Wood (2005)

Terms and Concepts for Wood (2005)

²⁶Mg* - This is the amount of ²⁶Mg that is created by radioactive decay of ²⁶Al. It is not the same thing as the total amount of ²⁶Mg, as some of this isotope was created by fusion reactions inside stars and inherited by our solar system. To infer the amount of ²⁶Mg created by decay of ²⁶Al, we have to see a correlation between the amount of ²⁶Mg and the amount of total Al present in a sample. We'll say more about this later.

magnetorotation instability - This has to do with the instability that can occur in a rotating magnetic field. In the context used by Wood, it refers to the stability of the magnetic field transferred to the nebula from the protosun. It depends in part on the amount of gas that is ionized and the rate at which the nebular gas revolves around the sun relative to the rate at which the magnetic field is rotating. Ionization will occur in the inner edge of the protoplanetary disk via irradiation from the protosun, as well as in the outer edges of the disk via irradiation from other stars.

Figure 2 - Note that in this plot, distance from the sun increases to the right (increasing "AU" or astronomical units).

Keplerian velocity - This is the velocity of a orbiting body determined by gravity. Kepler showed that for planets, there is a relationship between orbital period (the time to make one orbit) and the semi-major axis of an orbit. This means that the Keplerian orbital velocity depends only on the distance between the sun and the orbiting object, with objects closer to the sun moving faster. The same relationship holds for moons in orbit around a planet (faster movement closer to the planet). For the nebula, this means that bodies too large to be affected by non-gravitational effects (e.g., by gas drag, radiation pressure, or by the Poynting-Robertson Effect or Yarkovsky Effect) would move at Keplerian velocity as they orbit the sun.

density waves and spiral density waves - These are wave-like fluctuations in particle density caused by gravitational effects. They are observed in spiral galaxies and the rings of Saturn and could have been important in the solar nebula, with waves in the protoplanetary disk generated by the sun or by protoplanets in the disk. The density wave pattern moves at a velocity that is not the same as the Keplerian velocity. This means that for most locations (depending on orbital location), there will be a difference in velocity between the wave pattern and the speed at which particles are moving, and this difference can result in shock heating as the waves pass through the particles in the disk. According to Wood, the relative velocity and thus the strength of the shocks increases as one moves closer to the sun (if the pattern is centered on the sun) or away from Jupiter (if the pattern is centered on Jupiter). An analogy to an astrophysical density wave is what I call a traffic accordion. Imagine that cars are driving at normal speed (analogous to Keplerian velocity) until they encounter traffic congestion (analogous to the density wave) and are forced to slow down. Once the congestion eases up, the cars can resume normal speed. Depending on the number of cars and their speeds inside and outside the congestion pulse, it is possible for the location of the pulse itself to move, at a speed that is different than the speed that the cars are moving. One can extend the analogy further by imagining that the cars will experience frictional heating as they enter the traffic pulse (by application of brakes, analogous to shock heating). Whether or not any significant heating occurs depends on the properties of the medium and the strength of the shock. Shock wave heating is a leading candidate to explain the melting process recorded by chondrules.

Color-enhanced image of galaxy M81 showing an obvious spiral wave pattern that represents concentrations of stars.

Image from http://en.wikipedia.org/wiki/Density_wave_theory . This link also has more information about density waves.

Image of the A ring of Saturn obtained by the Cassini orbiter, showing density waves, spiral density waves, and the Encke Gap. Bright areas represent concentrations of ring particles. The density waves seen here have been attributed to the effects of various moons, including Mimas, Prometheus, and Pan. A resonance between ring particles and Mimas creates the Encke Gap, which is analogous to the Kirkwood Gaps seen in the asteroid belt. Although most ring particles have been removed from the Encke Gap, the moon Pan orbits within it.
Credit: NASA.

Snow line - The distance from the protosun in the protoplanetary disk corresponding to the position that water ice becomes stable (unstable closer to the protosun as it is warmer; stable further away).