Description of the Binary Evolution Simulation [Ultracompact Millisecond Pulsars]

You are about to see a simulation of what happens when a 1.5 solar mass star (similar to our own Sun) is in a very close orbit with a neutron star. In fact the orbit is so tight that the (donor) star continuously loses mass to its neutron star companion. Such a system is known as an interacting binary and several of these binaries have been discovered by astronomers (mostly using X-ray telescopes). As soon as the (red) donor star starts to lose hydrogen-rich gas from its surface, that gas forms an accretion disk centered around the neutron star companion. The hydrogen-rich gas eventually spirals inwards because of viscosity and is captured by the neutron star.

Neutron stars are very exotic because they contain more mass than the Sun yet are only about 10 km in radius. For this reason neutron stars have densities in excess of 1 billion tons per teaspoonful! The gas that is captured by the neutron star hits its surface at an angle causing the neutron star to be spun up very quickly. The rotational period with which a neutron star can make one rotation about its axis can be as small as one millisecond (i.e., 1000 rotations per second). If mass transfer is greatly reduced (transient behavior), the fast-spinning neutron star can emit X-ray radiation (and radiation from other parts of the spectrum). Only within the past five have astronomers been able to detect these "accreting millisecond pulsars" using orbiting X-ray telescopes located far above the Earth's atmosphere. By the end of 2003, four of these objects had been discovered.

As can be seen from the chronometer on the upper right-hand side of the animation, it takes thousands of megayears (Myr) before the star fills its critical Roche lobe and mass transfer commences. During the mass transfer phase of the evolution, the mass of the red donor star decreases substantially while its orbital period and separation remain approximately constant. The distance between the donor star and its neutron star companion can be estimated using the ruler superimposed on the screen. One unit on the ruler is equivalent to a distance corresponding to the radius of our Sun (i.e., a solar radius). Also note that the zero-point on the ruler is fixed to the position of the center of mass of the binary. The position of the observer (you!) is attached to the corotating frame of reference (i.e., moving with the orbital motion of the binary system) and thus the two binary components are not seen to exhibit any angular (orbital) motion.

As the donor evolves it continuously burns hydrogen (converting it into helium) in a region located at its center. If there is not sufficient time for hydrogen to be completely depleted at the center, a core of helium gas is not formed (unlike the BMSP evolution seen in the previous animation). When the mass of the donor has been reduced to approximately 0.1 solar masses (one-tenth of the mass of the Sun), the radius of donor contracts rapidly. This leads to a very sharp decrease in the orbital separation and concomitantly the orbital period. The orbital period attains a minimum value of about 30 minutes and then starts to increase. This increase coincides with an increase in the radius of the donor. The donor expands because the pressure that supports it from collapsing changes from an "ideal gas pressure" to an "electron degeneracy pressure". A White Dwarf is a good example of a type of star that depends on electron degeneracy pressure to support its structure.

© 2003 Lorne Nelson

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