Bulletin of the American Physical Society
APS April Meeting 2021
Volume 66, Number 5
Saturday–Tuesday, April 17–20, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y10: Pulsars and Neturon StarsLive
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Sponsoring Units: DAP Chair: Rodrigo Fernandez, Univ Alberta |
Tuesday, April 20, 2021 1:30PM - 1:42PM Live |
Y10.00001: How the $\omega_0$ condensate can spike the speed of sound cold, quarkyonic matter Robert Pisarski I consider the effects of a coupling $\sim + \omega_\mu^2 \vec{\phi}^2$ between the $\omega_\mu$ meson and the $O(4)$ chiral field, $\vec{\phi}$. A condensate for $\omega_0$ is automatically generated at nonzero baryon density. I assume that with increasing density, a decrease of the chiral condensate and the effective $\omega_0$ mass gives a stiff equation of state. In order to match that onto a soft equation of state for quarkyonic matter, I consider an $O(N)$ field at large $N$. At nonzero temperature, Tsvelik, Valgushev, and myself showed that at nonzero temperature quantum fluctuations disorder any putative pion ``condensate'' into a pion quantum spin liquid. Here I show that the pion quantum spin liquid persists at zero temperature. If valid qualitatively at $N=4$, the $\omega_0$ mass goes up sharply and suppresses the $\omega_0$ condensate. This could generate a spike in the speed of sound at high density, which is strongly suggested by experimental data on neutron stars. [Preview Abstract] |
Tuesday, April 20, 2021 1:42PM - 1:54PM Live |
Y10.00002: Effects of Dark Matter on the Nuclear Equation of State and Neutron Star Structure with Crust Adrian Abac, Christopher Bernido, Jose Perico Esguerra We investigate the effects of dark matter (DM) on the nuclear equation of state (EoS) and neutron star structure, in the relativistic mean field theory using the path integral formulation, both in the absence and presence of a crust. The simplest $\sigma $-$\omega $ model was modified by adding a WIMP-DM component, which interacts with nucleonic matter through the Higgs portal, considering only up to the h$^{\mathrm{2}}$ term of the Higgs potential. This simple model still agrees well with previous studies which utilized either a more complicated nuclear model or higher-order terms of the Higgs potential, in that DM softens the EoS, resulting in stars with lower maximum masses. This effect becomes more prominent as we increase the DM Fermi momentum. We then extended this by confining DM in the star's core. The region of instability corresponding to negative pressure values in the low-energy density regime of the EoS was replaced by an ideal gas EoS, interpreted as an atmosphere, and then by the Friedman-Pandharipande-Skyrne EoS, corresponding to a crust. The addition of the atmosphere and crust significantly affects the mass-radius relation of neutron stars, particularly in the low-mass regime, by increasing the radius of the star corresponding to the mass. [Preview Abstract] |
Tuesday, April 20, 2021 1:54PM - 2:06PM Live |
Y10.00003: Magnetohydrodynamic stability of magnetars with ultra-strong fields Peter Rau, Ira Wasserman We study the magnetohydrodynamic stability of a neutron star core threaded by magnetar-strength magnetic fields $10^{14}$--$10^{17}$ G, where quantum electrodynamical effects and Landau quantization of fermions are important. Using the canonical energy principle and the Euler--Heisenberg--Fermi--Dirac Lagrangian for a strongly magnetized fluid, we determine the local stability criterion for a fluid slab as a stand-in for a segment of a neutron star core, accounting for magnetic buoyancy and realistic species fraction gradient buoyancy. We find that, for sufficiently strong fields, the magnetized fluid can be unstable to magnetosonic instabilities, caused by the density-dependence of the magnetic $H$-field. This mechanism could thus be relevant in setting an upper limit on field strengths or determining stable field configurations in magnetar cores. [Preview Abstract] |
Tuesday, April 20, 2021 2:06PM - 2:18PM Live |
Y10.00004: Antineutrino Emission via the Direct Urca Process in Strongly Magnetized Neutron-Stars Grant Mathews, Tomoyuki Maruyama, A. Baha Balantekin, Myung-Ki Cheoun, Toshitaka Kajino, Motohiko Kusakabe We have applied a relativistic quantum framework to analyze the antineutrino emission via the direct Urca process in strongly magnetized neutron stars, i.e. magnetars. We calculate the neutrino emissivity from the direct Urca process whereby a neutron converts to a proton, an electron and an antineutrino. We solve for the exact wave functions for protons and electrons in the states described by Landau levels. We find that the direct Urca process in the presence of strong magnetic fields can occur in density regions where this process would not normally occur because of the kinematical condition. [Preview Abstract] |
Tuesday, April 20, 2021 2:18PM - 2:30PM Live |
