Artist's depiction of two white dwarf stars in the process of merging.
The issue of merging stars and their effects, including on astrophysical processes, has already been discussed at the end of a previous post for the Laboratory Astrophysics prize talk at the 236th AAS meeting, e.g.
AAS 236: Laboratory Astrophysics Prize Lecture...
This was in the context of one way to account for the r -process, as depicted in one slide:
In this case the collision theorized (bottom of slide to right) is either between two neutron stars (NS) or between a neutron star and black hole. But what about the possible merger of two white dwarfs? Recall here that the white dwarf in terms of the Sun's evolution, represents the final stage after it has expended all its hydrogen. It helps here to review the physical changes in the later stage evolution.
When only ten percent hydrogen remains (X= 0.10, Y = 0.87 for fractional abundances, say), the Sun is no longer able to generate sufficient energy from its core nuclear reactions to balance the weight of overlying layers. According to a well-known physical principle (the virial theorem), the Sun’s core must contract. The contraction converts gravitational potential energy into thermal (heat) energy that heats the core.[1] By now, hydrogen burning has moved to a peripheral shell around the core, and is ignited by the core heating process.
The ignition creates radiation pressure that forces the outer shells, layers to expand. This same radiation, however, is now emitted from a much larger surface area. The result of this combination of circumstances is that the Sun becomes a Red Giant. However, this stage also cannot be sustained and as the helium nuclear fuel must eventually be exhausted the pressure-gravity balance for the Red Giant, e.g. dP/dr = - G M(r) r / r2
(where P is the outward gas pressure and r is the density), can no longer continue. In this case the force of gravitational contraction wins out with the end phase being a "white dwarf". This object (the nearest to us is the companion to Sirius, or Sirius B) is mostly constituted of electron-degenerate matter. (Isaac Asimov in his book, The Collapsing Universe', one indicated a teaspoonful of its mass would be 15 tons.)
The latest work on merging white dwarfs is from Dr Mark Holland, lead author of a study on the white dwarf, and a physicist from the University of Warwick. He postulates that a collision of two such dying stars (see graphic) about 13 billion years ago has produced a star heavier than the Sun. This evidence, if confirmed, could help astrophysicists better understand the kind of cosmic events that make life on Earth possible.
The massive star from the merger is currently one of a handful of merged white dwarfs to be identified so far and the first to be discovered from the composition of its gases. Thus, using the tools and techniques of stellar spectroscopy, the team discovered the star in question (identified as WDJO551-4135) was made up of hydrogen and carbon as opposed to hydrogen and helium. This is crucial since it discloses the star has already progressed through hydrogen and helium burning and now has passed carbon burning. (Since the nuclear fuel accessed occurs in stages: hydrogen, then helium, then carbon)
But is this not proof enough? No, what is now needed is to learn more about this merged product's core composition. Only then can the merger hypothesis be proven. A key clue may be that while the average white dwarf mass is 0.6 times the mass of the Sun, this merged product is nearly twice the average mass for white dwarfs. A key aspect of the current finding is that this merged star has a mass of 1.14 the mass of the Sun, or less the the Chandrasekhar limit . The limit is computed to be 1.44 solar masses for a stellar collapsed core, below which the core cannot give rise to an exploding star or supernova.
Another theory related to that of the Univ. of Warwick team is that the star (WDJO551-4135) may have formed from a merger because of its age and temperature. In this sense, the star was found to be traveling faster than 99 % of nearby white dwarfs that appear to have the same cooling age. (The faster a star travels, according to Dr. Hollande, the older and colder it is because it has spent more time extracting energy from its neighbors and burning its available fuel. This suggests the star is older than it appears.)
What we will eagerly anticipate now is forthcoming results about the core composition, particularly as the merged star may be older than it appears.
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[1] According to the virial theorem: 2K + W = 0 for any spherical system in equilibrium, where K is the gas kinetic energy (K = 3/2(y-1)U) and W is the gravitational potential energy. From this one can obtain the binding (or total) energy of a star as: E(S)= K + W. Combining the two equations, E(S) = W/ 2 = -K. Thus, the total energy of the star is negative and equal to half the gravitational potential energy, or the negative of the gas kinetic. Hence, if E(S) decreases, K increases, but W decreases, i.e. contraction.
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