IK Pegasi - Future Evolution

Future Evolution

In a 1993 paper, David Wonnacott, Barry J. Kellett and David J. Stickland identified this system as a candidate to evolve into a Type Ia supernova or a cataclysmic variable. At a distance of 150 light years, this makes it the nearest known candidate supernova progenitor to the Earth. However, in the time it will take for the system to evolve to a state where a supernova could occur, it will have moved a considerable distance from Earth but may yet pose a threat.

At some point in the future, IK Pegasi A will consume the hydrogen fuel at its core and start to evolve away from the main sequence to form a red giant. The envelope of a red giant can grow to significant dimensions, extending up to a hundred times its previous radius (or larger). Once IK Pegasi A expands to the point where its outer envelope overflows the Roche lobe of its companion, a gaseous accretion disk will form around the white dwarf. This gas, composed primarily of hydrogen and helium, will then accrete onto the surface of the companion. This mass transfer between the stars will also cause their mutual orbit to shrink.

On the surface of the white dwarf, the accreted gas will become compressed and heated. At some point the accumulated gas can reach the conditions necessary for hydrogen fusion to occur, producing a runaway reaction that will drive a portion of the gas from the surface. This would result in a (recurrent) nova explosion—a cataclysmic variable star—and the luminosity of the white dwarf rapidly would increase by several magnitudes for a period of several days or months. An example of such a star system is RS Ophiuchi, a binary system consisting of a red giant and a white dwarf companion. RS Ophiuchi has flared into a (recurrent) nova on at least six occasions, each time accreting the critical mass of hydrogen needed to produce a runaway explosion.

It is possible that IK Pegasi B will follow a similar pattern. In order to accumulate mass, however, only a portion of the accreted gas can be ejected, so that with each cycle the white dwarf would steadily increase in mass. Thus, even should it behave as a recurring nova, IK Pegasus B could continue to accumulate a growing envelope.

An alternate model that allows the white dwarf to steadily accumulate mass without erupting as a nova is called the close-binary supersoft x-ray source (CBSS). In this scenario, the mass transfer rate to the close white dwarf binary is such that a steady fusion burn can be maintained on the surface as the arriving hydrogen is consumed in thermonuclear fusion to produce helium. This category of super-soft sources consist of high-mass white dwarfs with very high surface temperatures (0.5 × 106 to 1 × 106 K).

Should the white dwarf's mass approach the Chandrasekhar limit of 1.38 solar masses it will no longer be supported by electron degeneracy pressure and it will undergo a collapse. For a core primarily composed of oxygen, neon and magnesium, the collapsing white dwarf is likely to form a neutron star. In this case, only a fraction of star's mass will be ejected as a result. If the core is instead made of carbon-oxygen, however, increasing pressure and temperature will initiate carbon fusion in the center prior to attainment of the Chandrasekhar limit. The dramatic result is a runaway nuclear fusion reaction that consumes a substantial fraction of the star within a short time. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion.

Such a supernova event may pose some threat to life on the Earth. It is thought that the primary star, IK Pegasi A, is unlikely to evolve into a red giant in the immediate future. As shown previously, the space velocity of this star relative to the Sun is 20.4 km/s. This is equivalent to moving a distance of one light year every 14,700 years. After 5 million years, for example, this star will be separated from the Sun by more than 500 light years. A Type Ia supernova within a thousand parsecs (3300 light-years) is thought to be able to affect the Earth, but it must be closer than about 10 parsecs (around thirty light-years) to cause a major harm to the terrestrial biosphere.

Following a supernova explosion, the remnant of the donor star (IK Pegasus A) would continue with the final velocity it possessed when it was a member of a close orbiting binary system. The resulting relative velocity could be as high as 100–200 km/s, which would place it among the high-velocity members of the galaxy. The companion will also have lost some mass during the explosion, and its presence may create a gap in the expanding debris. From that point forward it will evolve into a single white dwarf star. The supernova explosion will create a remnant of expanding material that will eventually merge with the surrounding interstellar medium.