45 research outputs found
Radiative cooling in numerical astrophysics: the need for adaptive mesh refinement
Energy loss through optically thin radiative cooling plays an important part
in the evolution of astrophysical gas dynamics and should therefore be
considered a necessary element in any numerical simulation. Although the
addition of this physical process to the equations of hydrodynamics is
straightforward, it does create numerical challenges that have to be overcome
in order to ensure the physical correctness of the simulation. First, the
cooling has to be treated (semi-)implicitly, owing to the discrepancies between
the cooling timescale and the typical timesteps of the simulation. Secondly,
because of its dependence on a tabulated cooling curve, the introduction of
radiative cooling creates the necessity for an interpolation scheme. In
particular, we will argue that the addition of radiative cooling to a numerical
simulation creates the need for extremely high resolution, which can only be
fully met through the use of adaptive mesh refinement.Comment: 11 figures. Accepted for publication in Computers & Fluid
Multi-dimensional models of circumstellar shells around evolved massive stars
Massive stars shape their surrounding medium through the force of their
stellar winds, which collide with the circumstellar medium. Because the
characteristics of these stellar winds vary over the course of the evolution of
the star, the circumstellar matter becomes a reflection of the stellar
evolution and can be used to determine the characteristics of the progenitor
star. In particular, whenever a fast wind phase follows a slow wind phase, the
fast wind sweeps up its predecessor in a shell, which is observed as a
circumstellar nebula. We make 2-D and 3-D numerical simulations of fast stellar
winds sweeping up their slow predecessors to investigate whether numerical
models of these shells have to be 3-D, or whether 2-D models are sufficient to
reproduce the shells correctly. We focus on those situations where a fast
Wolf-Rayet (WR) star wind sweeps up the slower wind emitted by its predecessor,
being either a red supergiant or a luminous blue variable. As the fast WR wind
expands, it creates a dense shell of swept up material that expands outward,
driven by the high pressure of the shocked WR wind. These shells are subject to
a fair variety of hydrodynamic-radiative instabilities. If the WR wind is
expanding into the wind of a luminous blue variable phase, the instabilities
will tend to form a fairly small-scale, regular filamentary lattice with thin
filaments connecting knotty features. If the WR wind is sweeping up a red
supergiant wind, the instabilities will form larger interconnected structures
with less regularity. Our results show that 3-D models, when translated to
observed morphologies, give realistic results that can be compared directly to
observations. The 3-D structure of the nebula will help to distinguish
different progenitor scenarios.Comment: Accepted for publication in A&A. All figures in low resolution. v2:
language corrections and addition of DOI numbe
Luminous Blue Variables & Mass Loss near the Eddington Limit
During the course of their evolution, massive stars lose a substantial
fraction of their initial mass, both through steady winds and through
relatively brief eruptions during their Luminous Blue Variable (LBV) phase.
This talk reviews the dynamical driving of this mass loss, contrasting the
line-driving of steady winds to the potential role of continuum driving for
eruptions during LBV episodes when the star exceeds the Eddington limit. A key
theme is to emphasize the inherent limits that self-shadowing places on
line-driven mass loss rates, whereas continuum driving can in principle drive
mass up to the "photon-tiring" limit, for which the energy to lift the wind
becomes equal to the stellar luminosity. We review how the "porosity" of a
highly clumped atmosphere can regulate continuum-driven mass loss, but also
discuss recent time-dependent simulations of how base mass flux that exceeds
the tiring limit can lead to flow stagnation and a complex, time-dependent
combination of inflow and outflow regions. A general result is thus that
porosity-mediated continuum driving in super-Eddington phases can explain the
large, near tiring-limit mass loss inferred for LBV giant eruptions.Comment: Conference proceedings, Massive Stars as Cosmic Engines, IAU Symp
250, ed. F. Bresolin, P. A. Crowther, & J. Puls (Cambridge Univ. Press
Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?
