2,825 research outputs found
Landform classification of the Mamaku Plateau using a digital elevation model.
The Mamaku Plateau is a large, forest-covered ignimbrite plateau in the North Island of New Zealand. This study was launched to aid inquiries into the evolu-tion of the plateau by classifying its valleys (which con-stitute Mamaku’s main geomorphological feature).An automatic classification of the valley forms in the Mamaku Plateau was made using a Digital Elevation Model (DEM). A new algorithm was created for the Arc/Info Geographic Information System, to extract the valleys from the DEM. Preliminary results from this phase of the inquiry indi-cated that it was difficult to do a computer-aided classi-fication of valleys based only on valley form. Conven-tional classification of the plateau’s valleys had also been done using stereoscopy to assist in the geomorphological interpretation. The results of these classifications were compared, and it was concluded that DEMs are useful in landscape analysis especially if combined with traditional geomorphological analysis
Singular Vertices and the Triangulation Space of the D-sphere
By a sequence of numerical experiments we demonstrate that generic
triangulations of the sphere for contain one {\it singular}
simplex. The mean number of elementary simplices sharing this
simplex increases with the volume of the triangulation according to a simple
power law. The lower dimension subsimplices associated with this
simplex also show a singular behaviour. Possible consequences for the
DT model of four-dimensional quantum gravity are discussed.Comment: 15 pages, 9 figure
Infrared High-Resolution Spectroscopy of Post-AGB Circumstellar Disks. I. HR 4049 - The Winnowing Flow Observed?
High-resolution infrared spectroscopy in the 2.3-4.6 micron region is
reported for the peculiar A supergiant, single-lined spectroscopic binary HR
4049. Lines from the CO fundamental and first overtone, OH fundamental, and
several H2O vibration-rotation transitions have been observed in the
near-infrared spectrum. The spectrum of HR 4049 appears principally in emission
through the 3 and 4.6 micron region and in absorption in the 2 micron region.
The 4.6 micron spectrum shows a rich 'forest' of emission lines. All the
spectral lines observed in the 2.3-4.6 micron spectrum are shown to be
circumbinary in origin. The presence of OH and H2O lines confirm the
oxygen-rich nature of the circumbinary gas which is in contrast to the
previously detected carbon-rich material. The emission and absorption line
profiles show that the circumbinary gas is located in a thin, rotating layer
near the dust disk. The properties of the dust and gas circumbinary disk and
the spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolar
and M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocity
of 1 km/s. The severe depletion of refractory elements but near-solar
abundances of volatile elements observed in HR 4049 results from abundance
winnowing. The separation of the volatiles from the grains in the disk and the
subsequent accretion by the star are discussed. Contrary to prior reports, the
HR 4049 carbon and oxygen isotopic abundances are typical AGB values:
12C/13C=6^{+9}_{-4} and 16O/17O>200.Comment: 42 pages, 14 figures, Accepted by Ap
A Spectroscopic and Photometric Study of the Metal-Poor, Pulsating, Post-AGB Binary HD 46703
The metal-poor post-AGB star HD 46703 is shown to be a single-line
spectroscopic binary with a period of 600 days, a high velocity of -94 km/s,
and an orbital eccentricity of 0.3. Light curve studies show that it also
pulsates with a period of 29 days. High-resolution, high signal-to-noise
spectra were used for a new abundance study. The atmospheric model determined
is T(eff) = 6250 K, log(g) = 1.0, V(t) = 3.0 km/s, and a metal abundance of
[M/H] = -1.5. A low carbon abundance and lack of s-process element enhancement
indicate that the star has not experienced third dredge-up on the AGB. The
sulfur and zinc abundances are high compared with iron, and the chemical
abundances show a clear anti-correlation with condensation temperature. The
abundance depletion pattern is similar to that seen in other post-AGB binaries,
and, like them, is attributed to the chemical fractionation of refractory
elements onto dust stored in a circumbinary disk and the re-accretion of
volatiles in the stellar atmosphere. The infrared excess is small but the
excess energy distribution is very similar to what can expected from a disk. HD
46703 joins the growing list of depleted, post-AGB stars which are likely
surrounded by a dusty and stable circumbinary disk.Comment: Machine readable files not include
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
