Brown dwarfs
| France Allard and Derek Homeier (2007), Scholarpedia, 2(12):4475. | doi:10.4249/scholarpedia.4475 | revision #91086 [link to/cite this article] |
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Brown Dwarfs vs. Stars: Common Properties and Differences
A brown dwarf is a failed star, not massive enough (\(\le 0.075\) solar mass) to sustain stable hydrogen fusion in its core, which only fades away with time. Brown dwarfs are formed like stars by gravitational contraction of a cloud of gas and dust. The Spitzer Space Telescope has detected infrared emission from accretion disks around brown dwarfs in young stellar clusters, thus confirming this formation process. Contraction provides the thermal energy that powers the brown dwarf luminosity. Depending on the mass of the brown dwarf, core temperatures can reach \(10^4\) to \(6 \times 10^6 \) \(\mathrm{K}\), while core densities range from \(10\) to \(10^3\,\mathrm{g\,cm}^{-3}\ .\) It also provides angular momentum that brown dwarfs, unlike sun-like stars, retain during their evolution since magnetic breaking is inefficient due to their cooler, neutrally-charged atmosphere. Some brown dwarfs have been observed to rotate at velocities close to disruption by centrifugal forces (breakup period = 2 hours)! Jupiter, for comparison, has a rotation period of 10 hours.
During the first few million years of their evolution, brown dwarfs experience deuterium burning and are so bright (\( \le 4 \times 10^{-2}\) solar luminosity) and warm (\(T_{\rm eff}\le 3600\,\mathrm{K}\)), where \(T_{\rm eff}\) is the effective temperature, or temperature of a blackbody emitting the same integrated energy flux) despite their smaller masses, that they can be mistaken for very low mass stars. Lithium detection in the spectral energy distribution helps distinguish brown dwarfs from M or L dwarf stars. Lithium is an element that is converted into helium by thermonuclear fusion in the hot cores of low mass dwarfs, and eventually completely destroyed due to mixing in their convective interior. As a result, low mass stars as well as the most massive (\(> 0.055\) solar mass) brown dwarfs are fully depleted of their lithium early in their evolution, while this is not the case with less massive brown dwarfs. For brown dwarfs with masses between \(0.055\) and \(0.075\) solar mass there is thus no simple spectral diagnostic, and neither is there for very young objects that did not have time to burn their lithium. Differences in gravitational acceleration due to mass difference between brown dwarfs and stars can, however, cause systematic changes in the spectral energy distribution, especially in the collisionally broadened wings of absorption lines.
Atmospheric Properties and Evolution
The atmospheres of the warmer brown dwarfs and M dwarf stars have a gas composition similar to that of the sunspots, with molecular hydrogen, carbon monoxide and water vapour locking up most hydrogen, carbon and oxygen. In cooler brown dwarfs the carbon is found in methane. The spectral energy distribution is governed by molecular absorption bands of calcium hydride (\(\mathrm{CaH}\)), titanium oxyde (\(\mathrm{ TiO}\)), vanadium oxide (\( \mathrm{ VO}\)) in the optical to red part of the spectrum, and by water vapor (\(\mathrm{ H_2O}\)), carbon monoxide (\(\mathrm{CO}\)) and pressure-induced \(\mathrm{H_2}\) absorption in the infrared which makes their spectral properties very different from those of blackbodies. Only resonance transitions of the most abundant chemical elements (alkali metal elements, \(\mathrm{Ca}\ ,\) \(\mathrm{Al}\ ,\) \(\mathrm{Ti}\ ,\) \(\mathrm{Fe}\)) can absorb through the molecular absorption continuum. Brown dwarfs are brightest between the molecular absorption bands of water which also absorb in Earth's atmosphere. The peak flux is therefore found at wavelengths corresponding to the center of standard broadband photometric filters \(Y\) (\(1.02 ~{\mu}m\)), \(J\) (\(1.22 ~{\mu}m\)), \(H\) (\(1.63 ~{\mu}m\)), \(K\) (\(2.19 ~{\mu}m\)), \(M\) (\(4.75 ~{\mu}m\)) and \(N\) (\(10.2 ~{\mu}m\)).
