arXiv subjects

Armen Sedrakian

Publications and source records attributed to Armen Sedrakian.

At least 127 records · Page 7Linked to original sources

arXiv2013

Hypernuclear matter in strong magnetic field

Compact stars with strong magnetic fields (magnetars) have been observationally determined to have surface magnetic fields of order of 10^{14}-10^{15} G, the implied internal field strength being several orders larger. We study the equation of state and composition of dense hypernuclear matter in strong magnetic fields in a range expected in the interiors of magnetars. Within the non-linear Boguta-Bodmer-Walecka model we find that the magnetic field has sizable influence on the properties of matter for central magnetic field B \ge 10^{17} G, in particular the matter properties become anisotropic. Moreover, for the central fields B \ge 10^{18} G, the magnetized hypernuclear matter shows instability, which is signaled by the negative sign of the derivative of the pressure parallel to the field with respect to the density, and leads to vanishing parallel pressure at the critical value B_{\rm cr} \simeq 10^{19} G. This limits the range of admissible homogeneously distributed fields in magnetars to fields below the critical value B_{\rm cr}.

astro-ph.HE↗

arXiv2013

Dense QCD and phenomenology of compact stars

I discuss three topics in physics of massive (two solar-mass and larger) neutron stars containing deconfined quark matter: (i) the equation of state of deconfined dense quark matter and its color superconducting phases, (ii) the thermal evolution of stars with quark cores, (iii) color-magnetic flux tubes in type-II superconducting quark matter and their dynamics driven by Aharonov-Bohm interactions with unpaired fermions.

astro-ph.HE↗

arXiv2012

Phase diagram of dilute nuclear matter: Unconventional pairing and the BCS-BEC crossover

We report on a comprehensive study of the phase structure of cold, dilute nuclear matter featuring a SD condensate at non-zero isospin asymmetry, within wide ranges of temperatures and densities. We find a rich phase diagram comprising three superfluid phases, namely a Larkin-Ovchinnikov-Fulde-Ferrell phase, the ordinary BCS phase, and a heterogeneous, phase-separated BCS phase, with associated crossovers from the latter two phases to a homogeneous or phase-separated Bose-Einstein condensate of deuterons. The phase diagram contains two tricritical points (one a Lifshitz point), which may degenerate into a single tetracritical point for some degree of isospin asymmetry.

nucl-th↗

arXiv2012

Selected problems in astrophysics of compact objects

I review three problems in astrophysics of compacts stars: (i) the phase diagram of warm pair-correlated nuclear matter a sub-saturation densities at finite isospin asymmtery; (ii) the Standard Model neutrino emission from superfluid phases in neutron stars within the Landau theory of Fermi (superfluid) liquids; (iii) the beyond Standard Model physics of axionic cooling of compact stars by the Cooper pair-breaking processes.

nucl-th↗

arXiv2012

Axions from cooling compact stars: pair-breaking processes

Once formed in a supernova explosion, a neutron star cools rapidly via neutrino emission during the first 10^4-10^5 yr of its life-time. Here we compute the axion emission rate from baryonic components of a star at temperatures below their respective critical temperatures T_c for normal-superfluid phase transition. The axion production is driven by a charge neutral weak process, associated with Cooper pair breaking and recombination. The requirement that the axion cooling does not overshadow the neutrino cooling puts a lower bound on the axion decay constant f_a > 6 10^9 T_{c9}^{-1} GeV, with T_{c9} = T_c/10^9 K. This translates into a upper bound on the axion mass m_a < 10^{-3} T_{c9} eV.

astro-ph.CO↗

arXiv2012

Spontaneous symmetry breaking in rotating condensates of ultracold atoms

We describe an equilibrium state of a rotating trapped atomic condensate, which is characterized by a non-zero internal circulation and spontaneous breaking of the rotational O(2) symmetry with all three major semiaxes of the condensate having different values. The macroscopic rotation of the condensate is supported by a mesh of quantized vortices, whose number density is a function of internal circulation. The oscillation modes of this state are computed and the Goldstone mode associated with the loss of the symmetry is identified. The possible avenues for experimental identification this state are discussed.

cond-mat.quant-gas↗

arXiv2012

Vertex renormalization of weak interactions in compact stars: beyond leading order

Neutrino emission rate from baryonic matter in neutron stars via weak neutral vector interaction is computed up to order O(v_F^6), where v_F is the Fermi velocity in units of speed of light. The vector current polarization tensors are evaluated with full vertices which include resummed series in the particle-hole channel. The neutrino emissivity is enhanced compared to the O(v_F^4) order up of 10% for values v_F< 0.4 characteristic to baryons in compact stars.

