Orbital motions and the conservation-law/preferred-frame $\alpha_3$ parameter

Abstract

We analytically calculate some orbital effects induced by the Lorentz-invariance/momentum-conservation PPN parameter $\alpha_3$ in a gravitationally bound binary system made of a compact primary orbited by a test particle. We neither restrict ourselves to any particular orbital configuration nor to specific orientations of the primary's spin axis $\boldsymbol{\hat{\psi}}$. We use our results to put preliminary upper bounds on $\alpha_3$ in the weak-field regime by using the latest data from Solar System's planetary dynamics. By linearly combining the supplementary perihelion precessions $\Delta\dot\varpi$ of the Earth, Mars and Saturn, determined with the EPM2011 ephemerides, we infer $|\alpha_3|\lesssim 6\times 10^{-10}$. Our result is about 3 orders of magnitude better than the previous weak-field constraints existing in the literature, and of the same order of magnitude of the bound expected from the future BepiColombo mission to Mercury. It is, by construction, independent of the other preferred-frame PPN parameters $\alpha_1,\alpha_2$, both preliminarily constrained down to a $\approx 10^{-6}$ level. The wide pulsar-white dwarf binary PSR J0407+1607 yields a preliminary upper bound on the strong-field version $\hat{\alpha}_3$ of the Lorentz-invariance/momentum-conservation PPN parameter of the order of $3\times 10^{-17}$. It relies upon certain assumptions on the unknown values of the pulsar's spin axis orientation $\boldsymbol{\hat{\psi}}$, the orbital node $\Omega$ and the inclination $I$. Neither the pulsar's proper motion, still undetected, nor a possible value of the pulsar's mass $m_{\rm p}$ up to two solar masses substantially affect our result.