28,350 research outputs found
Surveyor landing radar test program review Final report
Test program evaluation and modifications for Surveyor radar altimeter and Doppler velocity sensor syste
Partial suppression of the radial orbit instability in stellar systems
It is well known that the simple criterion proposed originally by Polyachenko
and Shukhman (1981) for the onset of the radial orbit instability, although
being generally a useful tool, faces significant exceptions both on the side of
mildly anisotropic systems (with some that can be proved to be unstable) and on
the side of strongly anisotropic models (with some that can be shown to be
stable). In this paper we address two issues: Are there processes of
collisionless collapse that can lead to equilibria of the exceptional type?
What is the intrinsic structural property that is responsible for the sometimes
noted exceptional stability behavior? To clarify these issues, we have
performed a series of simulations of collisionless collapse that start from
homogeneous, highly symmetrized, cold initial conditions and, because of such
special conditions, are characterized by very little mixing. For these runs,
the end-states can be associated with large values of the global pressure
anisotropy parameter up to 2K_r/K_T \approx 2.75. The highly anisotropic
equilibrium states thus constructed show no significant traces of radial
anisotropy in their central region, with a very sharp transition to a radially
anisotropic envelope occurring well inside the half-mass radius (around 0.2
r_M). To check whether the existence of such almost perfectly isotropic
"nucleus" might be responsible for the apparent suppression of the radial orbit
instability, we could not resort to equilibrium models with the above
characteristics and with analytically available distribution function; instead,
we studied and confirmed the stability of configurations with those
characteristics by initializing N-body approximate equilibria (with given
density and pressure anisotropy profiles) with the help of the Jeans equations.Comment: 26 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Seasonal variations in Greenland Ice Sheet motion : Inland extent and behaviour at higher elevations
Peer reviewedPreprin
Mapping isoprene emissions over North America using formaldehyde column observations from space
We present a methodology for deriving emissions of volatile organic compounds (VOC) using space-based column observations of formaldehyde (HCHO) and apply it to data from the Global Ozone Monitoring Experiment (GOME) satellite instrument over North America during July 1996. The HCHO column is related to local VOC emissions, with a spatial smearing that increases with the VOC lifetime. Isoprene is the dominant HCHO precursor over North America in summer, and its lifetime (≃1 hour) is sufficiently short that the smearing can be neglected. We use the Goddard Earth Observing System global 3-D model of tropospheric chemistry (GEOS-CHEM) to derive the relationship between isoprene emissions and HCHO columns over North America and use these relationships to convert the GOME HCHO columns to isoprene emissions. We also use the GEOS-CHEM model as an intermediary to validate the GOME HCHO column measurements by comparison with in situ observations. The GEOS-CHEM model including the Global Emissions Inventory Activity (GEIA) isoprene emission inventory provides a good simulation of both the GOME data (r2 = 0.69, n = 756, bias = +11%) and the in situ summertime HCHO measurements over North America (r2 = 0.47, n = 10, bias = −3%). The GOME observations show high values over regions of known high isoprene emissions and a day-to-day variability that is consistent with the temperature dependence of isoprene emission. Isoprene emissions inferred from the GOME data are 20% less than GEIA on average over North America and twice those from the U.S. EPA Biogenic Emissions Inventory System (BEIS2) inventory. The GOME isoprene inventory when implemented in the GEOS-CHEM model provides a better simulation of the HCHO in situ measurements than either GEIA or BEIS2 (r2 = 0.71, n = 10, bias = −10%)
Sources and budgets for CO and O-3 in the northeastern Pacific during the spring of 2001: Results from the PHOBEA-II Experiment
Abstract. Ground and airborne measurements of CO, ozone, and aerosols were obtained in th
Defining the interaction of the protease CpaA with its type II secretion chaperone CpaB and its contribution to virulence in Acinetobacter species
The NuMAX Long Baseline Neutrino Factory Concept
A Neutrino Factory where neutrinos of all species are produced in equal
quantities by muon decay is described as a facility at the intensity frontier
for exquisite precision providing ideal conditions for ultimate neutrino
studies and the ideal complement to Long Baseline Facilities like LBNF at
Fermilab. It is foreseen to be built in stages with progressively increasing
complexity and performance, taking advantage of existing or proposed facilities
at an existing laboratory like Fermilab. A tentative layout based on a
recirculating linac providing opportunities for considerable saving is
discussed as well as its possible evolution toward a muon collider if and when
requested by Physics. Tentative parameters of the various stages are presented
as well as the necessary R&D to address the technological issues and
demonstrate their feasibility.Comment: JINST Special Issue on Muon Accelerators. arXiv admin note: text
overlap with arXiv:1308.0494, arXiv:1502.0164
Six-Dimensional (1,0) Superconformal Models and Higher Gauge Theory
We analyze the gauge structure of a recently proposed superconformal field
theory in six dimensions. We find that this structure amounts to a weak
Courant-Dorfman algebra, which, in turn, can be interpreted as a strong
homotopy Lie algebra. This suggests that the superconformal field theory is
closely related to higher gauge theory, describing the parallel transport of
extended objects. Indeed we find that, under certain restrictions, the field
content and gauge transformations reduce to those of higher gauge theory. We
also present a number of interesting examples of admissible gauge structures
such as the structure Lie 2-algebra of an abelian gerbe, differential crossed
modules, the 3-algebras of M2-brane models and string Lie 2-algebras.Comment: 31+1 pages, presentation slightly improved, version published in JM
The 100-month Swift catalogue of supergiant fast X-ray transients I. BAT on-board and transient monitor flares
We investigate the characteristics of bright flares for a sample of
supergiant fast X-ray transients and their relation to the orbital phase. We
have retrieved all Swift/BAT Transient Monitor light curves, and collected all
detections in excess of from both daily- and orbital-averaged light
curves in the time range of 2005-Feb-12 to 2013-May-31. We also considered all
on-board detections as recorded in the same time span and selected those within
4 arcmin of each source in our sample and in excess of . We present a
catalogue of over a thousand BAT flares from 11 SFXTs, down to 15-150keV fluxes
of erg cm s (daily timescale) and
erg cm s (orbital timescale, averaging
s) and spanning 100 months. The great majority of these flares are
unpublished. This population is characterized by short (a few hundred seconds)
and relatively bright (in excess of 100mCrab, 15-50keV) events. In the hard
X-ray, these flares last in general much less than a day. Clustering of hard
X-ray flares can be used to indirectly measure the length of an outburst, even
when the low-level emission is not detected. We construct the distributions of
flares, of their significance (in terms of sigma) and their flux as a function
of orbital phase, to infer the properties of these binary systems. In
particular, we observe a trend of clustering of flares at some phases as
increases, as consistent with a progression from tight, circular
or mildly eccentric orbits at short periods, to wider and more eccentric orbits
at longer orbital periods. Finally, we estimate the expected number of flares
for a given source for our limiting flux and provide the recipe for calculating
them for the limiting flux of future hard X-ray observatories. (Abridged).Comment: Accepted for publication in Astronomy and Astrophysics. 23 pages, 8
figures. Full catalog files will be available at CDS and at
http://www.ifc.inaf.it/sfxt/ Fixed typos and updated reference
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