Quantum geometry of 2d gravity coupled to unitary matter
We show that there exists a divergent correlation length in 2d quantum
gravity for the matter fields close to the critical point provided one uses the
invariant geodesic distance as the measure of distance. The corresponding
reparameterization invariant two-point functions satisfy all scaling relations
known from the ordinary theory of critical phenomena and the KPZ exponents are
determined by the power-like fall off of these two-point functions. The only
difference compared to flat space is the appearance of a dynamically generated
fractal dimension d_h in the scaling relations. We analyze numerically the
fractal properties of space-time for Ising and three-states Potts model coupled
to 2d dimensional quantum gravity using finite size scaling as well as small
distance scaling of invariant correlation functions. Our data are consistent
with d_h=4, but we cannot rule out completely the conjecture d_H =
-2\alpha_1/\alpha_{-1}, where \alpha_{-n} is the gravitational dressing
exponent of a spin-less primary field of conformal weight (n+1,n+1). We compute
the moments and the loop-length distribution function and show that the
fractal properties associated with these observables are identical, with good
accuracy, to the pure gravity case.Comment: LaTeX2e, 38 pages, 13 figures, 32 eps files, added one referenc
The circumstellar shell of the post-AGB star HD 56126: the CN/CN isotope ratio and fractionation
We have detected circumstellar absorption lines of the CN and
CN Violet and Red System in the spectrum of the post-AGB star HD~56126.
From a synthetic spectrum analysis, we derive a Doppler broadening parameter of
km~s, CN/CN, and a lower limit of
on CN/CN and CN/CN. A simple
chemical model has been computed of the circumstellar shell surrounding
HD~56126 that takes into account the gas-phase ion-molecule reaction between CN
and C. From this we infer that this reaction leads to isotopic
fractionation of CN. Taking into account the isotopic exchange reaction and the
observed CN/CN we find C/C (for K). Our analysis suggests that CN has a somewhat higher
rotational temperature than CN: and
K respectively. We identify possible causes for this difference in excitation
temperature, among which the dependence of the isotopic exchange
reaction.Comment: 24 pages, 6 figues Apj accepte
Spectroscopic Study of IRAS 19285+0517(PDS 100): A Rapidly Rotating Li-Rich K Giant
We report on photometry and high-resolution spectroscopy for IRAS 19285+0517.
The spectral energy distribution based on visible and near-IR photometry and
far-IR fluxes shows that the star is surrounded by dust at a temperature of
250 K. Spectral line analysis shows that the star is a K
giant with a projected rotational velocity = 9 2 km s.
We determined the atmospheric parameters: = 4500 K, log =
2.5, = 1.5 km s, and [Fe/H] = 0.14 dex. The LTE abundance
analysis shows that the star is Li-rich (log (Li) = 2.50.15),
but with essentially normal C, N, and O, and metal abundances. Spectral
synthesis of molecular CN lines yields the carbon isotopic ratio
C/C = 9 3, a signature of post-main sequence evolution and
dredge-up on the RGB. Analysis of the Li resonance line at 6707 \AA for
different ratios Li/Li shows that the Li profile can be fitted best
with a predicted profile for pure Li. Far-IR excess, large Li abundance,
and rapid rotation suggest that a planet has been swallowed or, perhaps, that
an instability in the RGB outer layers triggered a sudden enrichment of Li and
caused mass-loss.Comment: To appear in AJ; 40 pages, 9 figure
Spectral variability of the binary HR 4049
The H profile varies with the orbital period. The two strong shell
type emission peaks are identified as from one single broad emission feature
with an absorption centered around -7.5~\kms. The intensity variations are
largely attributed to a differential amount of reddening towards the H
emitting region and the stellar continuum. The radial velocities suggest that
the H emission moves in phase with the primary, but with a slightly
lower velocity amplitude.
We propose two competing models that could account for the observed velocity
and intensity variations of the H profile. Model I: light from the
primary reflects on a localized spot near the inner radius of the circumbinary
disc which is closest to the primary. Model II: H emission originates
in the outer layers of the extended atmosphere of the primary due to activity.
These activities are locked to the position of the primary in its orbit.
We discuss the similarities of variability and shape of the H
emission of HR~4049 with those of early type T-Tauri stars (e.g SU~Aur).Comment: 14 pages + 8 pages appendix, 17 figures, 4 tables A&A accepte
- …