The effect of low-velocity, low-mass intruders (collisionless gas) on the dynamical evolution of a binary system

Abstract

More than 20,000 computer-simulated encounters between low-mass (compared to the binary components) intruders and binary systems have been made. These calculations were made in the important (and difficult) limit where the intruder velocity is small compared to the orbital velocity of the binary. The dependence of the encounters on orbital eccentricity, on the mass ratios of the binary components, and on the impact parameter was found. These encounters increase the binding energy of the binary. At zero impact parameter, the average increase in the binding energy is proportional to the square of the binary orbital eccentricity. However, the increase in the binding energy decreases much faster with increasing impact parameter as the eccentricity increases. The net result is that the total cross section for increasing the binding energy of a binary through encounters with low-mass intruders is independent of the orbital eccentricity of the binary. The average increase in the binding energy per encounter is directly proportional to the mass of the intruder (in the limit where its mass is small compared to the binary mass). If the binary components are equally massive, the probability that the intruder is captured into a long-lived orbit which persists for at least 1.5more » x 10/sup 5/ integration steps or about 700 revolutions of the original binary is only about 1% at zero impact parameter. However, the probability of long-term capture climbs rapidly for encounters in which the closest approach, R/sub min/, of the intruder to the binary is greater than 0.5 of the semimajor axis a/sub 0/ of the orbit. It climbs to about 20% at R/sub min//a/sub 0/ = 1.9 and then falls off rapidly at larger R/sub min/. Here the capture is due to the tidal field of the binary. The probability of long-term capture increases as the orbital eccentricity increases. 14 references, 9 figures, 4 tables.« less