91,488 research outputs found
Cool bottom processes on the thermally-pulsing AGB and the isotopic composition of circumstellar dust grains
(Abridged) We examine the effects of cool bottom processing (CBP) on several
isotopic ratios in the convective envelope during the TP-AGB phase of evolution
in a 1.5 M_sun initial-mass star of solar initial composition. We use a
parametric model which treats extra mixing by introducing mass flow between the
convective envelope and the underlying radiative zone. The parameters of this
model are the mass circulation rate (Mdot) and the maximum temperature (T_P)
experienced by the circulating material. The effects of nuclear reactions in
the flowing matter were calculated using a set of structures of the radiative
zone selected from a complete stellar evolution calculation. The compositions
of the flowing material were obtained and the resulting changes in the envelope
determined. Abundant ^26Al was produced by CBP for log T_P > 7.65. While
^26Al/^27Al depends on T_P, the isotopic ratios in CNO elements depend
dominantly on the circulation rate. The correspondence is shown between models
of CBP as parameterized by a diffusion formalism within the stellar evolution
model and those using the mass-flow formalism employed here. The isotopic
ratios are compared with the data on circumstellar dust grains. It is found
that the ratios ^{18}O/^{16}O, ^{17}O/^{16}O, and ^26Al/^27Al observed for
oxide grains formed at C/O < 1 are reasonably well-understood. However, the
^15N/^14N, ^12C/^13C, and ^26Al/^27Al in carbide grains (C/O > 1) require many
stellar sources with ^14N/^15N at least a factor of 4 below solar. The rare
grains with ^12C/^13C < 10 cannot be produced by any red-giant or AGB source.Comment: 35 pages, plus 18 included figures. Scheduled for January 10, 2003
issue of Ap
Models for the soft X-ray emission of post-outburst classical novae
A hydrostatic and stationary white dwarf envelope model has been developed
for the study of the post-outburst phases of classical novae and their soft
X-ray emission. Several white dwarf masses and chemical compositions typical
for classical novae have been considered. The results show that the luminosity,
maximum effective temperature and envelope masses depend on the white dwarf
mass and on the chemical composition. Envelope masses for which equilibrium
solutions exist are pretty small ~10^{-7}-10^{-6} Msun, thus leading to a short
duration of the soft X-ray emitting phase of classical novae, in agreement with
most of the observations. The models presented provide a useful tool for the
determination of the white dwarf properties from observable parameters in the
X-ray range.Comment: 15 pages, 11 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
Aluminum-, Calcium- And Titanium-Rich Oxide Stardust In Ordinary Chondrite Meteorites
We report isotopic data for a total of 96 presolar oxide grains found in
residues of several unequilibrated ordinary chondrite meteorites. Identified
grain types include Al2O3, MgAl2O4, hibonite (CaAl12O19) and Ti oxide. This
work greatly increases the presolar hibonite database, and is the first report
of presolar Ti oxide. O-isotopic compositions of the grains span previously
observed ranges and indicate an origin in red giant and asymptotic giant branch
(AGB) stars of low mass (<2.5 MSun) for most grains. Cool bottom processing in
the parent AGB stars is required to explain isotopic compositions of many
grains. Potassium-41 enrichments in hibonite grains are attributable to in situ
decay of now-extinct 41Ca. Inferred initial 41Ca/40Ca ratios are in good
agreement with model predictions for low-mass AGB star envelopes, provided that
ionization suppresses 41Ca decay. Stable Mg and Ca isotopic ratios of most of
the hibonite grains reflect primarily the initial compositions of the parent
stars and are generally consistent with expectations for Galactic chemical
evolution, but require some local interstellar chemical inhomogeneity. Very
high 17O/16O or 25Mg/24Mg ratios suggest an origin for some grains in binary
star systems where mass transfer from an evolved companion has altered the
parent star compositions. A supernova origin for the hitherto enigmatic
18O-rich Group 4 grains is strongly supported by multi-element isotopic data
for two grains. The Group 4 data are consistent with an origin in a single
supernova in which variable amounts of material from the deep 16O-rich interior
mixed with a unique end-member mixture of the outer layers. The Ti oxide grains
primarily formed in low-mass AGB stars. They are smaller and rarer than
presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes.Comment: Accepted for publication in ApJ; 47 pages, 13 figure
Stellar Origins of Extremely - and -enriched Presolar SiC Grains: Novae or Supernovae?
