Region of motion in the Sitnikov four-body problem when the fourth mass is finite

This article deals with the region of motion in the Sitnikov four-body problem where three bodies (called primaries) of equal masses fixed at the vertices of an equilateral triangle. Fourth mass which is finite confined to moves only along a line perpendicular to the instantaneous plane of the motions of the primaries. Contrary to the Sitnikov problem with one massless body the primaries are moving in non-Keplerian orbits about their centre of mass. It is investigated that for very small range of energy h the motion is possible only in small region of phase space. Condition of bounded motions has been derived. We have explored the structure of phase space with the help of properly chosen surfaces of section. PoincarS surfaces of section for the energy range -0.480a parts per thousand currency signha parts per thousand currency signa'0.345 have been computed. We have chosen the plane (q (1),p (1)) as surface of section, with q (1) is the distance of a primary from the centre of mass. We plot the respective points when the fourth body crosses the plane q (2)=0. For low energy the central fixed point is stable but for higher value of energy splits in to an unstable and two stable fixed points. The central unstable fixed point once again splits for higher energy into a stable and three unstable fixed points. It is found that at h=-0.345 the whole phase space is filled with chaotic orbits.