Starting from the second post-Keplerian (2PK) Hamiltonian describing the conservative part of the two-body dynamics in massless scalar-tensor (ST) theories, we build an effective-one-body (EOB) Hamiltonian which is a ν-deformation (where ν = 0 is the test mass limit) of the analytically known ST Hamiltonian of a test particle. This ST-EOB Hamiltonian leads to a simple (yet canonically equivalent) formulation of the conservative 2PK two-body problem, but also defines a resummation of the dynamics which is well-suited to ST regimes that depart strongly from general relativity (GR) and which may provide information on the strong-field dynamics, in particular, the ST innermost stable circular orbit (ISCO) location and associated orbital frequency. Results will be compared and contrasted with those deduced from the ST-deformation of the (5PN) GR-EOB Hamiltonian previoulsy obtained in [Phys. Rev. D95, 124054 (2017)].1 or, adopting the terminology of [3], 2.5 post-Keplerian (PK) order, to highlight the fact that (strong) self-gravity effects are encompassed.
arXiv:1709.09742v1 [gr-qc] 27 Sep 2017With the same motivation this paper proposes a mapping where the ST-EOB Hamiltonian reduces, in contrast with what was done in paper 1, to the scalar-tensor one-body Hamiltonian in the test mass limit, which describes the motion of a test particle in the metric and scalar field generated by a central SSS body. Although the conservative dynamics derived from this Hamiltonian and that proposed in paper 1 (and from the Mirshekari-Will Lagrangian) are the same at 2PK order, when taken as being exact, they define different resummations and hence, a priori different dynamics in the strong field regime which is reached near the last stable orbit. In particular, we shall highlight the fact that our new, ST-centered, EOB Hamiltonian is well-suited to investigate ST regimes that depart strongly from general relativity.The paper is organised as follows : In section II we present the Hamiltonian describing the motion of a test particle orbiting in the metric and scalar field generated by a central body (when written in Just coordinates) in scalar-tensor theories, henceforth refered as the real one-body Hamiltonian. In order for the paper to be self-contained, in section III we recall the expression of the two-body Hamiltonian in the centre-of-mass frame obtained in paper 1 at 2PK order. In section IV we then reduce the two-body problem to an EOB ν-deformed version of the ST one-body problem, by means of a canonical transformation and imposing the EOB mapping relation between their Hamiltonians. We finally study the resummed dynamics it defines ; in particular, we compute the innermost stable circular orbit (ISCO) location and associated orbital frequency in the case of Jordan-Fierz-Brans-Dicke theory. Corrections to general relativity ISCO predictions are compared to the results obtained in paper 1.