Dwarfs Brown

Two completely different scenarios attempt to explain the presence of large Li abundances among the RGB stars. One is the result of an external contamination (pollution) produced by the engulfing of near giant planets or brown dwarfs companions. The second one is the result of an internal action known as the Cameron-Fowler 7Be mechanism. Here, we will make a brief discussion of both. 

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

Disregarding for a moment the bright and dark bands that decorate the spectrum of a heavenly body at specific wavelengths, the overall hue of that spectrum can tell us the surface temperature of the object. A blue star is thus hotter than a yellow one, and a yellow star is hotter than a red one. The Sun is hotter at the surface than the red star Antares, which in turn stands as a torrid desert before the brown dwarfs or interstellar clouds. The stars go red with cold. 

Concerning gas losses, we must subtract gas transformed into stars and the matter imprisoned in stellar corpses. The latter occur in three forms white dwarfs, neutron stars and black holes. We must also include matter going into planets and aborted stars (brown dwarfs), forever frozen and permanently withdrawn from the (nuclear) chemical evolution of the Galaxy. 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

Collision-induced absorption spectra are also of considerable interest for studies of the atmospheres of late stars [241, 307, 112] certain white dwarfs [366, 300, 239, 240] low-mass stars, brown dwarfs, certain cool white dwarfs [240] and the hypothetical population III stars [306, 372],... 

Mass determines a star s luminosity and temperature. You can estimate a star s relative luminosity Lr (with respect to the Sun) from the star s relative mass Mr (with respect to the Sun) by L = Mf5. Very small, cool stars would be below the chart. For example, a brown dwarf is an astronomical object with characteristics that are intermediate between a planet and a star—and it is 50 times smaller than our Sun. The biggest stable stars have masses about 70 times the Sun s mass. ... 

Astronomers have found about 450 young red dwarf stars within 80 light-years of the Earth. This means that there are more red dwarfs closer to Earth than all the other types of stars combined. Some astronomers suggest that red dwarfs are the most common type of star in the Universe.12 Others suggest that even cooler brown dwarfs are the most common star, but they are so dim they are more difficult to detect (figure 8.7). 

Brown dwarfs Brown dwarfs are star-like objects whose masses are too small to permit nuclear fusion to occur at their cores. (The temperature and pressure at its core are insufficient for fusion.) Brown dwarfs are very abundant in the Universe and are about 0.01 to 0.08 solar masses. 

We ve been discussing the possibility of life in a future so distant that stars no longer shine. However, even today, stars are not necessary to support life or produce light. For example, light may be emitted by chemical processes on a planet far away from a sun. A more intriguing idea is the possibility of life on brown dwarfs—the warm planet-like objects far away from suns and therefore without sunlight. 

Incidentally, it s likely that brown dwarfs account for a small portion of the dark matter in our Universe. There seems to be ten times more matter in the Universe than astronomers can account for by studying observable stars. For example, galaxies near the Milky Way appear to be rotating faster than would... 

There are also brown dwarfs to consider. Brown dwarfs are astronomical objects somewhat between a planet and a star and have a mass less than 0.08 times the mass of our sun and a surface temperature below 2,500 K. (As comparison, the cool red dwarfs are about 3,000-3,400 K). A large number of brown dwarfs would not change how bright the Galaxy appears in optical observations but would change its total mass quite substantially. 

I discuss brown dwarf priests and related matters in my book, The Science of Aliens. 

Throughout this book we ve discussed brown dwarfs. These objects are too massive to be planets yet too small to be stars. These dwarfs probably arise the same way stars do, from the gravitational collapse of giant gas clouds, but these dwarfs are too small to sustain fusion. 

In 2000, researchers conducted a deep infrared photometric survey in the Orion nebula when they found that 30 percent of their 500 infrared sources were brown dwarfs. [For more information, see Linda Rowan, Free-floating planets in Orion, Science 288(5467) 773 (May 5,2000).]... 

The coolest stars with just enough mass to fuse hydrogen are the M-dwarfs (see chapter 3). Two new classes of brown dwarfs have been added to the cool end of the stellar spectrum. The L-dwarfs (1,300 to 2,000 Kelvin) are slightly cooler and less massive than M-dwarfs. T-dwarfs are cooler and lighter than the L-dwarfs. Both of these new dwarfs cannot sustain hydrogen fusion. Researchers have recently discovered hundreds of T-dwarfs and tens of L-dwarfs. Even the cool T-dwarfs may have magnetic fields that create occasional stellar flares. [For more information, see Linda Rowan, Cooler dwarf stars, Science 289(5480) 697 (August 4, 2000).]... 

Some astronomers believe that the growing population of extrasolar planets may be misleading. Nearly half of the so-called planets recently discovered orbiting around other stars may actually be brown dwarfs. The standard method for detecting extrasolar planets can only detect the minimum mass of an orbiting object. The actual mass may be much greater. [For more information, see Ron Cowen, Are most extrasolar planets hefty impostors Science News 158(18) 227 (October 28, 2000).]... 

Today, astronomers often analyze the spectra of faint objects in the sky when hunting for brown dwarfs. All stars burn lithium in their cores when a proton collides with the isotope lithium 7,... 

Figure 7.3 Sketch of a protoplanetary disk surrounding a solar-type star showing the regions in which different techniques probe grain and particle sizes. More luminous stars, like Herbig Ae-type stars, will stretch the scale by up to a factor of 10, while less luminous stars, like brown dwarfs, will shrink it by a comparable factor. See Table 8.1 for stellar parameters of young stars.

Thermal processing in protoplanetary nebulae Silicate emission features from six brown dwarf disks... 

Figure 8.1 Mid-infrared spectra reveal the ubiquitous presence of crystalline silicates, even around cool brown dwarfs. From Apai et al. (2005).

---