Implications of supermassive neutron stars for the form of the equation of state of hybrid stars
Hongyi Sun, De-Hua Wen
Abstract
The observations of PSR J0952-0607 $(M=2.{35}_{\ensuremath{-}0.17}^{+0.17}\phantom{\rule{0.16em}{0ex}}{M}_{\ensuremath{\bigodot}})$ and the second object in the GW190814 event $(M=2.{59}_{\ensuremath{-}0.09}^{+0.08}\phantom{\rule{0.16em}{0ex}}{M}_{\ensuremath{\bigodot}})$ indicate the possible existence of supermassive neutron stars. In this work, by using the constant-sound-speed (CSS) parametrization to describe the equation of state (EOS) of quark matter, the constraints on the EOS parameters from supermassive hybrid stars are investigated through the Maxwell and Gibbs constructions. It is shown that to support a supermassive hybrid star (e.g., $M=2.5\phantom{\rule{0.16em}{0ex}}{M}_{\ensuremath{\bigodot}}$), a lower transition energy density $({\ensuremath{\varepsilon}}_{\mathrm{tran}})$, a smaller energy density discontinuity $(\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon})$, and a higher sound speed of quark matter $({c}_{\mathrm{sq}})$ are favored. For the constructed hybrid star EOS model, the maximum mass of the corresponding hybrid stars will not meet the lower mass limit of the second object in the GW190814 event if $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}$ takes a value higher than 180 MeV ${\mathrm{fm}}^{\ensuremath{-}3}$. Moreover, it is confirmed that the supermassive neutron star observation can also rule out the existence of twin stars as a supermassive hybrid star requires a relatively small $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}$. Finally, we give a rough estimate of the lower limit of the dimensionless tidal deformability of neutron stars that ranges from 2 to 3.