The sound speed and core of massive compact stars: A manifestation of hadron-quark duality

Ma, Yong-Liang; Rho, Mannque

doi:10.48550/arxiv.2104.13822

“…) strictly at all densities leads to a strong tension with astrophysical measurements of neutron-star masses M  2 M e (Antoniadis et al 2013;Cromartie et al 2019;Fonseca et al 2021), which favor stiff EOSs with  c 1 3 s 2 at densities n s . In addition, various theoretical approaches, such as QCD at large isospin density (Carignano et al 2017), two-color QCD (Hands et al 2006), quarkyonic matter (McLerran & Reddy 2019Duarte et al 2021;Margueron et al 2021), models for high-density QCD (Leonhardt et al 2020;Ma & Rho 2021;Braun & Schallmo 2022;Pal et al 2022), and models based on the gauge/gravity duality (Ecker et al 2017;Demircik et al 2021;Kovensky et al 2022…”

Section: Introductionmentioning

confidence: 99%

A General, Scale-independent Description of the Sound Speed in Neutron Stars

Ecker

¹

,

Rezzolla

²

2022

ApJL

View full text Add to dashboard Cite

Using more than a million randomly generated equations of state that satisfy theoretical and observational constraints, we construct a novel, scale-independent description of the sound speed in neutron stars, where the latter is expressed in a unit cube spanning the normalized radius, r/R, and the mass normalized to the maximum one, M/M TOV. From this generic representation, a number of interesting and surprising results can be deduced. In particular, we find that light (heavy) stars have stiff (soft) cores and soft (stiff) outer layers, or that the maximum of the sound speed is located at the center of light stars but moves to the outer layers for stars with M/M TOV ≳ 0.7, reaching a constant value of c s 2 = 1 / 2 as M → M TOV. We also show that the sound speed decreases below the conformal limit c s 2 = 1 / 3 at the center of stars with M = M TOV. Finally, we construct an analytic expression that accurately describes the radial dependence of the sound speed as a function of the neutron-star mass, thus providing an estimate of the maximum sound speed expected in a neutron star.

show abstract

“…There exist arguments that suggest a universal bound c 2 s < 1/3 (see, e.g., [8,9]), thus favouring scenario i) and ii). However, a number of counter examples to this bound are known in QCD at large isospin density [10], two-color QCD [11], quarkyonic matter [12][13][14][15], models for high-density QCD [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] and models based on the gauge/gravity duality [31][32][33][34][35][36][37]. All of these example favour scenario iii).…”

mentioning

confidence: 99%

On the Sound Speed in Neutron Stars

Altiparmak¹,

Ecker²,

Rezzolla³

2022

Preprint

View full text Add to dashboard Cite

Determining the sound speed cs in compact stars is an important open question with numerous implications on the behaviour of matter at large densities and hence on gravitational-wave emission from neutron stars. To this scope, we construct more than 10 7 equations of state (EOSs) with continuous sound speed and build more than 10 8 nonrotating stellar models consistent not only with nuclear theory and perturbative QCD, but also with astronomical observations. In this way, we find that EOSs with sub-conformal sound speeds, i.e., with c 2 s < 1/3 within the stars, are possible in principle but very unlikely in practice, being only 0.03% of our sample. Hence, it is natural to expect that c 2 s > 1/3 somewhere in the stellar interior. Using our large sample, we obtain estimates at 95% credibility of neutron-star radii for representative stars with 1.4 and 2.0 solar masses, R1.4 = 12.42 +0.52 −0.99 km, R2.0 = 12.12 +1.11 −1.23 km, and for the binary tidal deformability of the GW170817 event, Λ1.186 = 485 +225 −211 . Interestingly, our lower-bounds on the radii are in very good agreement with the prediction derived from very different arguments, namely, the threshold mass. Finally, we provide simple analytic expressions to determine the minimum and maximum values of Λ as a function of the chirp mass.

show abstract

“…Ultimately the understandings of QC 2 D have important applications to the physics of neutron stars [55]. In this context, recently the picture of quark-hadron continuity is actively discussed [23,[56][57][58][59][60][61][62][63][64][65]) to account for the interplay between the low density nuclear physics [66], the neutron star radii for 1.4- [67,68] and 2-solar mass (M ⊙ ) neutron stars [69][70][71][72], and the maximum mass ≃ 2.08 ± 0.07M ⊙ [73]. In particular the latest result by the NICER [69][70][71][72] shows that the radii of 1.4M ⊙ and 2M ⊙ neutron stars are close (12 − 13 km for both), disfavoring strong first order transitions from the nuclear to quark matter domain, and implying that there should be rather stiffening with the sound velocity exceeding the conformal value 2 , 1/3 [23,[74][75][76][77][78].…”

Section: Discussionmentioning

confidence: 99%

Delineating chiral separation effect in two-color dense QCD

Suenaga,

Kojo

2021

Preprint

View full text Add to dashboard Cite

We study the chiral separation effect (CSE) in two-color and two-flavor QCD (QC2D) to delineate quasiparticle pictures in dense matter from low to high temperatures. Both massless and massive quarks are discussed. We particularly focus on the high density domain where diquarks form a color singlet condensate with the electric charge 1/3. The condensate breaks baryon number and U (1)A axial symmetry, and induces the electromagnetic Meissner effects. Within a quark quasiparticle picture, we compute the chiral separation conductivity at one-loop. We have checked that Nambu-Goldstone modes, which should appear in the improved vertices as required by the Ward-Takahashi identities, do not contribute to the chiral separation conductivity due to their longitudinal natures. In the static limit, the destructive interferences in the particle-hole channel, as in usual Meissner effects, suppress the conductivity (in chiral limit, to 1/3 of the normal phase's) This locally breaks the universality of the CSE coefficients, provided quasiparticle pictures are valid in the bulk matter.

show abstract

The sound speed and core of massive compact stars: A manifestation of hadron-quark duality

Cited by 6 publications

References 46 publications

A General, Scale-independent Description of the Sound Speed in Neutron Stars

A General, Scale-independent Description of the Sound Speed in Neutron Stars

On the Sound Speed in Neutron Stars

Delineating chiral separation effect in two-color dense QCD

Contact Info

Product

Resources

About