655 research outputs found
Simulation of subseismic joint and fault networks using a heuristic mechanical model
Flow simulations of fractured and faulted reservoirs require representation of subseismic structures about which subsurface data are limited. We describe a method for simulating fracture growth that is mechanically based but heuristic, allowing for realistic modelling of fracture networks with reasonable run times. The method takes a triangulated meshed surface as input, together with an initial stress field. Fractures initiate and grow based on the stress field, and the growing fractures relieve the stress in the mesh. We show that a wide range of bedding-plane joint networks can be modelled simply by varying the distribution and anisotropy of the initial stress field. The results are in good qualitative agreement with natural joint patterns. We then apply the method to a set of parallel veins and demonstrate how the variations in thickness of the veins can be represented. Lastly, we apply the method to the simulation of normal fault patterns on salt domes. We derive the stress field on the bedding surface using the horizon curvature. The modelled fault network shows both radial and concentric faults. The new method provides an effective means of modelling joint and fault networks that can be imported to the flow simulator
Circinus X-1: survivor of a highly asymmetric supernova
We have analyzed the kinematical parameters of Cir X-1 to constrain the
nature of its companion star, the eccentricity of the binary and the
pre-supernova parameter space. We argue that the companion is most likely to be
a low-mass (< 2.0 M_sun) unevolved star and that the eccentricity of the orbit
is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova
explosion and we find it must have been asymmetric. On average, we find that a
kick of 740 km/s is needed to account for the recently measured radial velocity
of +430 km/s (Johnston, Fender & Wu) for this extreme system. The corresponding
minimum kick velocity is 500 km/s. This is the largest kick needed to explain
the motion of any observed binary system. If Cir X-1 is associated with the
supernova remnant G321.9-0.3 then we find a limiting minimum age of this
remnant of 60000 yr. Furthermore, we predict that the companion star has lost
10% of its mass as a result of stripping and ablation from the impact of the
supernova shell shortly after the explosion.Comment: 6 pages, 3 figues, 2 tables, accepted for publication in MNRA
Morphological Properties of PPNs: Mid-IR and HST Imaging Surveys
We will review our mid-infrared and HST imaging surveys of the circumstellar
dust shells of proto-planetary nebulae. While optical imaging indirectly probes
the dust distribution via dust-scattered starlight, mid-IR imaging directly
maps the distribution of warm dust grains. Both imaging surveys revealed
preferencially axisymmetric nature of PPN dust shells, suggesting that
axisymmetry in planetary nebulae sets in by the end of the asymptotic giant
branch phase, most likely by axisymmetric superwind mass loss. Moreover, both
surveys yielded two morphological classes which have one-to-one correspondence
between the two surveys, indicating that the optical depth of circumstellar
dust shells plays an equally important role as the inclination angle in
determining the morphology of the PPN shells.Comment: 6 pages + 8 figures, to appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorny.
Figures have been degraded to minimize the total file siz
Evolution of a 3 \msun star from the main sequence to the ZZ Ceti stage: the role played by element diffusion
The purpose of this paper is to present new full evolutionary calculations
for DA white dwarf stars with the major aim of providing a physically sound
reference frame for exploring the pulsation properties of the resulting models
in future communications. Here, white dwarf evolution is followed in a
self-consistent way with the predictions of time dependent element diffusion
and nuclear burning. In addition, full account is taken of the evolutionary
stages prior to the white dwarf formation. In particular, we follow the
evolution of a 3 \msun model from the zero-age main sequence (the adopted
metallicity is Z=0.02) all the way from the stages of hydrogen and helium
burning in the core up to the thermally pulsing phase. After experiencing 11
thermal pulses, the model is forced to evolve towards its white dwarf
configuration by invoking strong mass loss episodes. Further evolution is
followed down to the domain of the ZZ Ceti stars on the white dwarf cooling
branch. Emphasis is placed on the evolution of the chemical abundance
distribution due to diffusion processes and the role played by hydrogen burning
during the white dwarf evolution. Furthermore, the implications of our
evolutionary models for the main quantities relevant for adiabatic pulsation
analysis are discussed. Interestingly, the shape of the Ledoux term is markedly
smoother as compared with previous detailed studies of white dwarfs. This is
translated into a different behaviour of the Brunt-Vaisala frequency.Comment: 11 pages, 11 figures, accepted for publication in MNRA
Die Druckerhöhung bei der Implosion von Kavitationsscheiben im Dieseleinspritzsystem
 
Near-IR Spectra of Red Supergiants and Giants. I- Models with Solar and with Mixing-Induced Surface Abundance Ratios
We provide a grid of PHOENIX spectra of red giant and supergiant stars, that
extend through optical and near-IR wavelengths. For the first time, models are
also provided with modified surface abundances of C, N and O, as a step towards
accounting for the changes known to occur due to convective dredge-up (and to
be enhanced in the case of rotation). The aims are (i) to assess how well
current models reproduce observed spectra, (ii) to quantify the effects of the
abundance changes on the spectra, and (iii) to determine how these changes
affect estimates of fundamental stellar parameters.
