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1. General relativistic pulsations of ultra-massive ZZ Ceti stars

   https://doi.org/10.1093/mnras/stad2248  

   Córsico, Alejandro_H; Boston, S_Reece; Althaus, Leandro_G; Kilic, Mukremin; Kepler, S_O; Camisassa, María_E; Torres, Santiago (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the Gaia space mission. These dense and compact stellar remnants likely play a major role in Type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, general relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from 1.29 to $$1.369 \ \mathrm{M}_{\odot }$$ with the aim of assessing the impact of general relativity on the adiabatic gravity (g)-mode period spectrum of very high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt–Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the g-mode periods are shorter in the relativistic case than those in the Newtonian computations, with relative differences of up to ∼$50$ per cent for the highest mass models ($$1.369 \ \mathrm{M}_{\odot }$$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of general relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than ∼$$1.29 \ \mathrm{M}_{\odot }$$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars. 

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2. Stability and Instability of Self-Gravitating Relativistic Matter Distributions

   https://doi.org/10.1007/s00205-021-01647-2  

   Hadžić, Mahir; Lin, Zhiwu; Rein, Gerhard (May 2021, Archive for Rational Mechanics and Analysis)

   Abstract We consider steady state solutions of the massive, asymptotically flat, spherically symmetric Einstein–Vlasov system, i.e., relativistic models of galaxies or globular clusters, and steady state solutions of the Einstein–Euler system, i.e., relativistic models of stars. Such steady states are embedded into one-parameter families parameterized by their central redshift$$\kappa >0$$ $\kappa >0$. We prove their linear instability when$$\kappa $$ $\kappa$is sufficiently large, i.e., when they are strongly relativistic, and prove that the instability is driven by a growing mode. Our work confirms the scenario of dynamic instability proposed in the 1960s by Zel’dovich & Podurets (for the Einstein–Vlasov system) and by Harrison, Thorne, Wakano, & Wheeler (for the Einstein–Euler system). Our results are in sharp contrast to the corresponding non-relativistic, Newtonian setting. We carry out a careful analysis of the linearized dynamics around the above steady states and prove an exponential trichotomy result and the corresponding index theorems for the stable/unstable invariant spaces. Finally, in the case of the Einstein–Euler system we prove a rigorous version of the turning point principle which relates the stability of steady states along the one-parameter family to the winding points of the so-called mass-radius curve. 

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3. On the Asymptotic Behavior of Solutions to the Vlasov–Poisson System

   https://doi.org/10.1093/imrn/rnab155  

   Ionescu, Alexandru D; Pausader, Benoit; Wang, Xuecheng; Widmayer, Klaus (July 2021, International Mathematics Research Notices)

   Abstract We prove small data modified scattering for the Vlasov–Poisson system in dimension $d=3$, using a method inspired from dispersive analysis. In particular, we identify a simple asymptotic dynamics related to the scattering mass. 

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4. An Isolated White Dwarf with a 70 s Spin Period

   https://doi.org/10.3847/2041-8213/ac3b60  

   Kilic, Mukremin; Kosakowski, Alekzander; Moss, Adam G.; Bergeron, P.; Conly, Annamarie A. (December 2021, The Astrophysical Journal Letters)

   Abstract We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high-speed photometry of three ultramassive white dwarfs within 100 pc and discovered significant variability in one. SDSS J221141.80+113604.4 is a 1.27M_⊙(assuming a CO core) magnetic white dwarf that shows 2.9% brightness variations in the BG40 filter with a 70.32 ± 0.04 s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength ofB_d= 15 MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long-term monitoring of the spin evolution of J2211+1136 and other fast-spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the monoperiodic variables found within the instability strip may be fast-spinning white dwarfs impersonating ZZ Ceti pulsators. 

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5. Nonlinear Landau Damping for the Vlasov–Poisson System in $$\mathbb {R}^3$$: The Poisson Equilibrium

   https://doi.org/10.1007/s40818-023-00161-w  

   Ionescu, Alexandru D.; Pausader, Benoit; Wang, Xuecheng; Widmayer, Klaus (June 2024, Annals of PDE)

   We prove asymptotic stability of the Poisson homogeneous equilibrium among solu- tions of the Vlasov–Poisson system in the Euclidean space R3. More precisely, we show that small, smooth, and localized perturbations of the Poisson equilibrium lead to global solutions of the Vlasov–Poisson system, which scatter to linear solutions at a polynomial rate as t → ∞. The Euclidean problem we consider here differs signif- icantly from the classical work on Landau damping in the periodic setting, in several ways. Most importantly, the linearized problem cannot satisfy a “Penrose condition”. As a result, our system contains resonances (small divisors) and the electric field is a superposition of an electrostatic component and a larger oscillatory component, both with polynomially decaying rates. 

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