63 research outputs found
The role of dredge-up in double white dwarf mergers
We present the results of an investigation of the dredge-up and mixing during
the merger of two white dwarfs with different chemical compositions by
conducting hydrodynamic simulations of binary mergers for three representative
mass ratios. In all the simulations, the total mass of the two white dwarfs is
. Mergers involving a CO and a He white dwarf have
been suggested as a possible formation channel for R Coronae Borealis type
stars, and we are interested in testing if such mergers lead to conditions and
outcomes in agreement with observations. Even if the conditions during the
merger and subsequent nucleosynthesis favor the production of , the merger must avoid dredging up large amounts of , or
else it will be difficult to produce sufficient to explain
the oxygen ratio observed to be of order unity. We performed a total of 9
simulations using two different grid-based hydrodynamics codes using fixed and
adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that
in most of the simulations, of is
indeed dredged up during the merger. However, in SPH simulations where the
accretor is a hybrid He/CO white dwarf with a layer of
helium on top, we find that no is being dredged up, while in
the simulation of has been
brought up, making a WD binary consisting of a hybrid CO/He WD and a companion
He WD an excellent candidate for the progenitor of RCB stars.Comment: Accepted for publication in Ap
NuGrid: Toward High Precision Double-Degenerate Merger Simulations with SPH in 3D
We present preliminary results from recent high-resolution double-degenerate
merger simulations with the Smooth Particle Hydrodynamics (SPH) technique. We
put particular emphasis on verification and validation in our effort and show
the importance of details in the initial condition setup for the final outcome
of the simulation. We also stress the dynamical importance of including shocks
in the simulations. These results represent a first step toward a suite of
simulations that will shed light on the question whether double-degenerate
mergers are a viable path toward type 1a supernovae. In future simulations, we
will make use of the capabilities of the NuGrid collaboration in
post-processing SPH particle trajectories with a complete nuclear network to
follow the detailed nuclear reactions during the dynamic merger phase.Comment: To appear in the Conference Proceedings for the "10th Symposium on
Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island,
Michigan, US
A Comparison of Grid-based and SPH Binary Mass-transfer and Merger Simulations
There is currently a great amount of interest in the outcomes and astrophysical implications of mergers of double degenerate binaries. In a commonly adopted approximation, the components of such binaries are represented by polytropes with an index of n = 3/2. We present detailed comparisons of stellar mass-transfer and merger simulations of polytropic binaries that have been carried out using two very different numerical algorithms - a finite-volume grid code and a smoothed-particle hydrodynamics (SPH) code. We find that there is agreement in both the ultimate outcomes of the evolutions and the intermediate stages if the initial conditions for each code are chosen to match as closely as possible. We find that even with closely matching initial setups, the time it takes to reach a concordant evolution differs between the two codes because the initial depth of contact cannot be matched exactly. There is a general tendency for SPH to yield higher mass transfer rates and faster evolution to the final outcome. We also present comparisons of simulations calculated from two different energy equations: in one series, we assume a polytropic equation of state and in the other series an ideal gas equation of state. In the latter series of simulations, an atmosphere forms around the accretor, which can exchange angular momentum and cause a more rapid loss of orbital angular momentum. In the simulations presented here, the effect of the ideal equation of state is to de-stabilize the binary in both SPH and grid simulations, but the effect is more pronounced in the grid code
Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?
A leading formation scenario for R Coronae Borealis (RCB) stars invokes the
merger of degenerate He and CO white dwarfs (WD) in a binary. The observed
ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the
solar value of ~500. In this paper, we investigate whether such a low ratio can
be obtained in simulations of the merger of a CO and a He white dwarf. We
present the results of five 3-dimensional hydrodynamic simulations of the
merger of a double white dwarf system where the total mass is 0.9 Mdot and the
initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations
with a feature around the merged stars where the temperatures
and densities are suitable for forming 18O. However, more 16O is being
dredged-up from the C- and O-rich accretor during the merger than the amount of
18O that is produced. Therefore, on a dynamical time scale over which our
hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is
found. If the conditions found in the hydrodynamic simulations persist for 10^6
seconds the oxygen ratio drops to 16 in one case studied, while in a hundred
years it drops to ~4 in another case studied, consistent with the observed
values in RCB stars. Therefore, the merger of two white dwarfs remains a strong
candidate for the formation of these enigmatic stars.Comment: 42 pages, 19 figures. Accepted for publication in the Astrophysical
Journa
Very Large Excesses of 18O in Hydrogen-Deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
We have found that at least seven hydrogen-deficient carbon (HdC) and R
Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases
less than unity, values that are orders of magnitude lower than measured in
other stars (the Solar value is 500). Greatly enhanced 18O is evident in every
HdC and RCB we have measured that is cool enough to have detectable CO bands.
