2,703 research outputs found
Simulating stellar winds in AMUSE
We present stellar_wind.py, a module that provides multiple methods of
simulating stellar winds using smoothed particle hydrodynamics codes (SPH)
within the astrophysical multipurpose software environment (AMUSE) framework.
With the simple wind mode, we create SPH wind particles in a spherically
symmetric shell. We inject the wind particles with a velocity equal to their
terminal velocity. The accelerating wind mode is similar, but with this method
particles can be injected with a lower initial velocity than the terminal
velocity and they are accelerated away from the star according to an
acceleration function. With the heating wind mode, SPH particles are created
with zero initial velocity with respect to the star, but instead wind particles
are given an internal energy based on the integrated mechanical luminosity of
the star. This mode is designed to be used on longer timescales and larger
spatial scales compared to the other two modes and assumes that the star is
embedded in a gas cloud. For fast winds, we find that both the simple and
accelerating mode can reproduce the desired velocity, density and temperature
profiles. For slow winds, the simple wind mode is insufficient due to dominant
hydrodynamical effects that change the wind velocities. The accelerating mode,
with additional options to account for these hydrodynamical effects, can still
reproduce the desired wind profiles. We test the heating mode by simulating
both a normal wind and a supernova explosion of a single star in a uniform
density medium. The stellar wind simulation results matches the analytical
solution for an expanding wind bubble. The supernova simulation gives
qualitatively correct results, but the simulated bubble expands faster than the
analytical solution predicts. We conclude with an example of a triple star
system which includes the colliding winds of all three stars.Comment: Accepted for publication in A&
Simulating extreme-mass-ratio systems in full general relativity
We introduce a new method for numerically evolving the full Einstein field
equations in situations where the spacetime is dominated by a known background
solution. The technique leverages the knowledge of the background solution to
subtract off its contribution to the truncation error, thereby more efficiently
achieving a desired level of accuracy. We demonstrate the method by applying it
to the radial infall of a solar-type star into supermassive black holes with
mass ratios . The self-gravity of the star is thus consistently
modeled within the context of general relativity, and the star's interaction
with the black hole computed with moderate computational cost, despite the over
five orders of magnitude difference in gravitational potential (as defined by
the ratio of mass to radius). We compute the tidal deformation of the star
during infall, and the gravitational wave emission, finding the latter is close
to the prediction of the point-particle limit.Comment: 6 pages, 5 figures; added one figure, revised to match PRD RC versio
Simulating the Formation of Massive Protostars: I. Radiative Feedback and Accretion Disks
We present radiation hydrodynamic simulations of collapsing protostellar
cores with initial masses of 30, 100, and 200 M. We follow their
gravitational collapse and the formation of a massive protostar and
protostellar accretion disk. We employ a new hybrid radiative feedback method
blending raytracing techniques with flux-limited diffusion for a more accurate
treatment of the temperature and radiative force. In each case, the disk that
forms becomes Toomre-unstable and develops spiral arms. This occurs between
0.35 and 0.55 freefall times and is accompanied by an increase in the accretion
rate by a factor of 2-10. Although the disk becomes unstable, no other stars
are formed. In the case of our 100 and 200 M simulation, the star
becomes highly super-Eddington and begins to drive bipolar outflow cavities
that expand outwards. These radiatively-driven bubbles appear stable, and
appear to be channeling gas back onto the protostellar accretion disk.
Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our
30 M simulation shows a star with a mass of 5.48 M and a
disk of mass 3.3 M, while our 100 M simulation forms a 28.8
M mass star with a 15.8 M disk over the course of 41.6 kyr,
and our 200 M simulation forms a 43.7 M star with an 18
M disk in 21.9 kyr. In the absence of magnetic fields or other forms
of feedback, the masses of the stars in our simulation do not appear limited by
their own luminosities.Comment: 24 pages, 14 figures. Accepted to The Astrophysical Journa
Tidally-induced thermonuclear Supernovae
We discuss the results of 3D simulations of tidal disruptions of white dwarfs
by moderate-mass black holes as they may exist in the cores of globular
clusters or dwarf galaxies. Our simulations follow self-consistently the
hydrodynamic and nuclear evolution from the initial parabolic orbit over the
disruption to the build-up of an accretion disk around the black hole. For
strong enough encounters (pericentre distances smaller than about 1/3 of the
tidal radius) the tidal compression is reversed by a shock and finally results
in a thermonuclear explosion. These explosions are not restricted to progenitor
masses close to the Chandrasekhar limit, we find exploding examples throughout
the whole white dwarf mass range. There is, however, a restriction on the
masses of the involved black holes: black holes more massive than M swallow a typical 0.6 M dwarf before their tidal forces
can overwhelm the star's self-gravity. Therefore, this mechanism is
characteristic for black holes of moderate masses. The material that remains
bound to the black hole settles into an accretion disk and produces an X-ray
flare close to the Eddington limit of _\odot$), typically lasting for a few months. The combination
of a peculiar thermonuclear supernova together with an X-ray flare thus
whistle-blows the existence of such moderate-mass black holes. The next
generation of wide field space-based instruments should be able to detect such
events.Comment: 8 pages, 2 figures, EuroWD0
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