Deuterium fusion, also calleddeuterium burning, is anuclear fusion reaction that occurs in stars and somesubstellar objects, in which adeuteriumnucleus (deuteron) and aproton combine to form ahelium-3 nucleus. It occurs as the second stage of theproton–proton chain reaction, in which a deuteron formed from twoprotons fuses with another proton, but can also proceed fromprimordial deuterium.
Deuterium (2H) is the most easily fused nucleus available to accretingprotostars,[1] and such fusion in the center of protostars can proceed when temperatures exceed 106 K.[2] The reaction rate is so sensitive to temperature that the temperature does not rise very much above this.[2] The energy generated by fusion drives convection, which carries the heat generated to the surface.[1]
If there were no2H available to fuse, then stars would gain significantly less mass in the pre-main-sequence phase, as the object would collapse faster, and more intensehydrogen fusion would occur and prevent the object from accreting matter.[2]2H fusion allows further accretion of mass by acting as a thermostat that temporarily stops the central temperature from rising above about one million degrees, a temperature not high enough for hydrogen fusion, but allowing time for the accumulation of more mass.[3] When the energy transport mechanism switches from convective to radiative, energy transport slows, allowing the temperature to rise and hydrogen fusion to take over in a stable and sustained way. Hydrogen fusion will begin at107 K.
The rate of energy generation is proportional to theproduct of deuterium concentration, density and temperature. If the core is in a stable state, the energy generation will be constant. If one variable in the equation increases, the other two must decrease to keep energy generation constant. As the temperature is raised to the power of 11.8, it would require very large changes in either the deuterium concentration or its density to result in even a small change in temperature.[2][3] The deuterium concentration reflects the fact that the gases are a mixture of normal hydrogen, helium and deuterium.
The mass surrounding the radiative zone is still rich in deuterium, and deuterium fusion proceeds in an increasingly thin shell that gradually moves outwards as the radiative core of the star grows. The generation of nuclear energy in these low-density outer regions causes the protostar to swell, delaying the gravitational contraction of the object and postponing its arrival on the main sequence.[2] The total energy available by2H fusion is comparable to that released by gravitational contraction.[3]
Due to the scarcity of deuterium in the cosmos, a protostar's supply of it is limited. After a few million years, it will have effectively been completely consumed.[4]
Hydrogen fusion requires much higher temperatures and pressures than does deuterium fusion, hence, there are objects massive enough to burn2H but not massive enough to burn normal hydrogen. These objects are calledbrown dwarfs, and have masses between about 13 and 80 times the mass ofJupiter.[5] Brown dwarfs may shine for a hundred million years before their deuterium supply is burned out.[6]
Objects above the deuterium-fusion minimum mass (deuterium burning minimum mass, DBMM) will fuse all their deuterium in a very short time (~4–50 Myr), whereas objects below that will burn little, and hence, preserve their original2H abundance. "The apparent identification of free-floating objects, orrogue planets below the DBMM would suggest that the formation of star-like objects extends below the DBMM."[7]
The onset of deuterium burning is called deuterium flash.[8] Deuterium burning induced instability after this initial deuterium flash was proposed for very low-mass stars in 1964 by M. Gabriel.[9][10] In this scenario a low-mass star or brown dwarf that is fullyconvective will becomepulsationally unstable due to thenuclear reaction being sensitive to temperature.[10] This pulsation is hard to observe because the onset of deuterium burning is thought to begin at <0.5 Myrs for >0.1M☉ stars. At this timeprotostars are still deeply embedded in theircircumstellar envelopes. Brown dwarfs with masses between 20 and 80MJ should be easier targets because the onset of deuterium burning does occur at an older age of 1 to 10 Myrs.[10][11] Observations of very low-mass stars failed to detect variability that could be connected to deuterium-burning instability, despite these predictions.[12] Ruíz-Rodríguez et al. proposed that the ellipticalcarbon monoxide shell around the young brown dwarfSSTc2d J163134.1-24006 is due to a violent deuterium flash, reminiscent of ahelium shell flash in old stars.[11]
It has been shown that deuterium fusion should also be possible in planets. The mass threshold for the onset of deuterium fusion atop the solid cores is also at roughly 13 Jupiter masses (1MJ =1.889×1027 kg).[13][14]
Though fusion with a proton is the dominant way to consume deuterium, other reactions are possible. These include fusion with another deuteron to formhelium-3,tritium, or more rarelyhelium-4, or with helium to form variousisotopes oflithium.[15]Pathways include:[citation needed]
| 2H | + | 2H | → | 3H | (1.01 MeV) | + | p+ | (3.02 MeV) |
| → | 3He | (0.82 MeV) | + | n | (2.45 MeV) | |||
| 2H | + | 3H | → | 4He | (3.52 MeV) | + | n | (14.06 MeV) |
| 2H | + | 3He | → | 4He | (3.6 MeV) | + | p+ | (14.7 MeV) |