  Arbitrary amplitude dust acoustic solitary structures have been investigated
in a four component multi-species plasma consisting of negatively charged dust
grains, nonthermal ions, isothermally distributed electrons and positrons
including the effect of dust temperature. We have used the Sagdeev
pseudo-potential method to discuss the arbitrary amplitude steady state dust
acoustic solitary structures in the present plasma system. We have designed a
computational scheme to draw the existence domains of different dust acoustic
solitary structures. We have observed only negative potential solitary waves
for isothermal ions. But for strong nonthermality of ions the system supports
positive potential solitary waves, positive potential double layers and
coexistence of solitary waves of both polarities. The positive potential
solitary waves are restricted by the positive potential double layers but
negative potential double layer has not been found for any parameter regime.
The system does not support dust acoustic supersoliton of any polarity. The
concentration of positrons plays an important role in the formation of positive
potential double layers. Finally, the phase portraits of the dynamical system
have been presented to confirm the existence of different dust acoustic
solitary structures.
