Neutron stars with a generalized Proca hair and spontaneous vectorization

Ryotaro Kase; Masato Minamitsuji; Shinji Tsujikawa

doi:10.1103/PhysRevD.102.024067

paper

Neutron stars with a generalized Proca hair and spontaneous vectorization

arXiv:2001.10701 · doi:10.1103/PhysRevD.102.024067

Abstract

In a class of generalized Proca theories, we study the existence of neutron star solutions with a nonvanishing temporal component of the vector field $A_{μ}$ approaching 0 toward spatial infinity, as they may be the endpoints of tachyonic instabilities of neutron star solutions in general relativity with $A_{μ} = 0$ . Such a phenomenon is called spontaneous vectorization, which is analogous to spontaneous scalarization in scalar-tensor theories with nonminimal couplings to the curvature or matter. For the nonminimal coupling $β X R$ , where $β$ is a coupling constant and $X = - A_{μ} A^{μ} /2$ , we show that there exist both 0-node and 1-node vector-field solutions, irrespective of the choice of the equations of state of nuclear matter. The 0-node solution, which is present only for $β = - O (0.1)$ , may be induced by some nonlinear effects such as the selected choice of initial conditions. The 1-node solution exists for $β = - O (1)$ , which suddenly emerges above a critical central density of star and approaches the general relativistic branch with the increasing central density. We compute the mass $M$ and radius $r_{s}$ of neutron stars for some realistic equations of state and show that the $M$ - $r_{s}$ relations of 0-node and 1-node solutions exhibit notable difference from those of scalarized solutions in scalar-tensor theories. Finally, we discuss the possible endpoints of tachyonic instabilities.

19 pages, 5 figures, published version

Neutron stars with a generalized Proca hair and spontaneous vectorization

Abstract

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