astro-ph.SR↗

arXiv2012

Two solar-mass compact stars: structure, composition, and cooling

I discuss the structure and composition of massive (two solar-mass) neutron stars containing hypernuclear and deconfined quark matter in color superconducting states. Stable configurations featuring such matter are obtained if the equation of state of hadronic matter is stiff above the saturation density, the transition to quark matter takes place at a few times the nuclear saturation density, and the repulsive vector interactions in quark matter are substantial. I also discuss our recent progress in understanding the cooling of massive compact stars with color superconducting quark cores.

astro-ph.SR↗

arXiv2011

Composition and stability of hybrid stars with hyperons and quark color-superconductivity

The recent measurement of a $1.97\pm 0.04$ solar-mass pulsar places a stringent lower bound on the maximum mass of compact stars and therefore challenges the existence of any agents that soften the equation of state of ultra-dense matter. We investigate whether hyperons and/or quark matter can be accommodated in massive compact stars by constructing an equation of state based on a combination of phenomenological relativistic hyper-nuclear density functional and an effective model of quantum chromodynamics (the Nambu-Jona-Lasinio model). Stable configurations are obtained with $M \ge 1.97 M_{\sun}$ featuring hyper-nuclear and quark matter in color superconducting state if the equation of state of nuclear matter is stiff above the saturation density, the transition to quark matter takes place at a few times the nuclear saturation density, and the repulsive vector interactions in quark matter are substantial.

astro-ph.SR↗

arXiv2011

Thermal evolution of massive compact objects with dense quark cores

We examine the thermal evolution of a sequence of compact objects containing low-mass hadronic and high-mass quark-hadronic stars constructed from a microscopically motivated equation of state. The dependence of the cooling tracks in the temperature versus age plane is studied on the variations of the gaplessness parameter (the ratio of the pairing gap for red-green quarks to the electron chemical potential) and the magnitude of blue quark gap. The pairing in the red-green channel is modeled assuming an inhomogeneous superconducting phase to avoid tachionic instabilities and anomalies in the specific heat; the blue colored condensate is modeled as a Bardeen-Cooper-Schrieffer (BCS)-type color superconductor. We find that massive stars containing quark matter cool faster in the neutrino-cooling era if one of the colors (blue) is unpaired and/or the remaining colors (red-green) are paired in a inhomogeneous gapless superconducting state. The cooling curves show significant variations along the sequence, as the mass (or the central density) of the models is varied. This feature provides a handle for fine-tuning the models to fit the data on the surface temperatures of same-age neutron stars. In the late-time photon cooling era we observe inversion in the temperature arrangement of models, i.e., stars experiencing fast neutrino cooling are asymptotically hotter than their slowly cooling counterparts.

astro-ph.HE↗

arXiv2011

Kubo formulas for relativistic fluids in strong magnetic fields

Magnetohydrodynamics of strongly magnetized relativistic fluids is derived in the ideal and dissipative cases, taking into account the breaking of spatial symmetries by a quantizing magnetic field. A complete set of transport coefficients, consistent with the Curie and Onsager principles, is derived for thermal conduction, as well as shear and bulk viscosities. It is shown that in the most general case the dissipative function contains five shear viscosities, two bulk viscosities, and three thermal conductivity coefficients. We use Zubarev's non-equilibrium statistical operator method to relate these transport coefficients to correlation functions of equilibrium theory. The desired relations emerge at linear order in the expansion of the non-equilibrium statistical operator with respect to the gradients of relevant statistical parameters (temperature, chemical potential, and velocity.) The transport coefficients are cast in a form that can be conveniently computed using equilibrium (imaginary-time) infrared Green's functions defined with respect to the equilibrium statistical operator.

astro-ph.HE↗

arXiv2011

Vortex formation in a rotating two-component Fermi gas

A two-component Fermi gas with attractive s-wave interactions forms a superfluid at low temperatures. When this gas is confined in a rotating trap, fermions can unpair at the edges of the gas and vortices can arise beyond certain critical rotation frequencies. We compute these critical rotation frequencies and construct the phase diagram in the plane of scattering length and rotation frequency for different total number of particles. We work at zero temperature and consider a cylindrically symmetric harmonic trapping potential. The calculations are performed in the Hartree-Fock-Bogoliubov approximation which implies that our results are quantitatively reliable for weak interactions.

cond-mat.quant-gas↗

arXiv2011

BCS-BEC crossover in spatially modulated fermionic condensates

Several novel multi-component fermionic condensates show universal behavior under imbalance in the number of fermionic species. Here I discuss their phase structure, thermodynamics, and the transition from the weak (BCS) to strong (BEC) coupling regime. The inhomogeneous superconducting phases are illustrated on the example of the Fulde-Ferrell phase which appears in the weak coupling regime, at low temperatures and large asymmetries. The inhomogeneous phases persist through the crossover up to (and possibly beyond) the transition to the strong coupling regime.

