6,456 research outputs found
Sterkfontein at 75: review of paleoenvironments, fauna, dating and archaeology from the hominin site of Sterkfontein (Gauteng Province, South Africa).
Seventy-five years after Robert Broom’s discovery of the first adult Australopithecus in 1936, the Sterkfontein Caves (Gauteng Province, South Africa) remains one of the richest and most informative fossil hominin sites in the world. The deposits record hominin and African mammal evolution from roughly 2.6 million years (Ma) until the Upper Pleistocene. Earlier excavation efforts focused on the Member 4 australopithecine-bearing breccia and the Member 5 stone tool-bearing breccias of Oldowan and Early Acheulean age. Ronald J. Clarke’s 1997 programme of understanding the cave deposits as a whole led to the discovery of the near-complete StW 573 Australopithecus skeleton in the Member 2 deposit of the Silberberg Grotto, and the exploration of lesser known deposits such as the Jacovec Cavern, Name Chamber and the Lincoln Cave. Our aim is to produce a cogent synthesis of the environments, palaeodietary information, fauna and stone artefacts as recorded in the Sterkfontein sequence. We begin with an overview of the site and early accounts of the interpretations of the site-formation processes, after which we discuss each Member in turn and summarize the various types of evidence published so far. Finally, we review the most pertinent debates about the site, including the ages of Sterkfontein Member 2 and 4, and the types of habitats represented at the site through time
The imprints of AGN feedback within a supermassive black hole's sphere of influence
We present a new 300 ks Chandra observation of M87 that limits pileup to only
a few per cent of photon events and maps the hot gas properties closer to the
nucleus than has previously been possible. Within the supermassive black hole's
gravitational sphere of influence, the hot gas is multiphase and spans
temperatures from 0.2 to 1 keV. The radiative cooling time of the lowest
temperature gas drops to only 0.1-0.5 Myr, which is comparable to its free fall
time. Whilst the temperature structure is remarkably symmetric about the
nucleus, the density gradient is steep in sectors to the N and S, with
, and significantly shallower along the jet axis
to the E, where . The density structure within
the Bondi radius is therefore consistent with steady inflows perpendicular to
the jet axis and an outflow directed E along the jet axis. By putting limits on
the radial flow speed, we rule out Bondi accretion on the scale resolved at the
Bondi radius. We show that deprojected spectra extracted within the Bondi
radius can be equivalently fit with only a single cooling flow model, where gas
cools from 1.5 keV down below 0.1 keV at a rate of 0.03 M/yr. For the
alternative multi-temperature spectral fits, the emission measures for each
temperature component are also consistent with a cooling flow model. The lowest
temperature and most rapidly cooling gas in M87 is therefore located at the
smallest radii at ~100 pc and may form a mini cooling flow. If this cooling gas
has some angular momentum, it will feed into the cold gas disk around the
nucleus, which has a radius of ~80 pc and therefore lies just inside the
observed transition in the hot gas structure
Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment:Extreme events in the Himalaya
Recession of high‐mountain glaciers in response to climatic change frequently results in the development of moraine‐dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km2) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice‐contact moraine‐dammed lake to develop. The lake had a maximum volume of 5.5 × 105 m3 and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 105 m3 of material was removed from the terminal moraine during breach development. Numerical dam‐breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine‐dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m3 s‐1. Results from two‐dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s‐1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine‐dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine‐dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact
POWERFUL, ROTATING DISK WINDS from STELLAR-MASS BLACK HOLES
We present an analysis of ionized X-ray disk winds observed in the Fe K band
of four stellar-mass black holes observed with Chandra, including 4U 1630-47,
GRO J1655-40, H 1743-322, and GRS 1915+105. High-resolution photoionization
grids were generated in order to model the data. Third-order gratings spectra
were used to resolve complex absorption profiles into atomic effects and
multiple velocity components. The Fe XXV line is found to be shaped by
contributions from the intercombination line (in absorption), and the Fe XXVI
line is detected as a spin-orbit doublet. The data require 2-3 absorption
zones, depending on the source. The fastest components have velocities
approaching or exceeding 0.01c, increasing mass outflow rates and wind kinetic
power by orders of magnitude over prior single-zone models. The first-order
spectra require re-emission from the wind, broadened by a degree that is
loosely consistent with Keplerian orbital velocities at the photoionization
