Differentially rotating neutron stars in scalar-tensor theories of gravity

Abstract

We present the first numerical models of differentially rotating stars in alternative theories of gravity. We chose a particular class of scalar-tensor theories of gravity that is indistinguishable from GR in the weak field regime but can lead to significant deviations when strong fields are considered. We show that the maximum mass that a differentially rotating neutron star can sustain increases significantly for scalarized solutions and such stars can reach larger angular momenta. In addition, the presence of a nontrivial scalar field has the effect of increasing the required axis ratio for reaching a given value of angular momentum, when compared to a corresponding model of same rest mass in general relativity. We find that the scalar field also makes rapidly rotating models less quasi-toroidal than their general-relativistic counterparts. For large values of the angular momentum and values of the coupling parameter that are in agreement with the observations, we find a second turning point for scalarized models along constant angular momentum sequences, which could have interesting implications for the stability of remnants created in a binary neutron star merger. * Electronic address: daniela.doneva@uni-tuebingen.de † Electronic address: yazad@phys.uni-sofia.bg ‡ Electronic address: niksterg@auth.gr § Electronic address: kostas.kokkotas@uni-tuebingen.de

Gauss-Bonnet black holes with a massive scalar field

“…For this reason, in the present paper we will present only the solutions with no zeros of the scalar field. We should note, that the Schwarzschild solutions is also unstable for black hole masses below the mass corresponding to the bifurcation point [37]. The points of bifurcation for different masses of the scalar field can be best observed in Fig.…”

“…In our numerical solutions we shall use β = 12 but similar results are observed for other values of β as well. We refer the reader to [37] for an extensive discussion of the influence of the parameter β on the properties of the scalarized solutions in the massless case. As we know from the massless scalar field case [28,37], for such coupling in addition to the Schwarzschild solution additional scalarized solutions branch off the GR one that can be labeled The value of the scalar field on the horizon versus the rescaled mass of the black hole for the exponential coupling function (19).…”

Ferromagnetic neutron stars in scalar-tensor theories of gravity

“…Consideration of the values smaller than the lower limit from the observations is justified because in the present study which is the first investigation of the scalarized FM neutron stars, we are interested on the effects of the coupling constant on the scalarization of these stars. In addition, the calculations with β < −4.5 in the massless case give the upper limit for the deviations from GR in the massive case [38]. We also calculate the maximum value of the coupling constant at which the spontaneous scalarization takes place in NFM and FM neutron stars.…”

“…These theories which include one or more scalar fields coupled to matter, predict scalar gravitational wave that can be detected by the laser interferometer [22,23]. These theories of gravity have been extensively applied to investigate the relativistic compact objects [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. It has been confirmed that a wide class of STTs passes the weak-field gravitational tests indicating nonperturbative strong-field deviations away from GR in neutron stars [24].…”

“…Applying scalar-tensor theory with massive field with self-interaction term in the potential to the neutron star models demonestrates that the self-interaction term suppresses the scalar field and large deviations from pure GR is observed [37]. Using a class of scalar-tensor theories of gravity indistinguishable from GR in the weak field regime but with significant deviations in strong fields, it has been shown that the maximum mass of a differentially rotating neutron star increases significantly for scalarized solutions and such stars can reach larger angular momenta [38]. Regarding the above discussions, the EOS of ferromagnetic neutron matter can have significant effects on the scalarized neutron stars.…”