nucl-th↗

arXiv2010

Cold quark matter in astrophysics of compact stars

The appearance of quark matter in the centers of compact stars has a number of astrophysical implications. In this contribution I discuss the structure and stability of compact stars, gravitational radiation from non-axisymmetric deformations, and nucleation and dynamics of vortices (flux tubes) in the color superconducting phases.

astro-ph.HE↗

arXiv2010

Phase diagram of chiral quark matter: color and electrically neutral Fulde-Ferrell phase

The phase diagram of charge and color neutral two-flavor color superconducting quark matter is studied including the homogeneous two-flavor superconductor (2SC) and the inhomogeneous Fulde-Ferrell (FF) phases within the Nambu-Jona-Lasinio model. The low-temperature domain T < 5 MeV of the phase diagram contains the FF phase, which borders at high temperatures to the 2SC phase. The critical temperature of phase transition from the 2SC to the unpaired state is in the range 20-30 MeV. We derive the equation of state of matter and its composition and show that matter in mature compact stars should be in the inhomogeneous FF-like superconducting state. We briefly discuss the astrophysical implications of such a phase in compact stars.

nucl-th↗

arXiv2010

Color-magnetic flux tubes in quark matter cores of neutron stars

We argue that if color-superconducting quark matter exists in the core of a neutron star, it may contain a high density of flux tubes, carrying flux that is mostly color-magnetic, with a small admixture of ordinary magnetic flux. We focus on the two-flavor color-superconducting ("2SC") phase, and assume that the flux tubes are energetically stable, although this has not yet been demonstrated. The density of flux tubes depends on the nature of the transition to the color-superconducting phase, and could be within an order of magnitude of the density of magnetic flux tubes that would be found if the core were superconducting nuclear matter. We calculate the cross-section for Aharonov-Bohm scattering of gapless fermions off the flux tubes, and the associated collision time and frictional force on a moving flux tube. We discuss the other forces on the flux tube, and find that if we take in to account only the forces that arise within the 2SC core region then the timescale for expulsion of the color flux tubes from the 2SC core is of order 10^10 years.

astro-ph.SR↗

arXiv2010

Response functions of cold neutron matter: density fluctuations

We compute the finite temperature density response function of nonrelativistic cold fermions with an isotropic condensate. The pair-breaking contribution to the response function is evaluated in the limit of small three-momentum transfers q within an effective theory which exploits series expansion in powers of small q/p_F, where p_F is the Fermi momentum. The leading order O(q^2) contribution is universal and depends only on two fundamental scales, the Fermi energy and the pairing gap. The particle-hole Landau Fermi-liquid interaction contributes first at the next-to-leading-order O(q^4). The scattering contribution to the polarization tensor is nonperturbative (in the above sense) and is evaluated numerically. The spectral functions of density fluctuations are constructed and the relevance of the q^2 scaling for the pair-breaking neutrino emission from neutron stars is discussed.

nucl-th↗

arXiv2010

Anisotropic Hydrodynamics, Bulk Viscosities and R-Modes of Strange Quark Stars with Strong Magnetic Fields

In strong magnetic fields the transport coefficients of strange quark matter become anisotropic. We determine the general form of the complete set of transport coefficients in the presence of a strong magnetic field. By using a local linear response method, we calculate explicitly the bulk viscosities $\zperp$ and $\zpara$ transverse and parallel to the $B$-field respectively, which arise due to the non-leptonic weak processes $u+s\leftrightarrow u+d$. We find that for magnetic fields $B<10^{17}$ G, the dependence of $\zperp$ and $\zpara$ on the field is weak, and they can be approximated by the bulk viscosity for zero magnetic field. For fields $B>10^{18}$ G, the dependence of both $\zperp$ and $\zpara$ on the field is strong, and they exhibit de Haas-van Alphen-type oscillations. With increasing magnetic field, the amplitude of these oscillations increases, which eventually leads to negative $\zperp$ in some regions of parameter space. We show that the change of sign of $\zperp$ signals a hydrodynamic instability. As an application, we discuss the effects of the new bulk viscosities on the r-mode instability in rotating strange quark stars. We find that the instability region in strange quark stars is affected when the magnetic fields exceeds the value $B= 10^{17}$ G. For fields which are larger by an order of magnitude, the instability region is significantly enlarged, making magnetized strange stars more susceptible to $r$-mode instability than their unmagnetized counterparts.

astro-ph.HE↗