258 research outputs found
Rotochemical heating of neutron stars
No full textThe electrostatic potential that keeps approximate charge neutrality in neutron star matter is self-consistently introduced into the formalism for "rotochemical heating" presented in a previous paper by Fernandez and Reisenegger. Although the new formalism is more rigorous, we show that its observable consequences are indistinguishable from those of the previous one, leaving the conclusions of the previous paper unchanged
Internal heating of old neutron stars: contrasting different mechanisms
No full textContext. The standard cooling models of neutron stars predict
temperatures of T < 104 K for ages
t > 107 yr. However, the likely
thermal emission detected from the millisecond pulsar J0437-4715, of spin-down age
ts ~ 7 × 109 yr, implies a temperature
T ~ 105 K. Thus, a heating mechanism needs to be added to
the cooling models in order to obtain agreement between theory and observation.
Aims. Several internal heating mechanisms could be operating in neutron
stars, such as magnetic field decay, dark matter accretion, crust cracking, superfluid
vortex creep, and non-equilibrium reactions (“rotochemical heating”). We study these
mechanisms to establish which could be the dominant source of thermal emission from old
pulsars.
Methods. We show by simple estimates that magnetic field decay, dark
matter accretion, and crust cracking are unlikely to have a significant heating effect on
old neutron stars. The thermal evolution for the other mechanisms is computed with the
code of Fernández and Reisenegger. Given the dependence of the heating mechanisms on the
spin-down parameters, we study the thermal evolution for two types of pulsars: young,
slowly rotating “classical” pulsars and old, fast rotating millisecond pulsars.
Results. We find that magnetic field decay, dark matter accretion, and
crust cracking do not produce any detectable heating of old pulsars. Rotochemical heating
and vortex creep can be important both for classical pulsars and millisecond pulsars. More
restrictive upper limits on the surface temperatures of classical pulsars could rule out
vortex creep as the main source of thermal emission. Rotochemical heating in classical
pulsars is driven by the chemical imbalance built up during their early spin-down, and is
therefore strongly sensitive to their initial rotation period
Contrasting neutron star heating mechanisms with Hubble Space Telescope observations
No full textTesis (Magíster en Física)--Pontificia Universidad Católica de Chile, 2018Si las estrellas de neutrones se enfriaran pasivamente, se esperaría que se vuelvan
indetectables en un tiempo menor a 107 años, al alcanzar bajas temperaturas
T < 104 K. Sin embargo, radiación del tipo ultravioleta, que implica temperaturas
superficiales T ∼ 105 K, fue detectada desde los pulsares de Giga años PSR J0437-
4715 y PSR J2124-3358 y también desde el pulsar B0950+08 cuya edad es ∼ 107
años. Esta discrepancia puede ser explicada por un grupo de mecanismos de
calentamiento propuestos en la literatura.
Usando el código de Petrovich y Reisenegger se calcularon curvas de evolución
térmica considerando diferentes mecanismos de calentamiento. Estas fueron contrastadas con las temperaturas inferidas a partir de las observaciones de los pulsares para determinar cuál es la principal fuente de emisión térmica de las estrellas
de neutrones.
Encontramos que el calentamiento rotoquímico, reacciones nucleares en las capas
profundas de la corteza y el calor liberado por la fricción de vórtices superfluidos pueden mantener la estrella lo suficientemente caliente más allá del tiempo
estándar de enfriamiento pasivo y explicar las observaciones.Si las estrellas de neutrones se enfriaran pasivamente, se esperaría que se vuelvan
indetectables en un tiempo menor a 107 años, al alcanzar bajas temperaturas
T < 104 K. Sin embargo, radiación del tipo ultravioleta, que implica temperaturas
superficiales T ∼ 105 K, fue detectada desde los pulsares de Giga años PSR J0437-
4715 y PSR J2124-3358 y también desde el pulsar B0950+08 cuya edad es ∼ 107
años. Esta discrepancia puede ser explicada por un grupo de mecanismos de
calentamiento propuestos en la literatura.
Usando el código de Petrovich y Reisenegger se calcularon curvas de evolución
térmica considerando diferentes mecanismos de calentamiento. Estas fueron contrastadas con las temperaturas inferidas a partir de las observaciones de los pulsares para determinar cuál es la principal fuente de emisión térmica de las estrellas
de neutrones.
