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A two orbital model for the new Fe-based superconductors is studied using theLanczos method as well as pairing mean-field approximations. Our main goals are(i) to provide a comprehensive analysis of this model using numericaltechniques with focus on half-filling and on the state with two more electronsthan half-filling and (ii) to investigate the nodal structure of the mean-fieldsuperconducting state and compare the results with angle-resolved photoemissiondata. In particular, we provide evidence that at half-filling spin 'stripes',as observed experimentally, dominate over competing states.  Depending on parameters, the state with two more electrons added to halffilling is either triplet or singlet. Since experiments suggest spin singletpairs, our focus is on this state. Under rotation, it transforms as the B_2grepresentation of the D_4h group. We also show that the s+/- pairing operatortransforms as A_1g and becomes dominant only in an unphysical regime of themodel where the undoped state is an insulator. For robust values of theeffective electronic attraction producing the Cooper pairs, assumptioncompatible with recent angle-resolved photoemission (ARPES) results thatsuggesting small Cooper-pair size, the nodes of the two-orbital model are foundto be located only at the electron pockets. Since recent ARPES efforts havesearched for nodes at the hole pockets or only in a few directions at theelectron pockets, our results for the nodal distribution may help to guidefuture experiments. More in general, the investigations reported here aim toestablish several of the properties of the two orbital model. Only a detailedcomparison with experiments will clarify how far this simple model present avalid description of the Fe pnictides.

Properties of a two orbital model for oxypnictide superconductors: Magnetic order, B_2g spin-singlet pairing channel, and its nodal structure