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Justin's Astronomy

Brown Dwarfs
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History

Brown dwarfs were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (black dwarfs currently refer to something different).

Early stellar models suggested that a true star requires a mass at least 80 times that of Jupiter to support such fusion. Dense star-like objects with smaller masses, or "brown dwarfs," were hypothesized by the early 1960s -- formed much the way stars are formed, they would however be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise for a few decades after that to firmly identify any brown dwarfs.

More recently, it has been hypothesized that, depending on the compounds that make up a growing stellar object, the critical mass for star-like hydrogen fusion could be as large as 90 Jupiter masses; and on the other end of the spectrum, that substellar objects formed quickly from a collapsing nebula could produce brown dwarfs smaller than 13 Jupiter masses, which nevertheless experience no fusion at all.

Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. They are now believed to be the most numerous type of body in the Milky Way. Brown dwarfs close to Earth include Epsilon Indi Ba and Bb, a pair of dwarfs around 12 light-years from Sun.

In theory

Distinguishing heavy brown dwarfs from light stars

Lithium: Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of Lithium-7 and a proton producing two Helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pionereed by Rafael Rebolo and colleagues.

  • However, lithium is also seen in very young stars, which have not yet had a chance to burn it off. Heavier stars like our sun can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size.
  • Contrariwise, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs massing 65-80 MJ can burn off their lithium by the time they are half a billion years old[Kulkarni]. So this test is not perfect.

Methane: Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane. Dwarfs confirmed in this fashion include Gliese 229B.

Luminosity: Mainline stars cool, but eventually reach a minimum luminosity which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old dwarfs will be too faint to be a star.

Distinguishing light brown dwarfs from large planets

A remarkable property of brown dwarfs is that they are all roughly the same radius, more or less the radius of Jupiter. At the high end of their mass range (60-90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron degeneracy pressure, as it is in white dwarfs; at the low end of the range (1-10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10-15% over the range of possible masses. This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after something on the order of 10 million years. However, there are other ways to distinguish dwarfs from planets:

Density is a clear giveaway. Brown dwarfs are all about the same radius and volume; so anything that size with over 10 Jupiter masses is unlikely to be a planet.

X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet like temperatures (under 1000 K).

In practice

Typical atmospheres of known brown dwarfs range in temperature from 300 to over 3000 K, in comparison with stars, which cool to minimum temperatures of around 4000 K. Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool more slowly than less massive ones.

Observational techniques

Coronographs have recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.
Sensitive telescopes equipped with charge-coupled devices (CCDs) have been used to search distant star clusters for faint objects, including Teide 1.
Wide-field searches have identified individual faint objects, such as Kelu-1 (30 ly away)

Milestones

First methane brown dwarf verified. Gliese 229B is discovered orbiting red dwarf Gliese 229A (20 ly away) using an Adaptive Optics Chronograph to sharpen images from the 60 inch (1.5 m) reflecting telescope at Palomar Observatory on southern California's Mt. Palomar; followup infrared spectroscopy made with their 200 inch (5 m) Hale telescope shows an abundance of methane.
  • 1998: First X-ray-emitting brown dwarf found. Cha Halpha 1, an M8 object in the Chamaeleon I dark cloud, is determined to be an X-ray source, similar to convective late-type stars.
  • December 15, 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoring LP 944-20 (60 Jupiter masses, 16 ly away) via the Chandra X-ray observatory, catches a 2-hour flare.
  • 27 July 2000: First radio emission (in flare and quiescence) detected from a brown dwarf. A team of students at the Very Large Array reported their observations of LP 944-20 in the 15 March 2001 issue of the British journal Nature.

Recent developments

Recent observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely violent, comparable to but far exceeding Jupiter's famous storms.

X-ray flares detected from brown dwarfs since late 1999 suggest changing magnetic fields within them, similar to those in very low-mass stars.

A brown dwarf Cha 110913-773444 located 500 light years away in the constellation Chamaeleon may be in the process of forming a mini solar system. Astronomers from Pennsylvania State University have detected what they believe to be a disk of gas and dust similar to the one hypothized to have formed our own solar system. Cha 110913-773444 is the smallest brown dwarf found to date (8 Jupiter masses)and if it formed a solar system it would be the smallest object to have one. Their findings will be published in the Dec. 10 issue of the Astrophysical Journal Letters

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