Description of the Binary Evolution Simulation [Low-Mass BMSP]

You are about to see a simulation of what happens when a one 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). Once mass transfer stops, the fast-spinning neutron star can freely emit two cones (jets) of mostly radio-frequency radiation and is properly referred to as a radio pulsar. If the radiation is emitted in the direction of Earth, it is possible for astronomers to observe the pulses of radiation with a radio telescope (i.e., similar to what we would observe from a lighthouse).

As can be seen from the chronometer on the upper right-hand side of the animation, the mass-transfer phase of the evolution can last for millions of years (Myr). During this time, the mass of the red donor star decreases while its orbital period and separation increase. 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 in time its surface temperature changes (red indicates a "cool" temperature of about 3000 Celsius and blue indicates a temperature about ten times larger). During its evolution, the donor continuously burns hydrogen (converting it into helium) in a region located at its center. Once the hydrogen is depleted at the center a core of helium gas is formed. When the mass of the donor has been reduced to approximately 0.25 solar masses (one-quarter of the mass of the Sun), almost all of the available hydrogen has been burned and the donor begins to collapse around its helium core. Mass transfer also ceases at this point and thus the pulsar can "turn on". However as the remaining hydrogen-rich gas in the donor is compressed by gravity, it heats up and this leads to a brief but vigorous phase of nuclear burning known as a thermonuclear flash. Because this phenomenon happens so quickly, the new chronometer seen at the center of the screen measures time in years (yr), not millions of years. The donor becomes so large that it undergoes a new episode of mass loss but again collapses thereby ending mass transfer. Another very brief flash occurs but so little hydrogen remains that not enough energy can be generated to expand the donor noticeably. The donor then contracts for a final time and is destined to cool forever (similar to a piece of charcoal in a barbecue). Astronomers refer to the star as being a white dwarf. Even though this white dwarf star becomes fainter and fainter astronomers have been able to detect these helium-rich remnants in orbit about pulsars using extremely sensitive optical telescopes. Approximately 20 of them have been detected to date.

© 2003 Lorne Nelson

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