radius. This suggests that disk winds are rotating with the orbital velocity of
the underlying disk, and provides a new means of estimating launching radii --
crucial to understanding wind driving mechanisms. Some aspects of the wind
velocities and radii correspond well to the broad-line region (BLR) in active
galactic nuclei, suggesting a physical connection. We discuss these results in
terms of prevalent models for disk wind production and disk accretion itself,
and implications for massive black holes in active galactic nuclei
The inevitable youthfulness of known high-redshift radio galaxies
Radio galaxies can be seen out to very high redshifts, where in principle
they can serve as probes of the early evolution of the Universe. Here we show
that for any model of radio-galaxy evolution in which the luminosity decreases
with time after an initial rapid increase (that is, essentially all reasonable
models), all observable high-redshift radio-galaxies must be seen when the
lobes are less than 10^7 years old. This means that high-redshift radio
galaxies can be used as a high-time-resolution probe of evolution in the early
Universe. Moreover, this result helps to explain many observed trends of
radio-galaxy properties with redshift [(i) the `alignment effect' of optical
emission along radio-jet axes, (ii) the increased distortion in radio
structure, (iii) the decrease in physical sizes, (iv) the increase in radio
depolarisation, and (v) the increase in dust emission] without needing to
invoke explanations based on cosmology or strong evolution of the surrounding
intergalactic medium with cosmic time, thereby avoiding conflict with current
theories of structure formation.Comment: To appear in Nature. 4 pages, 2 colour figures available on request.
Also available at http://www-astro.physics.ox.ac.uk/~km
3D-electrical resistivity tomography monitoring of salt transport in homogeneous and layered soil samples
Monitoring transport of dissolved substances in soil deposits is particularly relevant where safety is concerned, as in the case of geo-environmental barriers. Geophysical methods are very appealing, since they cover a wide domain, localising possible preferential flow paths and providing reliable links between geophysical quantities and hydrological variables. This paper describes a 3D laboratory application of Electrical Resistivity Tomography (ERT) used to monitor solute transport processes. Dissolution and transport tests on both homogeneous and heterogeneous samples were conducted in an instrumented oedometer cell. ERT was used to create maps of electrical conductivity of the monitored domain at different time intervals and to estimate concentration variations within the interstitial fluid. Comparisons with finite element simulations of the transport processes were performed to check the consistency of the results. Tests confirmed that the technique can monitor salt transport, infer the hydro-chemical behaviour of heterogeneous geomaterials and evaluate the performances of clay barrier
Two-neutron knockout from neutron-deficient Ar, S, and Si
Two-neutron knockout reactions from nuclei in the proximity of the proton
dripline have been studied using intermediate-energy beams of neutron-deficient
Ar, S, and Si. The inclusive cross sections, and also the
partial cross sections for the population of individual bound final states of
the Ar, S and Si knockout residues, have been determined
using the combination of particle and -ray spectroscopy. Similar to the
two-proton knockout mechanism on the neutron-rich side of the nuclear chart,
these two-neutron removal reactions from already neutron-deficient nuclei are
also shown to be consistent with a direct reaction mechanism.Comment: Phys. Rev. C, rapid communication, in pres
THE ACCRETION DISK WIND IN THE BLACK HOLE GRS 1915+105
© 2016. The American Astronomical Society. All rights reserved. We report on a 120 ks Chandra/HETG spectrum of the black hole GRS 1915+105. The observation was made during an extended and bright soft state in 2015 June. An extremely rich disk wind absorption spectrum is detected, similar to that observed at lower sensitivity in 2007. The very high resolution of the third-order spectrum reveals four components to the disk wind in the Fe K band alone; the fastest has a blueshift of v = 0.03c. Broadened re-emission from the wind is also detected in the first-order spectrum, giving rise to clear accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum gives wind launching radii of r ≃ 102-4 GM/c2. Wind density values of n ≃ 1013-16 cm-3 are then required by the ionization parameter formalism. The small launching radii, high density values, and inferred high mass outflow rates signal a role for magnetic driving. With simple, reasonable assumptions, the wind properties constrain the magnitude of the emergent magnetic field to be B ≃ 103-4 G if the wind is driven via magnetohydrodynamic (MHD) pressure from within the disk and B ≃ 104-5 G if the wind is driven by magnetocentrifugal acceleration. The MHD estimates are below upper limits predicted by the canonical α-disk model. We discuss these results in terms of fundamental disk physics and black hole accretion modes
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