67 research outputs found
Is Betelgeuse the Outcome of a Past Merger?
We explore the possibility that the star alpha Orionis (Betelgeuse) is the
outcome of a merger that occurred in a low mass ratio (q = M2/M1 = 0.07 - 0.25)
binary system some time in the past hundreds of thousands of years. To that
goal, we present a simple analytical model to approximate the perturbed
internal structure of a post-merger object following the coalescence of a
secondary in the mass range 1-4 Msun into the envelope of a 15-17 Msun primary.
We then compute the long-term evolution of post-merger objects for a grid of
initial conditions and make predictions about their surface properties for
evolutionary stages that are consistent with the observed location of
Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger
occurred after the end of the primary's main-sequence phase, while it was
expanding toward becoming a red supergiant star and typically with radius ~200
- 300 Rsun, then it's envelope is spun-up to values which remain in a range
consistent with the Betelgeuse observations for thousands of years of
evolution. We argue that the best scenario that can explain both the fast
rotation of Betelgeuse and its observed large space velocity is one where a
binary was dynamically ejected by its parent cluster a few million years ago
and then subsequently merged. An alternative scenario in which the progenitor
of Betelgeuse was spun up by accretion in a binary and released by the
supernova explosion of the companion requires a finely tuned set of conditions
but cannot be ruled out.Comment: 20 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Detectability of Gravitational Waves from SN 1987A
We discuss the potential for detection of gravitational waves from a rapidly
spinning neutron star produced by supernova 1987A taking the parameters claimed
by Middleditch et al. (2000) at face value. Asssuming that the dominant
mechanism for spin down is gravitational waves emitted by a freely precessing
neutron star, it is possible to constrain the wobble angle, the effective
moment of inertai of the precessing crust and the crust cracking stress limit.
Our analysis, suggests that, if the interpretation of the Middleditch data is
correct, the compact remnant of SN 1987A may well provide a reliable and
predictable source of gravitational waves well within the capability of LIGO
II
Evolving R Coronae Borealis Stars with MESA
The R Coronae Borealis (RCB) stars are rare hydrogen--deficient, carbon--rich
supergiants. They undergo extreme, irregular declines in brightness of many
magnitudes due to the formation of thick clouds of carbon dust. It is thought
that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We
constructed post--merger spherically asymmetric models computed with the MESA
code, and then followed the evolution into the region of the HR diagram where
the RCB stars are located. We also investigated nucleosynthesis in the
dynamically accreting material of CO/He WD mergers which may provide a suitable
environment for significant production of 18O and the very low 16O/18O values
observed. We have also discovered that the N abundance depends sensitively on
the peak temperature in the He--burning shell. Our MESA modeling consists of
engineering the star by adding He--WD material to an initial CO--WD model, and
then following the post--merger evolution using a nuclear--reaction network to
match the observed RCB abundances as it expands and cools to become an RCB
star. These new models are more physical because they include rotation, mixing,
mass-loss, and nucleosynthesis within MESA. We follow the later evolution
beyond the RCB phase to determine the stars' likely lifetimes. The relative
numbers of known RCB and Extreme Helium (EHe) stars correspond well to the
lifetimes predicted from the MESA models. In addition, most of computed
abundances agree very well with the observed range of abundances for the RCB
class.Comment: 14 pages, 7 figures, MNRAS in pres
Numerical Simulations of Mass Transfer in Binaries with Bipolytropic Components
We present the first self-consistent, three dimensional study of hydrodynamic
simulations of mass transfer in binary systems with bipolytropic (composite
polytropic) components. In certain systems, such as contact binaries or during
the common envelope phase, the core-envelope structure of the stars plays an
important role in binary interactions. In this paper, we compare mass transfer
simulations of bipolytropic binary systems in order to test the suitability of
our numerical tools for investigating the dynamical behaviour of such systems.
