Was GW190814 a Black Hole–Strange Quark Star System?

We investigate the possibility that the low mass companion of the black hole in the source of GW190814 was a strange quark star. This possibility is viable within the so-called two-families scenario in which neutron stars and strange quark stars coexist. Strange quark stars can reach the mass range indicated by GW190814, $M\sim (2.5–2.67)M_{\odot }$ due to a large value of the adiabatic index, without the need for a velocity of sound close to the causal limit. Neutron stars (actually hyperonic stars in the two-families scenario) can instead fulfill the presently available astrophysical and nuclear physics constraints which require a softer equation of state. In this scheme it is possible to satisfy both the request of very large stellar masses and of small radii while using totally realistic and physically motivated equations of state. Moreover it is possible to get a radius for a $1.4M_{\odot }$ star of the order or less than 11 km, which is impossible if only one family of compact stars exists.

Figure 1

The triangular regions delimit the values of $\mathrm{\Delta }$ and $B^{1/4}$ which lead to $M_{\max }^Q\ge 2.6M_{\odot }$ (solid lines) and $M_{\max }^Q\ge 2.5M_{\odot }$ (dot-dashed lines) for three different values of $a_4$. The horizontal lines indicate the values of $B^{1/4}$ below which two-flavor QM is more stable than ${}^{56}\mathrm{Fe}$ and are thus excluded, see analogous figure in Ref. [47]. The dashed lines indicate the regions of the parameter space for which QM nucleation is possible for the $1.5M_{\odot }$ metastable HS configuration obtained with the RMF EOS. For the BHF EOS those lines are not shown because they do not further restrict the parameter space with respect to the constraint on $M_{\max }^Q$.

Figure 2

Mass-radius relations of NSs (thick black and blue lines correspond to the BHF and RMF EOS, respectively) and of QSs (thick red line), compared with several astrophysical data. Upper left panel: thermonuclear bursts [52]. Upper right panel: low mass x-ray binaries in quiescence [52]. Lower left panel: the NICER results for $\mathrm{J}0030+0451$ [53] and the RXTE results for the cooling tail spectra of 4U1702-429 [54]. We have modeled the mass-radius posterior distribution of 4U1702-429 by using a bivariate Gaussian with $\rho =0.9$ as in Ref. [55]. All constraints are at the 68% CI. Lower right panel: constraints on the two compact stars of GW170817, solid (dashed) lines correspond to the 90% (68%) CI [56].

Figure 3

Upper left panel: gravitational mass vs radius for HSs (black and blue lines) and QSs (red line). Also shown an example of jump (orange arrow) from the first to the second family of stars. Upper right panel: gravitational mass $M$ vs baryonic mass $M_b$ for HSs and QSs. QSs are more bound than HSs with the same baryonic mass. Lower left panel: Gibbs energy per baryon $g_H$ ($g_Q)$ for the hadronic (quark) phase vs pressure. Since, at fixed pressure, $g_Q<g_H$ then QM nucleation is allowed. Lower right panel: gravitational mass vs central pressure for HSs and QSs. The dots in all panels indicate the critical hadronic configuration, for each EOS model, and the final QS configuration having the same baryonic mass of the initial one. Notice that the final QS configurations are very close in mass (see text) and thus both are indicated by the same red dot.