60,469 research outputs found
Pb isotopic evidence for early Archaean crust in South Greenland
The results of an isotopic remote sensing study focussed on delineating the extent of Early Archean crust north and south of the Nuuk area and in south Greenland is presented. Contamination of the Late Archean Nuk gneisses and equivalents by unradiogenic Pb uniquely characteristic of Amitsoq gneiss was detected as far south as Sermilik about 70 km south of Nuuk and only as far north as the mouth of Godthabsfjord. This study was extended to the southern part of the Archean craton and the adjoining Early Proterozoic Ketilidian orogenic belt where the Pb isotopes suggest several episodes of reworking of older uranium depleted continental crust. The technique of using the Pb isotope character of younger felsic rocks, in this case Late Archean and Early Proterozoic gneisses and granites to sense the age and isotopic character of older components, is a particularly powerful tool for reconstructing the evolutionary growth and development of continental crust
Observations of the interplanetary magnetic field July 4-12, 1966
Explorer XXVIII and XXXIII and Pioneer VI MAGNETOMETRIC determination of general macrostructure of interplanetary magnetic field in cislunar spac
Measuring dark energy properties with 3D cosmic shear
We present parameter estimation forecasts for present and future 3D cosmic
shear surveys. We demonstrate that, in conjunction with results from cosmic
microwave background (CMB) experiments, the properties of dark energy can be
estimated with very high precision with large-scale, fully 3D weak lensing
surveys. In particular, a 5-band, 10,000 square degree ground-based survey to a
median redshift of zm=0.7 could achieve 1- marginal statistical errors,
in combination with the constraints expected from the CMB Planck Surveyor, of
w0=0.108 and wa=0.099 where we parameterize w by
w(a)=w0+wa(1-a) where a is the scale factor. Such a survey is achievable with a
wide-field camera on a 4 metre class telescope. The error on the value of w at
an intermediate pivot redshift of z=0.368 is constrained to
w(z=0.368)=0.0175. We compare and combine the 3D weak lensing
constraints with the cosmological and dark energy parameters measured from
planned Baryon Acoustic Oscillation (BAO) and supernova Type Ia experiments,
and find that 3D weak lensing significantly improves the marginalized errors. A
combination of 3D weak lensing, CMB and BAO experiments could achieve
w0=0.037 and wa=0.099. Fully 3D weak shear analysis avoids the
loss of information inherent in tomographic binning, and we show that the
sensitivity to systematic errors is much less. In conjunction with the fact
that the physics of lensing is very soundly based, this analysis demonstrates
that deep, wide-angle 3D weak lensing surveys are extremely promising for
measuring dark energy properties.Comment: 18 pages, 16 figures. Accepted to MNRAS. Figures now in grayscale.
Further discussions on non-Gaussianity and photometric redshift errors. Some
references adde
Mapping the 3-D Dark Matter potential with weak shear
We investigate the practical implementation of Taylor's (2002) 3-dimensional
gravitational potential reconstruction method using weak gravitational lensing,
together with the requisite reconstruction of the lensing potential. This
methodology calculates the 3-D gravitational potential given a knowledge of
shear estimates and redshifts for a set of galaxies. We analytically estimate
the noise expected in the reconstructed gravitational field, taking into
account the uncertainties associated with a finite survey, photometric redshift
uncertainty, redshift-space distortions, and multiple scattering events. In
order to implement this approach for future data analysis, we simulate the
lensing distortion fields due to various mass distributions. We create
catalogues of galaxies sampling this distortion in three dimensions, with
realistic spatial distribution and intrinsic ellipticity for both ground-based
and space-based surveys. Using the resulting catalogues of galaxy position and
shear, we demonstrate that it is possible to reconstruct the lensing and
gravitational potentials with our method. For example, we demonstrate that a
typical ground-based shear survey with redshift limit z=1 and photometric
redshifts with error Delta z=0.05 is directly able to measure the 3-D
gravitational potential for mass concentrations >10^14 M_\odot between
0.1<z<0.5, and can statistically measure the potential at much lower mass
limits. The intrinsic ellipticity of objects is found to be a serious source of
noise for the gravitational potential, which can be overcome by Wiener
filtering or examining the potential statistically over many fields. We examine
the use of the 3-D lensing potential to measure mass and position of clusters
in 3-D, and to detect clusters behind clusters.Comment: 21 pages, including 24 figures, submitted to MNRA
Timing the millisecond pulsars in 47 Tucanae
In the last 10 years 20 millisecond pulsars have been discovered in the
globular cluster 47 Tucanae. Hitherto, only 3 of these had published timing
solutions. Here we improve upon these 3 and present 12 new solutions. These
measurements can be used to determine a variety of physical properties of the
pulsars and of the cluster. The 15 pulsars have positions determined with
typical uncertianties of only a few milliarcsec and they are all located within
1.2 arcmin of the cluster centre. We have also measured the proper motions of 5
of the pulsars, which are consistent with the proper motion of 47 Tuc based on
Hipparcos data. The period derivatives measured for many of the pulsars are
dominated by the dynamical effects of the cluster gravitational field, and are
used to constrain the surface mass density of the cluster. All pulsars have
characteristic ages T > 170 Myr and magnetic fields B < 2.4e9 Gauss, and the
average T > 1 Gyr. We have measured the rate of advance of periastron for the
binary pulsar J0024-7204H, implying a total system mass 1.4+-0.8 solar masses.Comment: 17 pages, 11 included figures, accepted for publication in MNRA
- …