Many evolved stars travel through space at supersonic velocities, which leads
to the formation of bow shocks ahead of the star where the stellar wind
collides with the interstellar medium (ISM). Herschel observations of the bow
shock of -Orionis show that the shock is almost free of instabilities,
despite being, at least in theory, subject to both Kelvin-Helmholtz and
Rayleigh-Taylor instabilities. A possible explanation for the lack of
instabilities lies in the presence of an interstellar magnetic field. We wish
to investigate whether the magnetic field of the interstellar medium (ISM) in
the Orion arm can inhibit the growth of instabilities in the bow shock of
-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic
simulations of a circumstellar bow shock, using the wind parameters derived for
-Orionis and interstellar magnetic field strengths of , and G, which fall within the boundaries of the observed
magnetic field strength in the Orion arm of the Milky Way. Our results show
that even a relatively weak magnetic field in the interstellar medium can
suppress the growth of Rayleigh-Taylor and Kelvin-Helmholtz instabilities,
which occur along the contact discontinuity between the shocked wind and the
shocked ISM. The presence of even a weak magnetic field in the ISM effectively
inhibits the growth of instabilities in the bow shock. This may explain the
absence of such instabilities in the Herschel observations of -Orionis.Comment: 5 pages, including 7 figures. The published version will include 4
animations. Accepted for publication in A&
Shape and evolution of wind-blown bubbles of massive stars: on the effect of the interstellar magnetic field
The winds of massive stars create large (>10 pc) bubbles around their
progenitors. As these bubbles expand they encounter the interstellar coherent
magnetic field which, depending on its strength, can influence the shape of the
bubble. We wish to investigate if, and how much, the interstellar magnetic
field can contribute to the shape of an expanding circumstellar bubble around a
massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical
simulations of bubbles, using a single star model, combined with several
different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic
field. This covers the typical field strengths of the interstellar magnetic
fields found in the galactic disk and bulge. Furthermore, we present two
simulations that include both a 5 muG interstellar magnetic field and a 10,000
K interstellar medium and two different ISM densities to demonstrate how the
magnetic field can combine with other external factors to influence the
morphology of the circumstellar bubbles. Our results show that low magnetic
fields, as found in the galactic disk, inhibit the growth of the circumstellar
bubbles in the direction perpendicular to the field. As a result, the bubbles
become ovoid, rather than spherical. Strong interstellar fields, such as
observed for the galactic bulge, can completely stop the expansion of the
bubble in the direction perpendicular to the field, leading to the formation of
a tube-like bubble. When combined with a warm, high-density ISM the bubble is
greatly reduced in size, causing a dramatic change in the evolution of
temporary features inside the bubble. The magnetic field of the interstellar
medium can affect the shape of circumstellar bubbles. This effect may have
consequences for the shape and evolution of circumstellar nebulae and supernova
remnants, which are formed within the main wind-blown bubble.Comment: Proposed for acceptance for publication in Astronomy & Astrophysics.
The published version will contain animations of each simulatio
Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines
Long gamma-ray bursts are thought to be caused by a subset of exploding
Wolf-Rayet stars. We argue that the circumstellar absorption lines in early
supernova and in gamma-ray burst afterglow spectra may allow us to determine
the main properties of the Wolf-Rayet star progenitors which can produce those
two events. To demonstrate this, we first simulate the hydrodynamic evolution
of the circumstellar medium around a 40 Msun star up to the time of the
supernova explosion. Knowledge of density, temperature and radial velocity of
the circumstellar matter as function of space and time allows us to compute the
column density in the line of sight to the centre of the nebula, as a function
of radial velocity, angle, and time. Our column density profiles indicate the
possible number, strengths, widths and velocities of absorption line components
in supernova and gamma-ray burst afterglow spectra. Our example calculation
shows four distinct line features during the Wolf-Rayet stage, at about 0, 50,
150-700 and 2200 km/s, with only those of the lowest and highest velocity
present at all times. The 150-700 km/s feature decays rapidly as function of
time after the onset of the Wolf-Rayet stage. It consists of a variable number
of components, and, especially in its evolved stage, is depending strongly on
the particular line of sight. A comparison with absorption lines detected in
the afterglow of GRB 021004 suggests that the high velocity absorption
component in GRB 021004 may be attributed to the free streaming Wolf-Rayet
wind, which is consistent with the steep density drop indicated by the
afterglow light curve. The presence of the intermediate velocity components
implies that the duration of the Wolf-Rayet phase of the progenitor of GRB
021004 was much smaller than the average Wolf-Rayet life time.Comment: 13 pages, 13 figures, accepted by Astronomy & Astrophysics The newest
version contains the changes requested by the A&A style edito
Computing the dust distribution in the bowshock of a fast moving, evolved star
We study the hydrodynamical behavior occurring in the turbulent interaction
zone of a fast moving red supergiant star, where the circumstellar and
interstellar material collide. In this wind-interstellar medium collision, the
familiar bow shock, contact discontinuity, and wind termination shock
morphology forms, with localized instability development. Our model includes a
detailed treatment of dust grains in the stellar wind, and takes into account
the drag forces between dust and gas. The dust is treated as pressureless gas
components binned per grainsize, for which we use ten representative grainsize
bins. Our simulations allow to deduce how dust grains of varying sizes become
distributed throughout the circumstellar medium. We show that smaller dust
grains (radius <0.045 micro-meters) tend to be strongly bound to the gas and
therefore follow the gas density distribution closely, with intricate
finestructure due to essentially hydrodynamical instabilities at the
wind-related contact discontinuity. Larger grains which are more resistant to
drag forces are shown to have their own unique dust distribution, with
progressive deviations from the gas morphology. Specifically, small dust grains
stay entirely within the zone bound by shocked wind material. The large grains
are capable of leaving the shocked wind layer, and can penetrate into the
shocked or even unshocked interstellar medium. Depending on how the number of
dust grains varies with grainsize, this should leave a clear imprint in
infrared observations of bowshocks of red supergiants and other evolved stars.Comment: Accepted for publication in ApJL, 4 figure
Using numerical models of bow shocks to investigate the circumstellar medium of massive stars
Many massive stars travel through the interstellar medium at supersonic
speeds. As a result they form bow shocks at the interface between the stellar
wind. We use numerical hydrodynamics to reproduce such bow shocks numerically,
creating models that can be compared to observations. In this paper we discuss
the influence of two physical phenomena, interstellar magnetic fields and the
presence of interstellar dust grains on the observable shape of the bow shocks
of massive stars.