The initial, equilibrium binary models possess a core-envelope structure and
are obtained using the bipolytropic self-consistent field technique. We conduct
mass transfer simulations using two independent, fully three-dimensional,
Eulerian codes - Flow-ER and Octo-tiger. These hydrodynamic codes are compared
across binary systems undergoing unstable as well as stable mass transfer, and
the former at two resolutions. The initial conditions for each simulation and
for each code are chosen to match closely so that the simulations can be used
as benchmarks. Although there are some key differences, the detailed comparison
of the simulations suggests that there is remarkable agreement between the
results obtained using the two codes. This study puts our numerical tools on a
secure footing, and enables us to reliably simulate specific mass transfer
scenarios of binary systems involving components with a core-envelope
structure
A Numerical Method for Generating Rapidly Rotating Bipolytropic Structures in Equilibrium
We demonstrate that rapidly rotating bipolytropic (composite polytropic)
stars and toroidal disks can be obtained using Hachisu's self consistent field
technique. The core and the envelope in such a structure can have different
polytropic indices and also different average molecular weights. The models
converge for high cases, where T is the kinetic energy and W is the
gravitational energy of the system. The agreement between our numerical
solutions with known analytical as well as previously calculated numerical
results is excellent. We show that the uniform rotation lowers the maximum core
mass fraction or the Schnberg-Chandrasekhar limit for a
bipolytropic sequence. We also discuss the applications of this method to
magnetic braking in low mass stars with convective envelopes
Evolution of close white dwarf binaries
We describe the evolution of double degenerate binary systems, consisting of components obeying the zero-temperature mass-radius relationship for white dwarf stars, from the onset of mass transfer to one of several possible outcomes, including merger, tidal disruption of the donor, or survival as a semidetached AM CVn system. We use a combination of analytic solutions and numerical integrations of the standard orbit-averaged first-order evolution equations, including direct-impact accretion and the evolution of the components due to mass exchange. We include also the effects of mass loss during supercritical (super-Eddington) mass transfer and the tidal and advective exchanges of angular momentum between the binary components. With the caveat that our formalism does not include an explicit treatment of common-envelope phases, our results suggest that a larger fraction of detached double white dwarfs survive the onset of mass transfer than has been hitherto assumed, even if this mass transfer is initially unstable and rises to super-Eddington levels. In addition, as a consequence of the tidal coupling, systems that come into contact near the mass transfer instability boundary undergo a phase of oscillation cycles in their orbital period (and other system parameters). Unless the donor star has a finite entropy such that the effective mass-radius relationship deviates significantly from that of a zero-temperature white dwarf, we expect our results to be valid. Much of the formalism developed here would also apply to other mass-transferring binaries, and in particular to cataclysmic variables and Algol systems. © 2007. The American Astronomical Society. All rights reserved
Betelgeuse as a Merger of a Massive Star with a Companion
We investigate the merger between a 16 solar mass star, on its way to
becoming a red supergiant (RSG), and a 4 solar mass main-sequence companion.
Our study employs three-dimensional hydrodynamic simulations using the
state-of-the-art adaptive mesh refinement code Octo-Tiger. The initially
corotating binary undergoes interaction and mass transfer, resulting in the
accumulation of mass around the companion and its subsequent loss through the
second Lagrangian point (L2). The companion eventually plunges into the
envelope of the primary, leading to its spin-up and subsequent merger with the
helium core. We examine the internal structural properties of the post-merger
star, as well as the merger environment and the outflow driven by the merger.
Our findings reveal the ejection of approximately 0.6 solar mass of material in
an asymmetric and somewhat bipolar outflow. We import the post-merger stellar
structure into the MESA stellar evolution code to model its long-term nuclear
evolution. In certain cases, the post-merger star exhibits persistent rapid
equatorial surface rotation as it evolves in the H-R diagram towards the
observed location of Betelgeuse. These cases demonstrate surface rotation
velocities of a similar magnitude to those observed in Betelgeuse, along with a
chemical composition resembling that of Betelgeuse. In other cases, efficient
rotationally-induced mixing leads to slower surface rotation. This pioneering
study aims to model stellar mergers across critical timescales, encompassing
dynamical, thermal, and nuclear evolutionary stages.Comment: 28 pages, 19 figures, submitted to Ap
The Stability of Double White Dwarf Binaries Undergoing Direct Impact Accretion
We present numerical simulations of dynamically unstable mass transfer in a
double white dwarf binary with initial mass ratio, q = 0.4. The binary
components are approximated as polytropes of index n = 3/2 and the initially
synchronously rotating, semi-detached equilibrium binary is evolved
hydrodynamically with the gravitational potential being computed through the
solution of Poisson's equation. Upon initiating deep contact in our baseline
simulation, the mass transfer rate grows by more than an order of magnitude
over approximately ten orbits, as would be expected for dynamically unstable
mass transfer. However, the mass transfer rate then reaches a peak value, the
binary expands and the mass transfer event subsides. The binary must therefore
have crossed the critical mass ratio for stability against dynamical mass
transfer. Despite the initial loss of orbital angular momentum into the spin of
the accreting star, we find that the accretor's spin saturates and angular
momentum is returned to the orbit more efficiently than has been previously
suspected for binaries in the direct impact accretion mode. To explore this
surprising result, we directly measure the critical mass ratio for stability by
imposing artificial angular momentum loss at various rates to drive the binary
to an equilibrium mass transfer rate. For one of these driven evolutions, we
attain equilibrium mass transfer and deduce that effectively q_crit has evolved
to approximately 2/3. Despite the absence of a fully developed disk, tidal
interactions appear effective in returning excess spin angular momentum to the
orbit.Comment: 27 pages, 6 figures. Please see
http://www.phys.lsu.edu/faculty/tohline/astroph/mftd07/ for animations and
full resolution figures. Accepted for publication in the Astrophysical
Journa
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