LP 944-20

Robert Hurt, IPAC, NASA

Larger illustration: Sol; M,L,T dwarfs;
and Jupiter.

At one billion years in age, large brown
dwarfs are reddish like the smallest
M-type stars, but cooler, dimmer T-dwarfs
are more magenta in hue (more).

The Brown Dwarf

© Association of Universities for Research in
Astronomy, Inc. (AURA). All Rights Reserved.
Digital Sky Survey, ESO -- larger image

From left: nearby brown dwarf LP 944-20 in
blue (B), red (R) and near-infrared light at
center of each photo (ESO news release).

A dim celestial object, LP-944-20 is believed to be a young and relatively hot, but cooling, brown dwarf. It has less than six percent of Sol's mass -- less than 57 times Jupiter's mass (Pavlenko et al, 2007), about a tenth of Sol's diameter, and 14.5/100,000th of its luminosity (Tinney, 1998, in pdf). The brown dwarf appears to be roughly as enriched in elements heavier than hydrogen as Sol (I Ribas, 2003). LP 944-20 has a rotational period of less than five hours. It is probably around 320 +/- 80 million years old and may be a member of the Castor stellar moving group (Pavlenko et al, 2007; and I Ribas, 2003). In 1999, changes in the surface chemistry of the brown dwarf suggested that it had weather patterns with winds, clouds and storms -- similar to those found on a gas giant planet like Jupiter (press release). Some other useful catalogue numbers include: 2MASS J03393521-3525440, 2MASSW J0339352-352544, APMPM J0340-3526, BRI 0337-3535, BRI B0337-3535, LEHPM 3451, N3 9 (in the Astronomisches Rechen-Institut at Heidelberg's ARICNS), and SIPS J0339-3525.

© American Scientist



Artwork by Linda Huff
(for Martin et al, 1997)
used with permission.

Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to ignite core hydrogen fusion, the smallest true stars (red dwarfs) can have such cool atmospheric temperatures (below 4,000° K) that it is difficult to distinguish them from brown dwarfs. While Jupiter-class planets may be much less massive than brown dwarfs, they are about the same diameter and may contain many of the same atmospheric molecules.

LP 944-20's Flare

Brown dwarfs are believed to shine with only the heat of gravitational contraction for a comparatively short time after birth and then darken as they cool down to become dead cinders drifting through space. Previously, astronomers did not anticipate that a brown dwarf could flare like a true star that sustains nuclear fusion at its core. On December 15, 1999, however, LP 944-20 was observed -- by the Chandra X-Ray Observatory's Advanced CCD Imaging Spectrometer (ACIS) -- to emit a small solar-sized flare, which lasted for two hours and is thought to be very similar to the flares seen on our Sun (press release; and Rutledge et al, 2000).

NASA's Chandra X-Ray Observatory, University of California at Berkeley, Caltech, (Rutledge et al, 2000)
LP 944-20 is the first brown dwarf to have an observed flare (more from Chandra and Astronomy Picture of the Day).

Detected across 16 light-years as x-rays, astronomers estimated that the energy emitted in the brown dwarf flare was comparable to a small solar flare, which is a billion times greater than X-ray flares observed from Jupiter. The astronomers (Robert E. Rutledge, Gibor Basri, Eduardo L. Martin, and Lars Bildsten) observing LP-944-20 did not expect to detect any X-rays from the brown dwarf during their observations because the brown dwarf's atmosphere should be too cool to emit such radiation. Indeed, the fact that the Chandra X-Ray Observatory detected nothing for most of its 12-hour observation would seem to prove the hypothesis that LP 944-20 does not have an hot outer atmosphere found on true stars know as coronae.

LP 944-20's flare suggests, however, that brown dwarfs have magnetic fields which generate subsurface flares that can occasionally punch erupt into their outer atmosphere. The leader of the astronomer team that discovered the flare, Gibor Basri, speculates that the flare was generated about 60 miles (about 97 km) below the brown dwarf's surface, where its gas should be hot, turbulent, and highly magnetized. This subsurface flare heated LP 944-20's outer atmosphere so that flowing currents of gas would give rise to an x-ray flare, similar to a stroke of lightning. Its flare is also the strongest evidence found yet that brown dwarfs and possibly young giant planets have magnetic fields that can release a large amount of energy through a flare. The eruption of stellar flares are believed to occur when magnetic fields twist, occasionally cross, and arc like an electrical short. When this happens, gas is explosively heated, and a a flare erupts to inject high-energy particles into the upper atmosphere of the star. A number of astronomers are coming to the conclusion that the hot gas observed in the atmospheres of relatively low mass stars is produced only by flares.

In early 2001, a group of astronomers announced the discovery of quiescent and flaring radio emission from LP 944-20 (Berger et al, 2001). This first-ever detection of persistent radio emission from a brown dwarf was unexpected, and the astronomers concluded that LP 944-20 may be roiled by storms several times more powerful than the most energetic flares on the Sun. Follow-up observations suggested that the object's magnetic fields were extremely weak, which is a surprise because flares are normally powered by the energy in magnetic fields. Jeffrey Linsky, (an astrophysicist at the University of Colorado) suggested that, while the cooler cores of brown dwarfs might create less turbulence inside them leading to weaker magnetic fields, the result might be conformations, or geometries, that make the such fields more likely to reconnect and produce a few very large flares (James Glanz, New York Times, 2/16/01).

Brown Dwarfs or Planets?

When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.

© Anglo-Australian Telescope Board
(Image by Chris Tinney)



Wide field image with satellite trail.



"True-color" image of the brown
dwarf binary DENIS 1228-1547
(more information).

University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover the other nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.

Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.

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