TY - JOUR

T1 - Neutron stars with a generalized Proca hair and spontaneous vectorization

AU - Kase, Ryotaro

AU - Minamitsuji, Masato

AU - Tsujikawa, Shinji

N1 - Funding Information:
R. K. is supported by the Grant-in-Aid for Young Scientists B of the JSPS No. 17K14297. S. T. is supported by the Grant-in-Aid for Scientific Research Fund of the JSPS No. 19K03854 and MEXT KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “Cosmic Acceleration” (No. 15H05890). M. M. was supported by the research grant under the Decree-Law 57/2016 of August 29 (Portugal) through the Fundação para a Ciência e a Tecnologia. M. M. is also grateful for the hospitality at the Tokyo University of Science where this work was initiated.

PY - 2020/7/15

Y1 - 2020/7/15

N2 - In a class of generalized Proca theories, we study the existence of neutron star solutions with a nonvanishing temporal component of the vector field Aμ approaching 0 toward spatial infinity, as they may be the endpoints of tachyonic instabilities of neutron star solutions in general relativity with Aμ=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 βXR, where β is a coupling constant and X=-AμAμ/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 rs of neutron stars for some realistic equations of state and show that the M-rs relations of 0-node and 1-node solutions exhibit a notable difference from those of scalarized solutions in scalar-tensor theories. Finally, we discuss the possible endpoints of tachyonic instabilities.

AB - In a class of generalized Proca theories, we study the existence of neutron star solutions with a nonvanishing temporal component of the vector field Aμ approaching 0 toward spatial infinity, as they may be the endpoints of tachyonic instabilities of neutron star solutions in general relativity with Aμ=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 βXR, where β is a coupling constant and X=-AμAμ/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 rs of neutron stars for some realistic equations of state and show that the M-rs relations of 0-node and 1-node solutions exhibit a notable difference from those of scalarized solutions in scalar-tensor theories. Finally, we discuss the possible endpoints of tachyonic instabilities.

UR - http://www.scopus.com/inward/record.url?scp=85088710323&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85088710323&partnerID=8YFLogxK

U2 - 10.1103/PhysRevD.102.024067

DO - 10.1103/PhysRevD.102.024067

M3 - Article

AN - SCOPUS:85088710323

VL - 102

JO - Physical Review D

JF - Physical Review D

SN - 2470-0010

IS - 2

M1 - 024067

ER -