13,738 research outputs found
Neocarchean carbonate-associated sulfate records positive Δ^(33)S anomalies
Mass-independent fractionation of sulfur isotopes (reported as Δ^(33S) recorded in Archean
sedimentary rocks helps to constrain the composition of Earth’s early atmosphere and the
timing of the rise of oxygen ~2.4 billion years ago. Although current hypotheses predict
uniformly negative Δ^(33)S for Archean seawater sulfate, this remains untested through the
vast majority of Archean time.We applied x-ray absorption spectroscopy to investigate the
low sulfate content of particularly well-preserved Neoarchean carbonates and mass
spectrometry to measure their Δ^(33)S signatures. We report unexpected, large, widespread
positive Δ^(33)S values from stratigraphic sections capturing over 70 million years and diverse
depositional environments. Combined with the pyrite record, these results show that
sulfate does not carry the expected negative Δ^(33)S from sulfur mass-independent
fractionation in the Neoarchean atmosphere
Correspondence between solar fine-scale structures in the corona, transition region, and lower atmosphere from collaborative observations
The Soft X-Ray Imaging Payload and the High Resolution Telescope and Spectrograph (HRTS) instrument were launched from White Sands on 11 December 1987 in coordinated sounding rocket flights to investigate the correspondence of coronal and transition region structures, especially the relationship between X-ray bright points (XBPs) and transition region small spatial scale energetic events. The coaligned data from X-ray images are presented along with maps of sites of transition region energetic events observed in C IV (100,000 K), HRTS 1600 A spectroheliograms of the T sub min region and ground based magnetogram and He I 10830 A images
Identification of a nearby stellar association in the Hipparcos catalog: implications for recent, local star formation
The TW Hydrae Association (~55 pc from Earth) is the nearest known region of
recent star formation. Based primarily on the Hipparcos catalog, we have now
identified a group of 9 or 10 co-moving star systems at a common distance (~45
pc) from Earth that appear to comprise another, somewhat older, association
(``the Tucanae Association''). Together with ages and motions recently
determined for some nearby field stars, the existence of the Tucanae and TW
Hydrae Associations suggests that the Sun is now close to a region that was the
site of substantial star formation only 10-40 million years ago. The TW Hydrae
Association represents a final chapter in the local star formation history.Comment: 5 pages incl figs and table
Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency
A minimal model is constructed for two-dimensional fracture propagation. The
heterogeneous process zone is presumed to suppress stress relaxation rate,
leading to non-quasistatic behavior. Using the Yoffe solution, I construct and
solve a dynamical equation for the tip stress. I discuss a generic tip velocity
response to local stress and find that noise-free propagation is either at
steady state or oscillatory, depending only on one material parameter. Noise
gives rise to intermittency and quasi-periodicity. The theory explains the
velocity oscillations and the complicated behavior seen in polymeric and
amorphous brittle materials. I suggest experimental verifications and new
connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3
postscript figures upon request from author at [email protected] or
[email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm
Recommended from our members
Prediction of tilted capillary barrier performance
Capillary barriers, consisting of tilted fine-over-coarse layers under unsaturated conditions, have been suggested as landfill covers to divert water infiltration away from sensitive underground regions, especially for arid and semi-arid regions. The Hydrological Evaluation of Landfill Performance (HELP) computer code is an evaluation tool for landfill covers used by designers and regulators. HELP is a quasi-two-dimensional model that predicts moisture movement into and through the underground soil and waste layers. Processes modeled within HELP include precipitation, runoff, evapotranspiration, unsaturated vertical drainage, saturated lateral drainage, and leakage through liners. Unfortunately, multidimensional unsaturated flow phenomena that are necessary for evaluating tilted capillary barriers are not included in HELP. Differences between the predictions of the HELP and those from a multidimensional unsaturated flow code are presented to assess the two different approaches. Comparisons are presented for the landfill covers including capillary barrier configurations at the Alternative Landfill Cover Demonstration (ALCD) being conducted at Sandia
Posteruptive phenomena in coronal mass ejections and substorms: Indicators of a universal process?
