Eccentricity excitation and apsidal resonance capture in the planetary system v Andromedae

The orbits of the outer two known planets orbiting Andromedae are remarkably eccentric. Planet C possesses an orbital eccentricity of e(1) = 0.253. For the more distant planet D, e(2) = 0.308. Previous dynamical analyses strongly suggest that the two orbits are nearly coplanar and are trapped in an apsidal resonance in which Delta(ω) over tilde, the difference between their longitudes of periastron, undergoes a bounded oscillation about 0degrees. Here we elucidate the origin of these large eccentricities and of the apsidal alignment. Resonant interactions between a remnant circumstellar disk of gas lying exterior to the orbits of both planets can smoothly increase e(2). Secular interactions between planets D and C can siphon off the eccentricity of the former to increase that of the latter. Externally amplifying e(2) during the phase of the apsidal oscillation when e(2)/e(1) is smallest drives the apsidal oscillation amplitude toward zero. Thus, the substantial eccentricity of planet C and the locking of orbital apsides are both consequences of externally pumping the eccentricity of planet D over timescales exceeding apsidal precession periods of order 10(4) yr. We explain why the recently detected stellar companion to And is largely dynamically decoupled from the planetary system.