2016/06/30 by Eugeny Babichev, Kazuya Koyama, David Langlois +2 · 1 citation
Physics and Astronomy · #astro-ph.CO #gr-qc #hep-th
paper · pdf · doi:10.1088/0264-9381/33/23/235014
published as 2016 Class. Quantum Grav. 33 235014 · 13 pages, five figures. Updated to reflect published version. Results unchanged but added plots for positive Upsilon
arxiv created 2016/11/08 · arxiv updated 2016/11/16
This work studies relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter, focusing on a model based on the quartic beyond Horndeski Lagrangian. We self-consistently derive the scalar field profile for static spherically symmetric objects in asymptotically de Sitter space-time and show that the Vainshtein breaking branch of the solutions is the physical branch thereby resolving several ambiguities with non-relativistic frameworks. The geometry outside the star is shown to be exactly Schwarzschild-de Sitter and therefore the PPN parameter β\rm PPN=1, confirming that the external screening works at the post-Newtonian level. The Tolman-Oppenheimer-Volkoff (TOV) equations are derived and a new lower bound on the Vainshtein breaking parameter Υ1>-4/9 is found by requiring the existence of static spherically symmetric stars. Focusing on the unconstrained case where Υ1<0, we numerically solve the TOV equations for polytropic and realistic equations of state and find stars with larger radii at fixed mass. Furthermore, the maximum mass can increase dramatically and stars with masses in excess of 3M_\odot can be found for relatively small values of the Vainshtein breaking parameter. We re-examine white dwarf stars and show that post-Newtonian corrections are important in beyond Horndeski theories and therefore the bounds coming from previous analyses should be revisited.