Brown dwarfs

Post-publication activity

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 near breakup conditions (breakup period = 2 hours)! For comparison, Jupiter rotates in 11 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 dwarf stars. Lithium is an element that burns in the hotter cores of the completely convective M dwarf stars. As a result, M dwarf stars are completely depleted of their lithium, while this is not the case of the less massive (\(\le 0.060\) solar mass) brown dwarfs. For brown dwarfs with masses between \(0.060\) and \(0.075\) solar mass there is thus no simple spectral diagnostic. 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.

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 vapor 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 oxyde (\( \mathrm{ VO}\)) 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 resonnance 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 \(N\) (\(10.2 ~{\mu}m\)).

Figure 1: The radii and masses of four red dwarfs observed by VLTI; GJ 205, GJ 887, GJ 191 ("Kapteyn star") and Proxima Centaury (red dots), are compared to the Chabrier & Baraffe (2000) models at 400 Myrs (red curve) and 5 Gyrs (black curve). Jupiter is shown by a black triangle. Brown dwarfs have masses below the hydrogen burning minimum mass of 0.075 solar mass.

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 phenomenum 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 futher 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 patturn similar to that observed on Jupiter for exemple. 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).

Thanks to the French DENIS (Deep Near Infrared Survey), and American 2MASS and Sloan Digital Sky Survey (SDSS) photometric surveys among others, we know of some 400 brown dwarfs in young clusters and stellar associations, and in the solar neighbourhood. The coolest brown dwarf known has an effective temperature of \(700\,\mathrm{K}\). 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 determine the maximum mass of an extrasolar planet, just as brown dwarfs are 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 probably wrongly called free-floating planets or planemos (planetary mass objects) in the literature. --Allard 12:55, 23 August 2007 (EDT)