20 research outputs found
3D Smoothed Particle Hydrodynamics Models of Betelgeuse's Bow Shock
Betelgeuse, the bright red supergiant (RSG) in Orion, is a runaway star. Its
supersonic motion through the interstellar medium has resulted in the formation
of a bow shock, a cometary structure pointing in the direction of motion. We
present the first 3D hydrodynamic simulations of the formation and evolution of
Betelgeuse's bow shock. We show that the bow shock morphology depends
substantially on the growth timescale for Rayleigh-Taylor versus
Kelvin-Helmholtz instabilities. We discuss our models in light of the recent
Herschel, GALEX and VLA observations. If the mass in the bow shock shell is low
(~few x 0.001 Msun), as seems to be implied by the AKARI and Herschel
observations, then Betelgeuse's bow shock is very young and is unlikely to have
reached a steady state. The circular, smooth bow shock shell is consistent with
this conclusion. We further discuss the implications of our results, in
particular, the possibility that Betelgeuse may have only recently entered the
RSG phase.Comment: 9 pages, 4 figures, Betelgeuse workshop, November 2012, Paris. To be
published in the European Astronomical Society Publications Series, editors:
Pierre Kervella, Thibaut Le Bertre & Guy Perri
Wind bubbles within H II regions around slowly moving stars
Interstellar bubbles around O stars are driven by a combination of the star's
wind and ionizing radiation output. The wind contribution is uncertain because
the boundary between the wind and interstellar medium is difficult to observe.
Mid-infrared observations (e.g., of the H II region RCW 120) show arcs of dust
emission around O stars, contained well within the H II region bubble. These
arcs could indicate the edge of an asymmetric stellar wind bubble, distorted by
density gradients and/or stellar motion. We present two-dimensional,
radiation-hydrodynamics simulations investigating the evolution of wind bubbles
and H II regions around massive stars moving through a dense (n=3000 cm^{-3}),
uniform medium with velocities ranging from 4 to 16 km/s. The H II region
morphology is strongly affected by stellar motion, as expected, but the wind
bubble is also very aspherical from birth, even for the lowest space velocity
considered. Wind bubbles do not fill their H II regions (we find filling
factors of 10-20%), at least for a main sequence star with mass M~30 Msun.
Furthermore, even for supersonic velocities the wind bow shock does not
significantly trap the ionization front. X-ray emission from the wind bubble is
soft, faint, and comes mainly from the turbulent mixing layer between the wind
bubble and the H II region. The wind bubble radiates <1 per cent of its energy
in X-rays; it loses most of its energy by turbulent mixing with cooler
photoionized gas. Comparison of the simulations with the H II region RCW 120
shows that its dynamical age is <=0.4 Myr and that stellar motion <=4 km/s is
allowed, implying that the ionizing source is unlikely to be a runaway star but
more likely formed in situ. The region's youth, and apparent isolation from
other O or B stars, makes it very interesting for studies of massive star
formation and of initial mass functions.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics (new version corrects an error in the simulation postprocessing,
figs 6,7,11 are modified slightly, conclusions unchanged
Numerical models for the circumstellar medium around Betelgeuse
The nearby red supergiant (RSG) Betelgeuse has a complex circumstellar medium
out to at least 0.5 parsecs from its surface, shaped by its mass-loss history
within the past 0.1 Myr, its environment, and its motion through the
interstellar medium (ISM). In principle its mass-loss history can be
constrained by comparing hydrodynamic models with observations. Observations
and numerical simulations indicate that Betelgeuse has a very young bow shock,
hence the star may have only recently become a RSG. To test this possibility we
calculated a stellar evolution model for a single star with properties
consistent with Betelgeuse. We incorporated the resulting evolving stellar wind
into 2D hydrodynamic simulations to model a runaway blue supergiant (BSG)
undergoing the transition to a RSG near the end of its life. The collapsing BSG
wind bubble induces a bow shock-shaped inner shell which at least superficially
resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is
the larger-scale retreating bow shock generated by the now defunct BSG wind's
interaction with the ISM. We investigate whether this outer shell could explain
the bar feature located (at least in projection) just in front of Betelgeuse's
bow shock.Comment: 5 pages, 3 figures; to appear in proceedings of the Betelgeuse 2012
Workshop, Paris, Nov. 201
Density Conversion between 1-D and 3-D Stellar Models with 1D-MESA2HYDRO-3D
We present 1D-MESA2HYDRO-3D, an open source, Python-based software tool that
provides an accessible means of generating physically motivated initial
conditions (ICs) for hydrodynamical simulations from 1-D stellar structure
models. We test 1D-MESA2HYDRO-3D on five stellar models generated with the MESA
stellar evolution code and verify its capacity as an IC generator with the
Phantom smoothed-particle hydrodynamics code \citep{MESAIV, Phantom}.
Consistency between the input density profiles, the 1D-MESA2HYDRO-3D-rendered
particle distributions, and the state of the distributions after evolution over
dynamical timescales is found for model stars ranging in structure and
density from a radially extended supergiant to a white dwarf.Comment: Accepted to ApJ; 14 pages, 13 figure
A wonderful muse : Mass transfer in mira-type binaries
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