Encontramos que el calentamiento rotoquímico, reacciones nucleares en las capas
profundas de la corteza y el calor liberado por la fricción de vórtices superfluidos pueden mantener la estrella lo suficientemente caliente más allá del tiempo
estándar de enfriamiento pasivo y explicar las observaciones.2020-08-0
Magnetohydrodynamic equilibria in barotropic stars
No full textTo appear in “Magnetic Fields in the Universe IV (2013)”Although barotropic matter does not constitute a realistic model for magnetic stars, it would be interesting
to con rm a recent conjecture that states that magnetized stars with a barotropic equation of state would be
dynamically unstable (Reisenegger 2009). In this work we construct a set of barotropic equilibria, which can
eventually be tested using a stability criterion. A general description of the ideal MHD equations governing
these equilibria is summarized, allowing for both poloidal and toroidal magnetic eld components. A new
nite-di erence numerical code is developed in order to solve the so-called Grad-Shafranov equation describing
the equilibrium of these con gurations, and some properties of the equilibria obtained are brie
y discussed
Chemical Equilibrium and Stable Stratification of a Multicomponent Fluid: Thermodynamics and Application to Neutron Stars
No full text
Stable magnetic equilibria and their evolution in the upper main sequence, white dwarfs and neutron stars
No full textContext. Long-lived, large-scale magnetic field configurations exist in upper main
sequence, white dwarf, and neutron stars. Externally, these fields
have a strong dipolar component,
while their internal structure and evolution are uncertain
but highly relevant to several problems in
stellar and high-energy astrophysics.
Aims. We discuss the main properties expected for the
stable magnetic configurations in these stars from physical arguments and the ways these
properties may determine the modes of decay of these configurations.
Methods. We explain and emphasize the likely importance of the non-barotropic, stable stratification
of matter
in all these stars (due to entropy gradients in main-sequence envelopes and white dwarfs,
due to composition gradients in neutron stars). We first illustrate it in
a toy model involving a single, azimuthal magnetic flux tube.
We then discuss the effect of stable stratification or its
absence on more general configurations, such as axisymmetric equilibria involving poloidal
and toroidal field components. We argue that the main mode of
decay for these configurations are processes that lift the
constraints set by stable stratification, such as heat
diffusion in main-sequence envelopes and white dwarfs, and beta
decays or particle diffusion in neutron stars. We estimate the
time scales for these processes, as well as their interplay with
the cooling processes in the case of neutron stars.
Results. Stable magneto-hydrostatic equilibria appear to exist in stars whenever the
matter in their interior is stably stratified (not barotropic).
These equilibria are not force-free and not required to satisfy the
Grad-Shafranov equation, but they do
involve both toroidal and poloidal field components. In main sequence stars with radiative
envelopes and in white dwarfs, heat diffusion is not fast enough to make
these equilibria evolve over the stellar lifetime. In neutron stars, a strong enough field
might decay by overcoming the compositional stratification through beta decays
(at the highest field strengths) or through ambipolar diffusion (for somewhat weaker fields).
These processes convert magnetic
energy to thermal energy, and they occur at significant rates only once the latter is
less than the former; therefore, they substantially delay the cooling of the neutron
star, while slowly decreasing its magnetic energy
Magnetic field evolution in neutron stars: one-dimensional multi-fluid model
No full textAims. This paper is the first in a series that aims to understand the long-term evolution of neutron star magnetic fields.
Methods. We model the stellar matter as an electrically neutral and lightly-ionized plasma composed of three moving particle species: neutrons, protons, and electrons; these species can be converted into each other by weak interactions (beta decays), suffer binary collisions, and be affected by each other's macroscopic electromagnetic fields. Since the evolution of the magnetic field occurs over thousands of years or more, compared to dynamical timescales (sound and Alfven) of milliseconds to seconds, we use a slow-motion approximation in which we neglect the inertial terms in the equations of motion for the particles. This approximation leads to three nonlinear partial-differential equations describing the evolution of the magnetic field, as well as the movement of two fluids: the charged particles (protons and electrons) and the neutrons. These equations are first rather than second order in time (involving the velocities of the three species but not their accelerations).
Results. In this paper, we restrict ourselves to a one-dimensional geometry in which the magnetic field points in one Cartesian direction, but varies only along an orthogonal direction. We study the evolution of the system in three different ways: (i) estimating timescales directly from the equations, guided by physical intuition; (ii) a normal-mode analysis in the limit of a nearly uniform system; and (iii) a finite-difference numerical integration of the full set of nonlinear partial-differential equations. We find good agreement between our analytical normal-mode solutions and the numerical simulations. We show that the magnetic field and the particles evolve through successive quasi-equilibrium states, on timescales that can be understood by physical arguments. Depending on parameter values, the magnetic field can evolve by ohmic diffusion or by ambipolar diffusion, the latter being limited either by interparticle collisions or by relaxation to chemical quasi-equilibrium through beta decays. The numerical simulations are further validated by verifying that they satisfy the known conservation laws in highly nonlinear situations
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