects down to Jupiter-mass if closer than one light year.
We’re at the bridge between planets and stars here. L and T-class brown dwarfs include objects with temperatures down to 600 K (620 F), but WISE should be able to find numerous brown dwarfs that are cooler still, objects that may lead us to define a new spectral class. Current estimates are that the mission will uncover up to a thousand new brown dwarfs — we’ll see how close some of them may be when the mission goes to work after a launch scheduled for November of this year.
More from the brown dwarf front: The Hubble Space Telescope has been used in a study of 233 multiple star systems, which turned up only two brown dwarfs as companions to normal stars. Sergio Dieterich (Georgia State University) reported on this one yesterday at AAS, noting that the lack of brown dwarfs around solar-type stars seems to be telling us something:
“We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit. Especially when we consider the fact that brown dwarfs binaries do exist, the fact that there are very few binaries whose components lie on different sides of the hydrogen burning limit is significant.”
This work is part of the RECONS (Research Consortium on Nearby Stars) survey, which looks for the Sun’s neighbors within 10 parsecs (32.6 light years) of Earth. The current total of brown dwarfs in this range of space is twelve, compared with 239 identified red dwarf stars. Now the Hubble work shows that brown dwarfs don’t co-exist readily even with these small stars, an indication that mass is not the operative element in keeping these stellar types apart. “If mass ratio was the driving factor,” says Dietrich, “we would expect to find more brown dwarfs around small red stars than around solar type stars.”
AAS will also be the scene of another brown dwarf report, this one by Micaela Stumpf (Max-Planck-Institute for Astronomy, Heidelberg), which likewise implies that brown dwarfs do not tend to be found in the company of larger stars. Stumpf also reports that the brown dwarf binary Kelu-1 AB may actually be a triple system, accounting for the puzzling discrepancy between the masses of the two known dwarfs and the total mass of the system, which had been estimated on the basis of orbital dynamics. If confirmed, Kelu-1 AB may turn out to be the first known triple brown dwarf system. We have high hopes for WISE, but the deep infrared searches of the next decade may be what it takes to unravel the true size and character of the brown dwarf population.
On Kelu-1 AB, see Stumpf, “Kelu-1 AB – A possible brown dwarf triple system,” submitted to Astronomy & Astrophysics and available online.