[1] We examine phenomena associated with eruptions in the two different regimes of the solar corona and the terrestrial magnetosphere. We find striking similarities between the speeds of shrinking magnetic field lines in the corona and dipolarization fronts traversing the magnetosphere. We also examine the similarities between supra-arcade downflows observed during solar flares and bursty bulk flows seen in the magnetotail and find that these phenomena have remarkably similar speeds, velocity profiles, and size scales. Thus we show manifest similarities in the magnetic reconfiguration in response to the ejection of coronal mass ejections in the corona and the ejection of plasmoids in the magnetotail. The subsequent return of loops to a quasi-potential state in the corona and field dipolarization in the magnetotail are physical analogs and trigger similar phenomena such as downflows, which provides key insights into the underlying drivers of the plasma dynamics
Discovery of seven T Tauri stars and a brown dwarf candidate in the nearby TW Hydrae Association
We report the discovery of five T Tauri star systems, two of which are
resolved binaries, in the vicinity of the nearest known region of recent star
formation, the TW Hydrae Association. The newly discovered systems display the
same signatures of youth (namely high X-ray flux, large Li abundance and strong
chromospheric activity) and the same proper motion as the original five
members. These similarities firmly establish the group as a bona fide T Tauri
association, unique in its proximity to Earth and its complete isolation from
any known molecular clouds.
At an age of ~10 Myr and a distance of ~50 pc, the association members are
excellent candidates for future studies of circumstellar disk dissipation and
the formation of brown dwarfs and planets. Indeed, as an example, our speckle
imaging revealed a faint, very likely companion 2" north of CoD-33 7795 (TWA
5). Its color and brightness suggest a spectral type ~M8.5 which, at an age of
~10^7 years, implies a mass ~20 M(Jupiter).Comment: 6 pages, 4 figures and 1 table. AAS LaTeX aas2pp4.sty. To be
published in Ap
Resilient entangling gates for trapped ions
Constructing a large-scale ion trap quantum processor will require entangling gate operations that are robust in the presence of noise and experimental imperfection. We experimentally demonstrate how a new type of Mølmer-Sørensen gate protects against infidelity caused by heating of the motional mode used during the gate. Furthermore, we show how the same technique simultaneously provides significant protection against slow fluctuations and mis-sets in the secular frequency. Since this parameter sensitivity is worsened in cases where the ions are not ground-state cooled, our method provides a path towards relaxing ion cooling requirements in practical realizations of quantum computing and simulation
Restrictions and Stability of Time-Delayed Dynamical Networks
This paper deals with the global stability of time-delayed dynamical
networks. We show that for a time-delayed dynamical network with
non-distributed delays the network and the corresponding non-delayed network
are both either globally stable or unstable. We demonstrate that this may not
be the case if the network's delays are distributed. The main tool in our
analysis is a new procedure of dynamical network restrictions. This procedure
is useful in that it allows for improved estimates of a dynamical network's
global stability. Moreover, it is a computationally simpler and much more
effective means of analyzing the stability of dynamical networks than the
procedure of isospectral network expansions introduced in [Isospectral graph
transformations, spectral equivalence, and global stability of dynamical
networks. Nonlinearity, 25 (2012) 211-254]. The effectiveness of our approach
is illustrated by applications to various classes of Cohen-Grossberg neural
networks.Comment: 32 pages, 9 figure
Isotope shift calculations for atoms with one valence electron
This work presents a method for the ab initio calculation of isotope shift in
atoms and ions with one valence electron above closed shells. As a zero
approximation we use relativistic Hartree-Fock and then calculate correlation
corrections. The main motivation for developing the method comes from the need
to analyse whether different isotope abundances in early universe can
contribute to the observed anomalies in quasar absorption spectra. The current
best explanation for these anomalies is the assumption that the fine structure
constant, alpha, was smaller at early epoch. We test the isotope shift method
by comparing the calculated and experimental isotope shift for the alkali and
alkali-like atoms Na, MgII, K, CaII and BaII. The agreement is found to be
good. We then calculate the isotope shift for some astronomically relevant
transitions in SiII and SiIV, MgII, ZnII and GeII.Comment: 11 page
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