Y10.00005: Neutron starquakes caused by spin-down Javier Arenas Rencoret, Claudia Aguilera, Andreas Reisenegger Pulsars rotate at a precise frequency, from which a spin-down is also observed in time. The star has an ellipsoidal shape when it is rotating. It is composed by a fluid core and a thin solid crust, which goes towards a more spherical shape due to the spin-down. Previous works show that the interior of the crust displaces slightly more than the surface, producing a greater stress at the equator, where the solid should breaks. We show that, in a star with a denser fluid core, the interior of the crust-core interface moves much less than the surface (with a greater radial gradient hence radial strain towards the interior than in the previous works). This displacement moves the breaking point to the pole, where the stress is larger. This type of deformation increases the mountains, and accumulates more elastic energy than the former, which could eventually be released when the crust breaks or yields in a singular event called starquake. If angular momentum is conserved during the yields, starquakes produce spin-ups similar to glitches observed in pulsars. We show that the change in moment of inertia by spin-down and rigidity of the star is not capable of explaining all glitch activity, even the activity of small glitches. Thus, starquakes by spin-down are less frequent than glitches. [Preview Abstract] |
Tuesday, April 20, 2021 2:30PM - 2:42PM Live |
Y10.00006: The Fate of Twin Stars on the Unstable Branch: Implications for the Formation of Twin Stars Pedro Espino, Vasileios Paschalidis We consider the dynamics of compact stars with hybrid hadron-quark equations of state. When the surface tension between the hadronic and quark phases is sufficiently strong, a first-order phase transition can be sustained over a large range of energy densities, leading to the emergence of a third family of stable compact stars (hybrid hadron-quark stars). The branch of stable hybrid stars is separated from the stable neutron star branch by a branch of \textit{unstable} hybrid stars. Of particular interest are hybrid stars with the same masses as neutron stars but different radii (twin stars). We study the dynamics of unstable hybrid stars all the way down to the minimum mass twin star through 3-dimensional general relativistic hydrodynamic simulations of non-rotating and rotating unstable-branch twin stars. We find that unstable hybrid stars naturally migrate toward the hadronic regime and undergo strong (quasi)radial oscillations in the process. Our study suggests that it may be difficult to form stable twin stars, and hence it may be more likely that astrophysical hybrid stars have masses above the twin star regime. The oscillations between the two phases could provide a unique gravitational wave signature for a Quantum Chromodynamics deconfinement in hybrid star progenitors. [Preview Abstract] |
Tuesday, April 20, 2021 2:42PM - 2:54PM Live |
Y10.00007: Precision Strong-field Gravity Tests with the Double Pulsar Ingrid Stairs The only known double-pulsar system has a number of unique features, including an extremely relativistic 2.5-hour orbit and an orbital inclined nearly edge-on to the line of sight. Our collaboration has been timing this system for over 16 years with various telescopes and now need to take into account higher-order relativistic effects when modelling and interpreting the data. The new phenomena observed include relativistic deformation of the orbit and next-to-leading-order effects on the Shapiro delay (retardation) and the aberration (gravitational signal deflection). It is also necessary to account for the Lense-Thirring effect (relativistic spin-orbit coupling) when interpreting the advance of periastron, leading to a constraint on the equation of state of super-dense matter. We have achieved the most precise test to date of the general-relativistic quadrupolar decription of gravitational waves, finding agreement with GR at a level of 0.013\% with 95\% confidence. [Preview Abstract] |
Tuesday, April 20, 2021 2:54PM - 3:06PM Live |
Y10.00008: Impact of the neutron-star deformability on equation of state parameters C.Y. Tsang, M.B. Tsang, Pawel Danielewicz, W.G. Lynch, F.J. Fattoyev We use a Bayesian inference analysis to explore the sensitivity of Taylor expansion parameters of the nuclear equation of state (EOS) to the neutron star dimensionless tidal deformability ($\Lambda )$ on 1 to 2 solar masses neutron stars. A global power law dependence between tidal deformability and compactness parameter (M/R) is verified over this mass region. To avoid superfluous correlations between the expansion parameters, we use a correlation-free EOS model based on a recently published meta-modeling approach. We find that assumptions in the prior distribution strongly influence the constraints on $\Lambda $. The $\Lambda $ constraints obtained from the neutron star merger event GW170817 prefer low values of L$_{\mathrm{sym}}$ and K$_{\mathrm{sym}}$, for a canonical neutron star with 1.4 solar mass. For neutron star with mass \textless 1.6 solar mass, L$_{\mathrm{sym}}$ and K$_{\mathrm{sym}}$ are highly correlated with the tidal deformability. For more massive neutron stars, the tidal deformability is more strongly correlated with higher order Taylor expansion parameters. [Preview Abstract] |
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