Observed giant star spectra can be fitted very well at solar metallicity down
to about 3400K. Modified surface abundances are preferred in only a minority of
cases for luminosity class II, possibly indicating mixing in excess of standard
first dredge-up.
Supergiant stars show a larger variety of near-IR spectra, and good fits are
currently obtained for about one third of the observations only. Modified
surface abundances help reproducing strong CN bands, but do not suffice to
resolve all the difficulties. The effect of the abundance changes on the
estimated Teff depends on the wavelength range of observation and can amount
several 100K. Reasons for the remaining discrepancies are discussed.Comment: To be published in A&A. 19 p., 35 postscript figures, uses aa.cls.
Selected model spectra available through CD
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
Solar Neutrinos from CNO Electron Capture
The neutrino flux from the sun is predicted to have a CNO-cycle contribution
as well as the known pp-chain component. Previously, only the fluxes from beta+
decays of 13N, 15O, and 17F have been calculated in detail. Another neutrino
component that has not been widely considered is electron capture on these
nuclei. We calculate the number of interactions in several solar neutrino
detectors due to neutrinos from electron capture on 13N, 15O, and 17F, within
the context of the Standard Solar Model. We also discuss possible non-standard
models where the CNO flux is increased.Comment: 4 pages, 1 figure, submitted to Phys. Rev. C; v2 has minor changes
including integration over solar volume and addition of missing reference to
previous continuum electron capture calculation; v3 has minor changes
including addition of references and the correction of a small (about 1%)
numerical error in the table
Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars
The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB)
stars are discussed in terms of the merger of a carbon-oxygen white dwarf with
a helium white dwarf. The model is expressed as a linear mixture of the
individual layers of both constituent white dwarfs, taking account of the
specific evolution of each star. In developing this recipe from previous
versions, particular attention has been given to the inter-shell abundances of
the asymptotic giant branch star which evolved to become the carbon-oxygen
white dwarf. Thus the surface composition of the merged star is estimated as a
function of the initial mass and metallicity of its progenitor. The question of
whether additional nucleosynthesis occurs during the white dwarf merger has
been examined.
The high observed abundances of carbon and oxygen must either originate by
dredge-up from the core of the carbon-oxygen white dwarf during a cold merger
or be generated directly by alpha-burning during a hot merger. The presence of
large quantities of O18 may be consistent with both scenarios, since a
significant O18 pocket develops at the carbon/helium boundary in a number of
our post-AGB models.
The production of fluorine, neon and phosphorus in the AGB intershell
produces n overabundance at the surface of the merged stars, but generally not
in sufficient quantity. However, the evidence for an AGB origin for these
elements points to progenitor stars with initial masses in the range 1.9 - 3
solar masses.
There is not yet sufficient information to discriminate the origin (fossil or
prompt) of all the abundance anomalies observed in EHe and RCB stars. Further
work is required on argon and s-process elements in the AGB intershell, and on
the predicted yields of all elements from a hot merger.Comment: 20 pages, 8 figures, 3 tables, MNRAS in pres
Rare case of an adult male Montagu's Harrier Circus pygargus over-summering in West Africa, as revealed by GPS tracking
Over-summering within or near the African wintering range by immature, non-breeding individuals occurs regularly in several species of long-distance migratory raptors, yet the extent of over-summering in Africa by adult birds remains unclear. In this study, we describe a case of an adult Montagu's Harrier over-summering in Africa, as revealed by GPS tracking. By relating detailed knowledge of the bird's movements to remotely sensed environmental data (normalized difference vegetation index, NDVI), we show that over-summering in this case was likely related to an exceptionally difficult breeding season the previous year rather than an effect of adverse weather conditions encountered during the winter or a failed attempt to migrate. Various factors are discussed as potential driving forces behind the bird's intra-African movements. Finally, we relate the documented case to a large number of North European Montagu's Harriers studied by telemetry and show that over-summering in Africa by adult individuals is indeed a rare event
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