Dynamics of Four Triple Systems

Abstract

Orbital motions in four hierarchical stellar systems discovered by speckle interferometry are studied. Their inner orbits are relatively well constrained, while the long outer orbits are less certain. The eccentric and misaligned inner orbits in the early-type hierarchies ϵ Cha (B9V, central star of the 5 Myr old association, P = 6.4 yr, e = 0.73) and I 385 (A0V, P ∼ 300 yr, e ∼ 0.8) suggest past dynamical interactions. Their nearly equal masses could be explained by a dynamical decay of a 2+2 quadruple progenitor consisting of four similar stars. However, there is no evidence of the associated recoil, so similar masses could be just a consequence of accretion from the same core. The other two hiearchies, HIP 32475 (F0IV, inner period 12.2 yr) and HIP 42910 (K7V, inner period 6.8 yr), have smaller masses and are double twins where both inner and outer mass ratios are close to one. A double twin could either result from a merger of one inner pair in a 2+2 quadruple or can be formed by a successive fragmentation followed by accretion.