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arXiv · 2404.13887

The complete exterior spacetime of spherical Brans-Dicke stars

Bertrand Chauvineau·Hoang Ky Nguyen
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Abstract

We derive the complete expression for the Brans Class I exterior spacetime explicitly in terms of the energy and pressures profiles of a stationary spherisymmetric gravity source. This novel and generic expression is achieved in a parsimonious manner, requiring only a subset of the Brans-Dicke field equation and the scalar equation. For distant orbiting test particles, this expression promptly provides a simple, closed and exact formula of the [textgreek] \textgreek{g} Eddington parameter, which reads {\gamma}_{exact}=(({\omega}+1+({\omega}+2){\Theta})/({\omega}+2+({\omega}+1){\Theta})), where {\Theta} is the ratio of the star's "total pressure" integral over its energy integral. This non-perturbative result reproduces the usual Post-Newtonian (({\omega}+1)/({\omega}+2)) expression in the case of a "Newtonian star", in which the pressure is negligible with respect to the energy density. Furthermore, it converges to the General Relativity value ({\gamma}_{GR}=1) as the star's equation of state approaches that of ultra-relativistic matter (in which case {\Theta} approaches 1), a behavior consistent with broader studies on scalar-tensor gravity. Our derivation underscores the essence of these results involving (1) the key relevant portion of the Brans-Dicke field equations, (2) the uniqueness of the Brans Class I vacuum solution for the non-phantom action, viz. {\omega}>-3/2, and (3) the involvement of only two free parameters in this solution, hence requiring two quantities (energy and pressure integrals) of the mass source to fully characterize the solution. From a practical standpoint, it elucidates how a given stellar interior structure model determines the star's exterior gravitational field and impacts the motions of light objects (such as planets and accretion disks) orbiting it.

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BibTeXRIS

Bertrand Chauvineau, Hoang Ky Nguyen. 2024-04-22. The complete exterior spacetime of spherical Brans-Dicke stars. https://doi.org/10.1016/j.physletb.2024.138803

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