Neutron Star Properties with Hyperons

The recent observation of a 1.97±0.04M⊙ millisecond pulsar, PSR J1614-2230, by Demorest et al. [1] has set the most stringent limit on models of neutron star cores so far. This discovery has spurred a re-examination of the possibility of exotica such as hyperons, Bose condensates, and quark matter playing an important role in models of neutron star interiors, owing to a presumed softening of the equation of state (EoS) expected in the presence of additional degrees of freedom. Historically, this has led to expectations of reduced maximum neutron star masses for compact objects in hydrostatic equilibrium. In this Letter we build on the earlier work of Stone et al. [2], who already predicted the existence of neutron stars containing hyperons with masses as large as 2 M⊙ in 2007, to establish a conservative upper limit on the maximum mass of such a star. We work within the quark-meson coupling (QMC) model [3–5], which has the advantage of being derived from the quark level, with a very small number of adjustable parameters, while being consistent with a broad range of constraints derived from hypernuclei as well as normal nuclear properties. We find that the stability under variation of the very small number of adjustable parameters is such that if a star were discovered with a mass significantly above 2.1 M⊙, we would need to consider more exotic physics, because it could not be accommodated within the QMC model. We recall that QMC is based upon the self-consistent modification of the structure of a baryon embedded in nuclear matter. At Hartree level it involves only three adjustable parameters which describe the effective couplings of the σ, ω and ρ mesons to the u and d quarks. These are fixed by adjusting them to fit the properties of symmetric nuclear matter, namely its saturation density and binding energy as well as its symmetry energy. We note that the σ meson used here simply serves as a convenient representation of the scalar-isoscalar attraction arising from two-pion exchange. In the most recent development of the QMC model [5], the self-consistent inclusion of the gluonic hyperfine interaction led to a very successful description of the binding energies of �-hypernuclei—as well as the observed absence of medium and heavy mass �-hypernuclei—with no additional parameters. We stress that this is achieved without any coupling of the strange quark to the σ,ω and ρ mesons (which would be OZI suppressed) and without the need to introduce any further mesons. While the model could be supplemented with much heavier mesons containing strange quarks [6], Occam’s razor suggests that one should not introduce them if they are not needed. A clear connection has been established between the self-consistent treatment of in-medium hadron structure and the existence of many-body [7] or density dependent [8] effective forces. Dutra et al. [9] critically examined a variety of phenomenological Skyrme models of the effective density dependent nuclear force against the most up-to-date empirical constraints. Amongst the few percent of the Skyrme forces studied which satisfied all of these constraints, the Skyrme model SQMC700, derived from the QMC model, was unique in that it incorporated the effects of the internal structure of the nucleon and its modification in-medium.