We find that the interstellar magnetic field, though too weak to restrict the
general shape of the bow shock, reduces the size of the instabilities that
would otherwise be observed in the bow shock of a red supergiant. The
interstellar dust grains, due to their inertia can penetrate deep into the bow
shock structure of a main sequence O-supergiant, crossing over from the ISM
into the stellar wind. Therefore, the dust distribution may not always reflect
the morphology of the gas. This is an important consideration for infrared
observations, which are dominated by dust emission.
Our models clearly show, that the bow shocks of massive stars are useful
diagnostic tools that can used to investigate the properties of both the
stellar wind as well as the interstellar medium.Comment: 7 pages, 4 figures, to be published in the Journal of Physics:
Conference Series (JPCS) as part of the proceedings of the 13th Annual
International Astrophysics Conferenc
3-D simulations of shells around massive stars
As massive stars evolve, their winds change. This causes a series of
hydrodynamical interactions in the surrounding medium. Whenever a fast wind
follows a slow wind phase, the fast wind sweeps up the slow wind in a shell,
which can be observed as a circumstellar nebula.
One of the most striking examples of such an interaction is when a massive
star changes from a red supergiant into a Wolf-Rayet star. Nebulae resulting
from such a transition have been observed around many Wolf-Rayet stars and show
detailed, complicated structures owing to local instabilities in the swept-up
shells.
Shells also form in the case of massive binary stars, where the winds of two
stars collide with one another. Along the collision front gas piles up, forming
a shell that rotates along with the orbital motion of the binary stars. In this
case the shell follows the surface along which the ram pressure of the two
colliding winds is in balance.
Using the MPI-AMRVAC hydrodynamics code we have made multi-dimensional
simulations of these interactions in order to model the formation and evolution
of these circumstellar nebulae and explore whether full 3D simulations are
necessary to obtain accurate models of such nebulae.Comment: 5 Pages, 4 figures, Proceedings of the 39th Liege Astrophysical
Colloquium, held in Liege 12-16 July 201
Pinwheels in the sky, with dust: 3D modeling of the Wolf-Rayet 98a environment
The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric
surveys, since its identification as a "rotating pinwheel nebulae", where
infrared images display a spiral dust lane revolving with a 1.4 year
periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling
given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic
models for WR 98a, where dust creation and redistribution are self-consistently
incorporated. Our grid-adaptive simulations resolve details in the wind
collision region at scales below one percent of the orbital separation (~4 AU),
while simulating up to 1300 AU. We cover several orbital periods under
conditions where the gas component alone behaves adiabatic, or is subject to
effective radiative cooling. In the adiabatic case, mixing between stellar
winds is effective in a well-defined spiral pattern, where optimal conditions
for dust creation are met. When radiative cooling is incorporated, the
interaction gets dominated by thermal instabilities along the wind collision
region, and dust concentrates in clumps and filaments in a volume-filling
fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the
rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations
and confront photometric long-term monitoring. We predict an asymmetry in the
dust distribution between leading and trailing edge of the spiral, show that
ALMA and E-ELT would be able to detect fine-structure in the spiral indicative
of Kelvin-Helmholtz development, and confirm the variation in photometry due to
the orientation. Historic Keck images are reproduced, but their resolution is
insufficient to detect the details we predict.Comment: Accepted for publication in mnra