4,119 research outputs found
Deformation modes and geometries in the EPICA-DML ice core, Antarctica
Combination of physical-properties methods (crystal-orientation-fabrics, grain-elongation-data, line-scan stratigraphy-documentation) reveal evidences for five deformation geometry regimes:1. Random c-axes distributions and crystal elongation directions (~2020 m depth). Here bed-parallel simple shear deforms the ice causing folding and inclination of stratigraphic layers.5. A last change of geometries is observed at ~2370 m depth, with a locally very restricted (~10 m) backslide to girdle fabric, isoclinal z-folding and borehole closure. Below that an inclined single maximum fabric reoccurs.Simple shear can easily produce the observed small-scale folding of layers which however may belong to disturbances on a larger scale with possible overturning and thus age reversal of layers. Below ~2020 m the EDML climate record has to be interpreted with great care
A High-Velocity Narrow Absorption Line Outflow in the Quasar J212329.46-005052.9
We report on a variable high-velocity narrow absorption line outflow in the
redshift 2.3 quasar J2123-0050. Five distinct outflow systems are detected with
velocity shifts from -9710 to -14,050 km/s and CIV 1548,1551 line widths of
FWHM = 62-164 km/s. These data require five distinct outflow structures with
similar kinematics, physical conditions and characteristic sizes of order
0.01-0.02 pc. The most likely location is ~5 pc from the quasar. The
coordinated line variations in <0.63 yr (rest) are best explained by global
changes in the outflow ionization caused by changes in the quasar's ionizing
flux. The absence of strong X-ray absorption shows that radiative shielding is
not needed to maintain the moderate ionizations and therefore, apparently, it
is not needed to facilitate the radiative acceleration to high speeds. The
kinetic energy yield of this flow is at least two orders of magnitude too low
to be important for feedback to the host galaxy's evolution.Comment: 20 pages. In press with MNRA
Heterodisulfide reductase from methanogenic archaea: a new catalytic role for an iron-sulfur cluster
Comparison of Global and Local Adaptive Coordinates for Density Functional Calculations
A globally-adaptive curvilinear coordinate formalism is shown to be easily
convertible to a class of curvilinear transformations locally optimized around
atom sites by a few parameters. Parameter transferability is established for a
demanding test case, and the results of the two methods are shown to be
comparable. Computational efficiencies realized in the local method are
discussed.Comment: 21 pages, 4 figure
The progenitor of a type Ia supernova with a short delay time?
HD49798/RXJ0648.0-4418 is the only known X-ray binary composed of a hot
subdwarf and a massive white dwarf (M=1.28+/-0.05 Msun). This system, with an
orbital period of 1.55 days, is the outcome of a common envelope evolution,
most likely of a pair of stars with initial masses of about 8-10 Msun. When the
hot subdwarf, currently in a He-burning phase, will expand again and fill its
Roche-lobe, the enhanced mass transfer can rapidly bring the already massive
white dwarf above the Chandrasekhar limit. The possible final fate, either a
Type Ia supernova explosion or an accretion induced collapse, is particularly
interesting in view of the high rotational velocity of this star, which has the
shortest spin period (13 s) observed in a white dwarf.Comment: 4 pages, to appear in Proceedings of IAU Symp. 281, Binary Paths to
Type Ia Supernovae Explosions, ed. R. Di Stefano and M. Ori
The rapid evolution of the exciting star of the Stingray Nebula
SAO244567, the exciting star of the Stingray nebula, is rapidly evolving.
Previous analyses suggested that it has heated up from an effective temperature
of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic
giant branch evolution suggests a relatively high mass while previous analyses
indicate a low-mass star. Fitting line profiles from static and expanding
non-LTE model atmospheres to the observed UV and optical spectra, taken during
1988-2013, allowed us to study the temporal change of effective temperature,
surface gravity, mass-loss rate, and terminal wind velocity. In addition, we
determined the chemical composition of the atmosphere. We find that the central
star has steadily increased its effective temperature from 38kK in 1988 to a
peak value of 60kK in 2002. During the same time, the star was contracting, as
concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a
drop in luminosity. Simultaneously, the mass-loss rate declined from log
(dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from
1800km/s to 2800km/s. Since around 2002, the star stopped heating and has
cooled down again to 55kK by 2006. It has a largely solar surface composition
with the exception of slightly subsolar carbon, phosphorus, and sulfur. By
comparison with stellar-evolution calculations, we confirm that SAO244567 must
be a low-mass star (M < 0.55 Msun). However, the slow evolution of the
respective stellar evolutionary models is in strong contrast to the observed
fast evolution and the young planetary nebula with a kinematical age of only
about 1000 years. We speculate that the star could be a late He-shell flash
object. Alternatively, it could be the outcome of close-binary evolution. Then
SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the
common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&
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