As brown dwarfs further evolve, the increasing core density leads to strong plasma correlation (\(\Gamma = 1 - 50\ ,\) where \(\Gamma\) is the plasma parameter) and partial electron degeneracy (\(k_B T_F / k_B T \approx 1 - 20\ ,\) where \(k_B\) is the Boltzmann constant, \(T\) is the gas temperature, and \(T_F\) is the Fermi temperature) which stops contraction, leading to a nearly constant radius (\(R \propto M^{-1/8}\)) with value around the radius of Jupiter. Due to pressure ionization, the interior is composed of metallic hydrogen. Their luminosity and surface temperature decrease below the condensation temperature of metals and silicate crystals (\(\approx 1800\,\mathrm{K}\)), which form cloud layers in their atmospheres for \(T_{\rm eff} \le 2500\,\mathrm{K}\ .\) These dust clouds change profoundly the composition of the atmosphere and its spectral properties in two ways: 1) by condensation of molecular gas onto crystal grains causing the optical to red part of the spectral energy distribution to be freed of the main molecular absorbers except alkali metal element transitions, and 2) Rayleigh and Mie scattering by micron-size particles produce a powerful greenhouse effect increasing atmospheric temperatures. This phenomenon gave rise to an extension of the official spectral classification of stars, beyond type M, to spectral types L and T, L designating cloudy brown dwarfs while T designates cloud-free brown dwarfs. It was observed that L-type brown dwarfs are systematically brighter and emit more in near-infrared wavelengths then T-type brown dwarfs, indicating an evolution sequence between the two spectral types. As brown dwarfs fade below \(2500\,\mathrm{K}\ ,\) their spectral type changes from M to L. Cooling further below about \(1400\,\mathrm{K}\ ,\) the combined effects of reduced luminosity -- causing the convection zone to shrink and no longer extend into the photosphere -- and gravitational settling of dust crystal particles to deeper atmospheric layers, cause the cloud layers to sink below the photosphere. It is also possible that the cloud cover gradually disperse, producing with rapid rotation a surface pattern similar to that observed on Jupiter for example. Finally, as brown dwarfs cool below \(600\,\mathrm{K}\ ,\) it is expected that ice clouds (water ice first, and eventually ammonia ice) begin to form, leading to an additional spectral type (the Y spectral class).
Observations
Thanks to modern wide-field photometric surveys such as the French-led Deep Near Infrared Survey (DENIS), and the US-based Two Micron All Sky Survey (2MASS) and US-led Sloan Digital Sky Survey (SDSS), we know of some 600 brown dwarfs in young clusters and stellar associations, and in the solar neighbourhood. This number can be expected to increase yet again with the second generation of large angle surveys like the UKIRT Infrared Deep Sky Survey (UKIDSS). For an up-to-date list of L and T dwarfs, most of which are brown dwarfs, refer also to DwarfArchives.org. The coolest brown dwarfs known have effective temperatures of \(650 - 700\,\mathrm{K}\ .\) Brown dwarfs can form with masses as low as some \(\approx 5\) times the mass of Jupiter. Since the minimum mass for deuterium fusion is \(13\) times the mass of Jupiter, this criterion has been adopted to define the maximum mass of an extrasolar planet, just as the distinction between brown dwarfs and stars is defined by the minimum mass of hydrogen burning. Brown dwarfs can therefore have masses similar to those of planets. This is the reason why some young low-mass brown dwarfs are sometimes misleadingly called free-floating planets or planemos (planetary mass objects) in the literature. But since deuterium fusion provides no significant, sustained energy source, these objects do not differ from more massive brown dwarfs in any fundamental aspect of their formation history, physical properties or evolution.
Bibliography
Journal Articles
Kirkpatrick, J.D.: Annual Review of Astronomy & Astrophysics, vol. 43, Issue 1, pp.195-245 (2005)
Ackerman, A.S.; Marley, M.S.: The Astrophysical Journal, Volume 556, Issue 2, pp. 872-884. (2001)
Gibor, B.: Annual Review of Astronomy and Astrophysics, Vol. 38, p. 485-519 (2000)
Chabrier, G.; Baraffe, I.: Annual Review of Astronomy and Astrophysics, Vol. 38, p. 337-377 (2000)
Textbooks
UltraCool Dwarfs: New Spectral Types L and T, 2001, Editors: Hugh R. A. Jones and Iain A. Steele, Springer (ISBN: 3-540-42353-2)
Very Low Mass Stars and Brown Dwarfs, 2000, Editors: Rafael Rebolo and Maria Zapatero-Osorio, Cambridge University Press (ISBN: 0-521-66335-0)
New Light on Dark Stars: Red Dwarfs, Low-Mass Stars, Brown Dwarfs, 2000, Authors: I. Neill Reid and Suzanne L. Hawley, editor: Springer (ISBN: 3-540-25124-3)
