Abstract
It is well known that neutron stars can undergo a phase transition under a certain class of Scalar Tensor Theories of gravity (STT's) where a new order parameter, the scalar charge, appears within the star. This is the well known phenomenon of spontaneous scalarization (SC) discovered by Damour and Esposito-Farèse in 1993. Under such mechanism neutron stars can afford in principle a maximum mass larger than in general relativity (GR) for a given equation of state without taking into account additional observational constraints (e.g. binary systems). This opens the possibility that neutron stars might be formed with masses as large as ∼2M⊙ without the need of stiff, or more exotic, equations of state for the nuclear matter. Thus, STT's through SC may account for compact objects with large masses observed recently in the sky in the form of pulsars (PSR J0348+0432 with M=2.01M±0.04⊙ observed in 2013, PSR J1614-2230 with M=1.97±0.04M⊙ observed in 2010 or J0740+6620 M=2.14−0.09+0.10M⊙ observed in 2019). However, we argue that even if that was possible such maximum mass models within STT cannot be formed solely from the dynamic transition of an initial “isolated” unscalarized neutron star whose mass cannot exceed the maximum mass in GR. This is because SC, being an energetic-preferred configuration, produces a final static star with a mass lower that the initial one with a fixed baryon mass. The mass difference between the initial and final configurations is radiated away in the form of a scalar-field. Thus, maximum mass models of scalarized neutron stars, if present in nature, must have formed by a different process, perhaps of cosmological origin or by the subsequent accretion of additional scalar charge and mass.
Highlights
Relativistic theories of gravity different from general relativity (GR) [1,2,3,4,5] have received a major interest in recent years for various reasons
Following the Damour and Esposito-Farese (DEF) discovery, subsequent analyses showed that SC is robust in that it is independent of the equation of state (EOS) for the nuclear matter adopted to model a neutron star [24,25,26,27]
This is the most important contribution of this letter, we argue that if such massive neutron stars are explained by the SC transition, the latter must be accompanied by an additional process, maybe of cosmological origin or by accretion, because SC, at least in its standard conception, requires that the initial configuration has a mass that cannot be larger that the maximum mass allowed by the unscalarized neutron star which maybe lower than 2M⊙ if the EOS considered is not very stiff [32, 33]
Summary
Relativistic theories of gravity different from general relativity (GR) [1,2,3,4,5] have received a major interest in recent years for various reasons. Near the threshold of instability towards SC the initial configuration is perturbed by a Gaussian scalar-field profile, and a numerical evolution is performed until reaching the final state of the system which corresponds to a static scalarized neutron star with gravitational mass lower than the initial one but with the same initial baryon mass This analysis has several consequences, both theoretical and observational. A detailed description of the specific equations in spherical symmetry and their numerical implementation will be provided in a forthcoming investigation where a thorough study about this subject matter will be reported, which include the collapse of a neutron star into a black hole and the radiation in the form of scalar gravitational waves associated with the NMC field φ [53]. In a more detailed report g rr we plan to provide a more complete set of plots showing the evolution of other interesting variables
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
