1,545 research outputs found
Numerical methods for the study of super-Eddington mass transfer in double white dwarf binaries
We present a numerical method for the study of double white dwarf (DWD) binary systems at the onset of super-Eddington mass transfer. We incorporate the physics of ideal inviscid hydrodynamical flow, Newtonian self-gravity, and radiation transport on a three-dimensional uniformly rotating cylindrical Eulerian grid. Our new method conserves total energy to a higher degree of accuracy than recent smoothed particle hydrodynamics methods and our previous Eulerian grid based method. We present the results of verification tests and we simulate the first 20+ orbits of a binary system of mass ratio q=0.7 at the onset of dynamically unstable direct impact mass transfer. Although the mass transfer rate exceeds the critical Eddington limit by many orders of magnitude, it appears to have very little effect on the accretion flow. We also model over 20 orbits of a DWD of mass ratio q=0.4. In these simulations, the accretion stream detaches from the accretor after 4 orbits and an asymmetric accretion torus forms. We submit these DWD models as the first self-consistent three dimensional simulations of mass transferring DWDs incorporating radiation transport
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
What is the Shell Around R Coronae Borealis?
The hydrogen-deficient, carbon-rich R Coronae Borealis (RCB) stars are known
for being prolific producers of dust which causes their large iconic declines
in brightness. Several RCB stars, including R CrB, itself, have large extended
dust shells seen in the far-infrared. The origin of these shells is uncertain
but they may give us clues to the evolution of the RCB stars. The shells could
form in three possible ways. 1) they are fossil Planetary Nebula (PN) shells,
which would exist if RCB stars are the result of a final, helium-shell flash,
2) they are material left over from a white-dwarf merger event which formed the
RCB stars, or 3) they are material lost from the star during the RCB phase.
Arecibo 21-cm observations establish an upper limit on the column density of H
I in the R CrB shell implying a maximum shell mass of 0.3
M. A low-mass fossil PN shell is still a possible source of the shell
although it may not contain enough dust. The mass of gas lost during a
white-dwarf merger event will not condense enough dust to produce the observed
shell, assuming a reasonable gas-to-dust ratio. The third scenario where the
shell around R CrB has been produced during the star's RCB phase seems most
likely to produce the observed mass of dust and the observed size of the shell.
But this means that R CrB has been in its RCB phase for 10 yr.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A
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
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