The gravitational-wave event GW190814 contained a 2.502.50–2.67 M⊙2.67\,M_\odot compact object merging with a ∼23 M⊙\sim23\,M_\odot black hole, at the lower edge of the neutron-star–black-hole mass gap. Bombaci, Drago, Logoteta, Pagliara and Vidaña [Phys. Rev. Lett. 126, 162702 (2021)] proposed that this object is a strange quark star within the two-families scenario, in which soft hadronic stars and stiff self-bound quark stars coexist, and identified the quantitative work needed to test that interpretation: temperature-dependent hadron-to-quark nucleation rates fixing the critical conversion mass, nucleosynthesis and electromagnetic emission of mixed and binary quark-star mergers, tighter quark-equation-of-state constraints, and formation rates of massive quark stars in binaries. Subsequent work has advanced each strand — first general-relativistic binary strange-star merger simulations, a finite-temperature nucleation framework, Bayesian equation-of-state tests of the two-families scenario, and next-generation detector forecasts — without closing any of them. The interpretation remains quantitatively undetermined, with a live tension between the ≳2.5 M⊙\gtrsim2.5\,M_\odot quark-star maximum mass the proposal requires and a ≲2.35 M⊙\lesssim2.35\,M_\odot post-merger bound, and no established discriminant. The problem is open.

Background

GW170817 and GW190814 are gravitational-wave (GW) detections of compact-binary mergers by the LIGO/Virgo collaboration. GW170817 was a binary neutron star merger whose inspiral signal measured the tidal deformability — the dimensionless quadrupole deformation Λ\Lambda induced in each star by the companion's tidal field, scaling roughly as (R/M)5(R/M)^5 and therefore probing the stellar radius RR at fixed mass MM — and showed that the equation of state (EOS; the pressure–density relation) of cold matter above nuclear saturation density is relatively soft, with radii of 1.4 M⊙1.4\,M_\odot stars of order 1111–1313 km. GW190814 merged a ∼23 M⊙\sim23\,M_\odot black hole with a 2.502.50–2.67 M⊙2.67\,M_\odot compact object, heavier than any firmly established neutron star and lighter than any established black hole, at the lower edge of the hypothesized mass gap. An EOS soft enough to satisfy GW170817 and the ∼2 M⊙\sim2\,M_\odot radio pulsars can support 2.6 M⊙2.6\,M_\odot only if its sound speed approaches the causal limit, which is physically uncomfortable and in tension with heavy-ion-collision constraints. A long-standing alternative is the strange quark matter (SQM) hypothesis (Bodmer–Witten): bulk matter made of up, down, and strange quarks in nearly equal proportions could be the true ground state of baryonic matter, in which case some compact stars are strange quark stars (QSs) — self-bound objects held together by the binding energy of quark matter rather than by gravity alone, described by bag-model EOSs with the bag constant BB, the perturbative-QCD correction parameter a4a_4, the pairing gap Δ\Delta of the color–flavor locked (CFL) color-superconducting phase, and the strange-quark mass msm_s. In the two-families scenario of Bombaci, Drago, Logoteta, Pagliara, and Vidaña (2021), ordinary hadronic stars (HSs; containing nucleons and, at high density, hyperons — baryons carrying a strange quark — modeled microscopically by Brueckner–Hartree–Fock or relativistic mean field EOSs — two standard microscopic and effective mean-field nuclear many-body approaches) form one family with Mmax⁡∼2 M⊙M_{\max}\sim2\,M_\odot, small radii, and Λ1.4\Lambda_{1.4} compatible with GW170817, while QSs form a second, stiffer family capable of Mmax⁡Q≳2.5 M⊙M_{\max}^{Q}\gtrsim2.5\,M_\odot; a benchmark parameter set (B1/4=135B^{1/4}=135 MeV, a4=0.7a_4=0.7, Δ=80\Delta=80 MeV, ms=100m_s=100 MeV) gives Mmax⁡Q=2.58 M⊙M_{\max}^{Q}=2.58\,M_\odot. Massive HSs are metastable: once a critical-size quark droplet nucleates in the core — a first-order phase transition whose seeding time τ\tau falls steeply with central density — the star converts to a QS, releasing ∼1053\sim10^{53} erg. The paper fixed the critical density ncrn_{\rm cr} and critical mass McrM_{\rm cr} by the operational criterion that the central hyperon fraction reaches nY/nB=0.10n_Y/n_B=0.10 (giving critical baryonic masses of 1.671.67–1.68 M⊙1.68\,M_\odot), and posed the quantitative questions that bear on whether the GW190814 secondary was a ∼2.6 M⊙\sim2.6\,M_\odot QS: the temperature-dependent nucleation theory behind McrM_{\rm cr} and τ\tau; the nucleosynthesis and kilonova (a transient powered by the radioactive decay of rapid-neutron-capture, r-process, ejecta) consequences of HS–QS and QS–QS mergers; the reduction of the quark-EOS parameter degeneracy against QCD and laboratory constraints; the formation rates of ∼2.5\sim2.5–2.6 M⊙2.6\,M_\odot QSs in binaries, e.g. via hierarchical triples (stable three-body stellar systems in which an inner pair is orbited by a distant companion) or quark-deconfinement-powered supernovae (supernovae whose anomalous luminosity is powered by the energy released when hadronic matter converts to quark matter). Constructing quantitative observational discriminants between a high-mass QS and a low-mass black hole in GW190814-like events is a related direction.

Problem Statement

Determine, quantitatively, whether the secondary component of GW190814 can be a strange quark star of mass 2.52.5–2.6 M⊙2.6\,M_\odot within the two-families scenario. The interpretation is settled if a self-consistent two-families configuration exists — a quark-EOS parameter set and a formation channel that jointly admit such a quark star in a GW190814-like binary while remaining consistent with the nucleation and with the merger-nucleosynthesis and electromagnetic constraints — or if no such configuration exists. The quantitative open questions posed by the source paper and developed in the subsequent literature are: (a) a temperature-dependent theory of hadron-to-quark nucleation with matched flavor content, including finite-size and thermal composition-fluctuation effects, yielding the nucleation time τ(nc,T)\tau(n_c,T) and the critical mass Mcr(T)M_{\rm cr}(T) that replace the operational nY/nB=0.10n_Y/n_B=0.10 criterion; (b) nucleosynthesis and electromagnetic-emission predictions for HS–QS and QS–QS mergers — r-process yields, ejecta masses, kilonova light curves, and the effect of immediate versus delayed quark conversion — checked against the GW170817 ejecta constraints; (c) the quark-EOS parameter set (B,a4,Δ,ms)(B,a_4,\Delta,m_s) required for Mmax⁡Q≳2.5 M⊙M_{\max}^{Q}\gtrsim2.5\,M_\odot, with its degeneracy reduced by QCD-based calculations and astrophysical/laboratory constraints and reconciled with post-merger arguments that cap the non-rotating quark-star maximum mass near 2.35 M⊙2.35\,M_\odot; and (d) formation pathways and astrophysical rates of binaries containing ∼2.5\sim2.5–2.6 M⊙2.6\,M_\odot QSs (hierarchical triples producing merger-formed QSs, accretion-induced conversion, quark-deconfinement-powered supernovae) via population synthesis (statistical simulation of large synthetic stellar and binary populations under assumed evolutionary physics) and stellar evolution. Each of (a)–(d) states a necessary condition on the two-families interpretation: the determination is settled negatively as soon as any single strand is shown to be unsatisfiable, whereas a positive resolution requires all four to hold consistently. These questions are not settled by the cited literature, and their answers must be established quantitatively and made mutually consistent.

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