Extreme excesses of (/<10) and
(/<20) in rare presolar SiC grains have been considered
diagnostic of an origin in classical novae, though an origin in core collapse
supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data
for 14 submicron- to micron-sized - and -enriched presolar SiC
grains (/<16 and /<~100) from Murchison, and
their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains
are enriched in and , but with quite diverse Si isotopic
signatures. Four grains with excesses similar to those of type C
SiC grains likely came from CCSNe, which experienced explosive H burning
occurred during explosions. The independent coexistence of proton- and
neutron-capture isotopic signatures in these grains strongly supports
heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven
putative nova grains with excesses and depletions show
lower-than-solar / ratios that cannot be explained by classical
nova nucleosynthetic models. We discuss these signatures within the CCSN
scenario. For the remaining five putative nova grains, both nova and supernova
origins are viable because explosive H burning in the two stellar sites could
result in quite similar proton-capture isotopic signatures. Three of the grains
are sub-type AB grains that are also enriched, but have a range of
higher /. We found that -enriched AB grains
(~50</<~100) have distinctive isotopic signatures compared to
putative nova grains, such as higher /, lower
/, and lack of excess, indicating weaker
proton-capture nucleosynthetic environments.Comment: fix typo in one of the authors' name
The Significance of Multiple Saturation Points in the Context of Polybaric Near-fractional Melting
Experimental petrologists have successfully located basaltic liquid compositions parental to mid-ocean ridge basalt that are, within experimental resolution, multiply saturated with three-phase harzburgite or four-phase lherzolite assemblages on their liquidus at some elevated pressure. Such an experimental result is a necessary consequence of any paradigm in which erupted basalts derive from single-batch primary liquids that equilibrate with a mantle residue and undergo no subsequent magma mixing before differentiation and eruption. Here we investigate whether, conversely, such evidence of multiple saturation is sufficient to exclude dynamic melting models wherein increments of melt are mixed after segregation from residues, during melt transport or in magma chambers. Using two independent models of crystal–liquid equilibria to simulate polybaric near-fractional peridotite melting, we find that aggregate liquids from such melting processes can display near-intersections of liquidus surfaces too close to distinguish experimentally from exact multiple saturation points. Given uncertainties in glass compositions, fractionation corrections, experimental temperature and pressure conditions, and achievement of equilibrium, these results suggest that polybaric mixtures can in fact masquerade as mantle-equilibrated single-batch primary liquids. Multiple saturation points on the liquidus surfaces of primitive basalts do, however, preserve information about the average pressure of extraction of their constituent increments of liquid
Hexagonal High-Entropy Alloys
We report on the discovery of a high-entropy alloy with a hexagonal crystal
structure. Equiatomic samples in the alloy system Ho-Dy-Y-Gd-Tb were found to
solidify as homogeneous single-phase high-entropy alloys. The results of our
electron diffraction investigations and high-resolution scanning transmission
electron microscopy are consistent with a Mg-type hexagonal structure. The
possibility of hexagonal high-entropy alloys in other alloy systems is
discussed.Comment: Changes upon replacement: inserted submission date of manuscript to
journal. No other changes were mad
STELLAR ORIGINS OF EXTREMELY C-13- AND N-15-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?
Extreme excesses of 13C (12C/13C < 10) and 15N (14N/15N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized 13C- and 15N-enriched presolar SiC grains (12C/13C < 16 and 14N/15N < ~100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in 13C and 15N, but with quite diverse Si isotopic signatures. Four grains with 29,30Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with 30Si excesses and 29Si depletions show lower-than-solar 34S/32S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that 15N-enriched AB grains (~50 < 14N/15N < ~100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower 26Al/27Al, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments
New constraints on the major neutron source in low-mass AGB stars
We compare updated Torino postprocessing asymptotic giant branch (AGB)
nucleosynthesis model calculations with isotopic compositions of mainstream SiC
dust grains from low-mass AGB stars. Based on the data-model comparison, we
provide new constraints on the major neutron source, 13C({\alpha},n)16O in the
He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba
grain data can only be consistently explained by the Torino model calculations
that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This
observation provides strong support to the suggestion of deep mixing of H into
the He-intershell at low 13C concentrations as a result of efficient transport
of H through magnetic tubes.Comment: ApJ, accepte
Phase separation in hydrogen-helium mixtures at Mbar pressures
The properties of hydrogen-helium mixtures at Mbar pressures and intermediate
temperatures (4000 to 10000 K) are calculated with first-principles molecular
dynamics simulations. We determine the equation of state as a function of
density, temperature, and composition and, using thermodynamic integration, we
estimate the Gibbs free energy of mixing, thereby determining the temperature,
at a given pressure, when helium becomes insoluble in dense metallic hydrogen.
These results are directly relevant to models of the interior structure and
evolution of Jovian planets. We find that the temperatures for the demixing of
helium and hydrogen are sufficiently high to cross the planetary adiabat of
Saturn at pressures around 5 Mbar; helium is partially miscible throughout a
significant portion of the interior of Saturn, and to a lesser extent in
Jupiter.Comment: 6 pages, 7 figures. Published in "Proceedings of the National Academy
of Sciences USA
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