Rotating strange dwarfs and their indistinguishability from white dwarfs

TL;DR

This work addresses whether strange dwarfs (SDs)—white-dwarf-like crusts with self-bound quark-matter cores—can be distinguished from ordinary white dwarfs using global observables. It adopts a fully relativistic treatment, solving the TOV equations for static stars and employing the Hartle–Thorne slow-rotation formalism to model rotation, coupled to a hybrid EoS with a carbon crust and an MIT bag-model core explored across representative bag constants. The major finding is that rotation expands SD radii toward white-dwarf values, and for substantial fractions of the Kepler spin, SDs become observationally indistinguishable from WDs in mass–radius space, with microphysical variations contributing only a few percent at most. This implies that some WD catalogs could conceal SDs and highlights the need for diagnostics beyond mass and radius—such as tidal deformability or gravitational-wave signatures—to robustly identify exotic compact objects in future surveys.

Abstract

We investigate the structure of strange dwarfs, modeled as hybrid compact stars composed of a self bound strange quark matter core surrounded by a white dwarf like crust, within a fully relativistic framework. Static configurations are constructed by solving the Tolman Oppenheimer Volkoff equations, and uniformly rotating configurations are modeled within the Hartle Thorne slow rotation expansion (to ${\cal O}(Ω^2)$). We therefore interpret results at large fractional spins conservatively, and use the Kepler frequency mainly as a reference scale for comparing different masses and models. The stellar matter is described using a hybrid equation of state, in which the crust is modeled by a degenerate electron ion system and the core by the MIT Bag Model. By comparing strange dwarfs with conventional white dwarfs across a range of rotation rates, we show that rotation inflates the radius and can reduce (in a quantifiable way) the separation between the two families in the $(M,R)$ plane, potentially masking structural signatures associated with the presence of a quark core. Our results highlight the importance of accounting for rotational effects when interpreting mass radius measurements and other global observables in the context of searches for exotic compact objects in current and future high precision surveys.

Figures (4)

• Figure 1: Fluid pressure against the energy density (carbon crust + SQM core) for ${\cal B^{\rm 1/4}}=\{135,145,160\}\,{\rm MeV}$. Differences appear only above neutron-drip density, where the SQM core starts to play a significant role. These trends explain the small shifts observed in the $M$-$R$ curves in Fig. \ref{['fig:mass_radiuas_bag']}.
• Figure 2: Mass-radius relations for SDs computed with ${\cal B^{\rm 1/4}}=\{135,145,160\}\,{\rm MeV}$.
• Figure 3: Top: Radial energy-density profiles for a $1\,M_{\odot}$ configuration. The SD profile shows an extended near-constant-density quark core and an abrupt density jump at the core--crust interface (at fixed pressure), whereas the WD profile is smooth. Bottom: $M$--$R$ relations for a pure carbon crust and a quark-matter core, compared with observational data (blue circles from Madej2004 and red squares from Nalezyty2004). The key takeaway is that increasing rotation inflates the SD radius and drives the rotating SD sequences toward the WD locus in the $(M,R)$ plane, motivating a quantitative "indistinguishability" criterion discussed in the main text.
• Figure 4: Kepler (mass-shedding) angular frequency $\Omega_{K}(M)$ for the SD sequences, shown here as a reference spin scale. Within the Hartle--Thorne approach, $\Omega_K$ is used to normalize the rotation rate; statements very near $\Omega\sim\Omega_K$ should be interpreted conservatively given the slow-rotation truncation.