The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB
stars. These discoveries are important clues in determining the evolutionary
pathways of HdC and RCB stars, for which two models have been proposed: the
double degenerate (white dwarf (WD) merger), and the final helium-shell flash
(FF). No overproduction of 18O is expected in the FF scenario. We have
quantitatively explored the idea that HdC and RCB stars originate in the
mergers of CO- and He-WDs. The merger process is estimated to take only a few
days, with accretion rates of 150 Msun/ yr producing temperatures at the base
of the accreted envelope of 1.2 - 1.9 x 10^8 K. Analysis of a simplified
one-zone calculation shows that nucleosynthesis in the dynamically accreting
material may provide a suitable environment for a significant production of
18O, leading to very low values of 16O/18O, similar to those observed. We also
find qualitative agreement with observed values of 12C/13C and with the CNO
elemental ratios. H-admixture during the accretion process from the small
H-rich C/O WD envelope may play an important role in producing the observed
abundances. Overall our analysis shows that WD mergers may very well be the
progenitors of O18-rich RCB and HdC stars, and that more detailed simulations
and modeling are justified.Comment: 29 pages, 6 figure
Mariana Serpentinite Mud Volcanism Exhumes Subducted Seamount Materials: Implications for the Origin of Life.
The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life.
This article is part of a discussion meeting issue ‘Serpentine in the Earth system’
A lanthanide-rich kilonova in the aftermath of a long gamma-ray burst
Kilonovae are a rare class of astrophysical transients powered by the
radioactive decay of nuclei heavier than iron, synthesized in the merger of two
compact objects. Over the first few days, the kilonova evolution is dominated
by a large number of radioactive isotopes contributing to the heating rate. On
timescales of weeks to months, its behavior is predicted to differ depending on
the ejecta composition and merger remnant. However, late-time observations of
known kilonovae are either missing or limited. Here we report observations of a
luminous red transient with a quasi-thermal spectrum, following an unusual
gamma-ray burst of long duration. We classify this thermal emission as a
kilonova and track its evolution up to two months after the burst. At these
late times, the recession of the photospheric radius and the rapidly-decaying
bolometric luminosity () support the
recombination of lanthanide-rich ejecta as they cool.Comment: 47 pages, 14 figures, 9 tables; submitted; a minor typo fixe
A JWST Survey of the Supernova Remnant Cassiopeia A
We present initial results from a James Webb Space Telescope (JWST) survey of the youngest Galactic core-collapse supernova remnant, Cassiopeia A (Cas A), made up of NIRCam and MIRI imaging mosaics that map emission from the main shell, interior, and surrounding circumstellar/interstellar material (CSM/ISM). We also present four exploratory positions of MIRI Medium Resolution Spectrograph integral field unit spectroscopy that sample ejecta, CSM, and associated dust from representative shocked and unshocked regions. Surprising discoveries include (1) a weblike network of unshocked ejecta filaments resolved to ∼0.01 pc scales exhibiting an overall morphology consistent with turbulent mixing of cool, low-entropy matter from the progenitor’s oxygen layer with hot, high-entropy matter heated by neutrino interactions and radioactivity; (2) a thick sheet of dust-dominated emission from shocked CSM seen in projection toward the remnant’s interior pockmarked with small (∼1″) round holes formed by ≲0.″1 knots of high-velocity ejecta that have pierced through the CSM and driven expanding tangential shocks; and (3) dozens of light echoes with angular sizes between ∼0.″1 and 1′ reflecting previously unseen fine-scale structure in the ISM. NIRCam observations place new upper limits on infrared emission (≲20 nJy at 3 μm) from the neutron star in Cas A’s center and tightly constrain scenarios involving a possible fallback disk. These JWST survey data and initial findings help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, the metal and dust enrichment of galaxies, and the origin of compact remnant objects
The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults
Background: Popular beliefs that breakfast is the most important meal of the day are grounded in cross-sectional observations that link breakfast to health, the causal nature of which remains to be explored under real-life conditions.
Objective: The aim was to conduct a randomized controlled trial examining causal links between breakfast habits and all components of energy balance in free-living humans.
Design: The Bath Breakfast Project is a randomized controlled trial with repeated-measures at baseline and follow-up in a cohort in southwest England aged 21–60 y with dual-energy X-ray absorptiometry–derived fat mass indexes #11 kg/m2 in women (n = 21) and #7.5 kg/m2 in men (n = 12). Components of energy balance (resting metabolic rate, physical activity thermogenesis, energy intake) and 24-h glycemic responses were measured under free-living conditions with random allocation to daily breakfast ($700 kcal before 1100) or extended fasting (0 kcal until 1200) for 6 wk, with baseline and follow-up measures of health markers (eg, hematology/biopsies).
Results: Contrary to popular belief, there was no metabolic adaptation to breakfast (eg, resting metabolic rate stable within 11 kcal/d), with limited subsequent suppression of appetite (energy intake remained 539 kcal/d greater than after fasting; 95% CI: 157, 920 kcal/d). Rather, physical activity thermogenesis was markedly higher with breakfast than with fasting (442 kcal/d; 95% CI: 34, 851 kcal/d). Body mass and adiposity did not differ between treatments at baseline or follow-up and neither did adipose tissue glucose uptake or systemic indexes of cardiovascular health. Continuously measured glycemia was more variable during the afternoon and evening with fasting than with breakfast by the final week of the intervention (CV: 3.9%; 95% CI: 0.1%, 7.8%).
Conclusions: Daily breakfast is causally linked to higher physical activity thermogenesis in lean adults, with greater overall dietary energy intake but no change in resting metabolism. Cardiovascular health indexes were unaffected by either of the treatments, but breakfast maintained more stable afternoon and evening glycemia than did fasting
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