Lithium burning is a nucleosynthetic process in which lithium is depleted in a star. Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which by definition must achieve the high temperature (2.5 × 10^6 K) necessary for fusing hydrogen, rapidly deplete their lithium.
7Li
Burning of the most abundant isotope of lithium, lithium-7, occurs by a collision of 7Li 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 low-mass 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.
6Li
From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that lithium burning by the P-P chain, during the last highly convective and unstable stages during the pre–main sequence later phase of the Hayashi contraction may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium into deeper layers where it is destroyed. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum. This causes an increased rate of lithium loss with age. Lithium burning will also increase with higher temperatures and mass, and will last for at most a little over 100 million years.
The P-P chain for lithium burning is as follows
p + 6 3Li → 7 4Be (unstable)
7 4Be + e− → 7 3Li + ν
p + 7 3Li → 8 4Be (unstable)
8 4Be → 2 4 2He + energy
It will not occur in stars less than sixty times the mass of Jupiter. In this way, the rate of lithium depletion can be used to calculate the age of the star.
Lithium test
The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test. 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. Brown dwarfs at the high end of their mass range (60–75 MJ) can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 MJ can burn off their lithium by the time they are half a billion years old; thus, this test is not perfect.[1]
See also
Cosmological lithium problem
Dilithium
Halo nucleus
Isotopes of lithium
Lithium
References
Basri, G. (1998). Rafael Rebolo; Eduardo L. Martin; Maria Rosa Zapatero Osorio (eds.). The Lithium Test for Young Brown Dwarfs (invited review). Proceedings of a Workshop held in Puerto de la Cruz, Tenerife, Spain, 17–21 March 1997, ASP Conference Series #134. p. 394. Bibcode:1998ASPC..134..394B.
Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track
Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova
Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]
White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster
Hypothetical
Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar
Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova
Structure
Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism
Properties
Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram
Star systems
Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system
Earth-centric
observations
Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard
Lists
Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy
Related articles
Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event
vte
Nuclear processes
Radioactive decay
Alpha decay Beta decay Gamma radiation Cluster decay Double beta decay Double electron capture Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission
Deuterium fusion Lithium burning pp-chain CNO cycle α process Triple-α C burning Ne burning O burning Si burning r-process s-process p-process rp-process
Other
processes
Photodisintegration Photofission
Capture
Electron capture Neutron capture Proton capture
Exchange
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
