Eccentricity Excitation and Apsidal Resonance Capture in the Planetary System υ Andromedae

The orbits of the outer two known planets orbiting Upsilon Andromedae areremarkably eccentric. Planet C possesses an orbital eccentricity of e1 = 0.253.For the more distant planet D, e2 = 0.308. Previous dynamical analyses stronglysuggest that the two orbits are nearly co-planar and are trapped in an apsidalresonance in which the difference between their longitudes of periastronundergoes a bounded oscillation about 0 degrees. Here we elucidate the originof these large eccentricities and of the apsidal alignment. Resonantinteractions between a remnant circumstellar disk of gas lying exterior to theorbits of both planets can smoothly grow e2. Secular interactions betweenplanets D and C can siphon off the eccentricity of the former to grow that ofthe latter. Externally amplifying e2 during the phase of the apsidaloscillation when e2/e1 is smallest drives the oscillation amplitude towardszero. Thus, the substantial eccentricity of planet C and the locking of orbitalapsides are both consequences of externally pumping the eccentricity of planetD over timescales exceeding apsidal precession periods of order 1e4 yr. Weexplain why the recently detected stellar companion to Upsilon Andromedae islargely dynamically decoupled from the planetary system.Comment: accepted to Ap