Publication: On neutron stars in ƒ (R) theories: Small radii, large masses and large energy emitted in a merger
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2016-09
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Cruz Dombriz, Álvaro de la
Zapatero Castrillo, Víctor
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Elsevier B.V.
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In the context of ƒ (R) gravity theories, we show that the apparent mass of a neutron star as seen from an observer at infinity is numerically calculable but requires careful matching, first at the star’s edge, between interior and exterior solutions, none of them being totally Schwarzschild-like but presenting instead small oscillations of the curvature scalar R; and second at large radii, where the Newtonian potential is used to identify the mass of the neutron star. We find that for the same equation of state, this mass definition is always larger than its general relativistic counterpart. We exemplify this with quadratic R^2 and Hu-Sawicki-like modifications of the standard General Relativity action. Therefore, the finding of two-solar mass neutron stars basically imposes no constraint on stable ƒ (R) theories. However, star radii are in general smaller than in General Relativity, which can give an observational handle on such classes of models at the astrophysical level. Both larger masses and smaller matter radii are due to much of the apparent effective energy residing in the outer metric for scalar-tensor theories. Finally, because the ƒ (R) neutron star masses can be much larger than General Relativity counterparts, the total energy available for radiating gravitational waves could be of order several solar masses, and thus a merger of these stars constitutes an interesting wave source.
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© 2016 Elsevier B.V. or its licensors or contributors.
We thank authors of [31] (used in Section 5) and [32] (used in Sections 6 and 7) for providing us their numerical data for the corresponding equations of state. We would like to thank Antonio Dobado, Radouane Ganouji, Antonio L. Maroto and Sergei Odintsov for the comprehensive reading of the manuscript. This work has been supported by grants MINECO:FPA2014-53375-C2-1-P, MINECO:FIS2014-52837-P, UCM:910309, Consolider-Ingenio MULTIDARK CSD2009-00064 and CPAN. Additionally, A.d.l.C.D. acknowledges partial financial support from University of Cape Town Launching Grants Programme and National Research Foundation grant 99077 2016-2018, Ref. No. CSUR150628121624 and CSIC I-LINK1019.
A.d.l.C.D. also thanks the CANTATA/CA15117 action supported by COST (European Cooperation in Science and Technology) and the hospitality of the Yukawa Institute for Theoretical Physics (Kyoto, Japan) and the Instituto de Física Teórica (IFT UAM-CSIC, Madrid Spain) for its support via the KA107 action for international Mobility during the latest stages of the manuscript. F.J.Ll-E. thanks the hospitality of the Institute for Nuclear Theory of the Univ. of Washington, Seattle, with DOE support, as well as the Spanish Excellence Network on Hadronic Physics FIS2014-57026-REDT.
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