Rotational effects in quark stars: comparing different models

TL;DR

The paper evaluates whether self-bound strange quark matter can be distinguished observationally by comparing two representative EOSs for rotating strange stars: the density-dependent quark mass (DDQM) model and the vector MIT bag model. Using fully relativistic calculations of uniformly rotating sequences with the rns code, it maps mass–radius, moment of inertia, quadrupole moment, surface redshift, and Keplerian limits, together with a complete decomposition of the stellar energy budget into gravitational, internal, rotational, and binding energies. Rotation magnifies EOS differences: the vector MIT bag model supports higher maximum masses (≈ $M_{max}$ ≈ 3.3 M_sun) and higher rotational limits, while DDQM produces larger radii and lower maxima (≈ 2.8 M_sun) at comparable rotation; at a canonical mass of ≈ 1.4 M_sun, DDQM predicts larger radii and I, whereas MIT yields higher Z_p and ν_K. The study argues that coordinated multi-messenger observations—mass–radius from NICER, moment of inertia from pulsar timing, quadrupole from gravitational waves, and spin limits from pulsar surveys—can break EOS degeneracies and test the strange matter hypothesis by distinguishing stiff, self-bound MIT-like matter from softer, density-dependent DDQM behavior.

Abstract

We investigate the rotational properties of self-bound strange quark stars using two representative quark matter equations of state (EOS): the vector MIT bag model and the density-dependent quark mass (DDQM) model. Through general-relativistic calculations of uniformly rotating sequences, we analyze their mass--radius relations, moments of inertia, quadrupole moments, surface redshifts, Keplerian frequencies, and energy components. A central result of this work is the full decomposition of the stellar energy budget in rotating strange stars, separating gravitational, internal, rotational, and binding energy contributions. Rotation amplifies the intrinsic EOS differences: the MIT model supports more massive ($M_{\max} \gtrsim 3.3\,M_\odot$) compact stars with larger moments of inertia and greater resistance to deformation, while the DDQM model produces larger radii, less massive stars limited by mass-shedding at lower frequencies. Combined measurements of mass, radius, and frequency can thus break the EOS degeneracy; massive, rapidly rotating pulsars favors MIT-like EOS, whereas larger radii in canonical stars point to a DDQM-like model. These rotational observables, soon to be tightly constrained by NICER and next-generation gravitational-wave detectors, offer a means to test the existence and composition of self-bound quark matter in compact stars.

Figures (6)

• Figure 1: The gravitational mass as a function of the equatorial radius for the DDQM model on the upper panel, and for the vector MIT bag model on the bottom panel.
• Figure 2: The gravitational mass as a function of the stellar frequency for the DDQM model on the upper panel, and for the vector MIT bag model on the bottom panel. The color palette shows the ratio between the frequency of the star and the Keplerian frequency of a particle in circular orbit.
• Figure 3: The moment of inertia as a function of the gravitational mass for the DDQM model on the top panel, and for the vector MIT bag model on the bottom panel.
• Figure 4: The quadrupole moment as a function of the gravitational mass for the DDQM model on the top panel, and for the vector MIT bag model on the bottom panel.
• Figure 5: The polar redshift as a function of the gravitational mass for the DDQM model on the upper panel, and for the vector MIT bag model on the bottom panel.