How do you discover brown dwarfs?

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Finding brown dwarfs

Brown dwarfs are small, cool objects - they become very faint, very quickly. That makes them difficult to detect. Ideally, we would like to know how many there are in the general stellar population of the Milky Way - but the first step was simply finding one, and proving that brown dwarfs actually exist. That result wasn't established until a mere 4-5 years ago, and culminated from the application of several search techniques. Next

look in young star clusters Next

such as the Pleiades Next

or younger clusters, such as the Jewel Box Next

A complication: while young brown dwarfs are hotter and brighter than old brown dwarfs, they have the same colours and spectral energy distribution as ordinary, main-sequence M dwarfs. How can we distinguish substellar mass brown dwarfs from M dwarfs, which are simply low-mass, hydrogen-burning stars? Next

Nuclear reactions, combined with the internal structure of cool dwarfs, combined to provide a discrimination mechanism. Late-type M dwarfs are fully convective - circulation currents carry material from the surface of the star deep into the core and out again on relatively short timescales (few years). Hence, all the material that we see on the stellar surface has passed through the much higher temperatures within the stellar core. Next

One of the primordial elements present in every star and brown dwarf at formation is lithium. Lithium is a relatively fragile element, which is destroyed by nuclear fusion reactions at temperatures above about 2 million degrees Celsius
Hydrogen burns to helium at a temperature of about 3 million degrees Celsius
hence, if hydrogen fusion is taking place, lithium will be destroyed
the corollary is that if you find a late-type M dwarf with detectable lithium, it's a brown dwarf
Next

Lithium is difficult to detect in such faint objects, but several brown dwarf candidates from the early 1990s have been confirmed as brown dwarfs, including some objects in the Pleiades.
It was a different search technique, however, which turned up the first unequivocal example of a brown dwarf: searching for companions to stars known to be relatively close to the Sun. There were two major discoveries: GD 165B, a companion to a white dwarf, and Gl 229B, a companion to an early-type M dwarf. Next

GD 165 is a fairly anonymous white dwarf star, discovered in the mid 1960s:
distance ~ 100 light years, luminosity ~ 10^-3 solar luminosities
remnant core of a 1.5-3 solar mass star, T ~ 14,000 degrees, R ~ 0.01 R(sun) ~ R (Earth)
1988: Ben Zuckerman & Eric Becklin discovered that is has a very red companion Next

...with a weird spectrum! Did this reflect observational problems? atmospheric pollution? or a new class of object? Next

Palomar 60-inch dome

Palomar 60-inch telescope Next

Then came Gl 229B - discovered in 1995 by Tadashi Nakajima, Shri Kulkarni, Dave Golimowski and collaborators using a coronagraphic camera on the Palomar 60-inch telescope. Next

The primary star is an M dwarf only 17 light years from the Sun, L ~ 0.02 L(sun)
Gl 229B is 1000 times fainter at infrared wavelengths, and more than 10⁶ times fainter at visual wavelengths - clearly far too faint to be a star
But, while very red in optical/infrared colours, Gl 229B has blue colours at infrared wavelengths - why? Next

The answer is - methane absorption. Gl 229B has a temperature of only 1000 degrees, cool enough for methane to form, as in Jupiter's atmosphere. Gl 229B is closer in temperature to Jupiter and the gas giants than to M dwarfs, and is very obviously not a star! Next

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