2,559 research outputs found
X-shooter search for outgassing from Main Belt Comet P/2012 T1 (Pan-STARRS)
Context. Main Belt Comets are a recently identified population of minor bodies with stable asteroid-like orbits but cometary appearances. Sublimation of water ice is the most likely mechanism for their recurrent activity (i.e. dust tails and dust comae), although there has been no direct detection of gas. These peculiar objects could hold the key to the origin of water on Earth.
Aims. In this paper we present a search for the gas responsible for lifting dust from P/2012 T1 (Pan-STARRS), and review previous attempts at such measurements. To date such searches have mainly been indirect, looking for the common cometary gas CN rather than gasses related to water itself.
Methods. We use the VLT and X-shooter to search for emission from OH in the UV, a direct dissociation product of water.
Results. We do not detect any emission lines, and place an upper limit on water production rate from P/2012 T1 of 8 − 9 × 1025 molecules s−1. This is similar to limits derived from observations using the Herschel space telescope.
Conclusions. We conclude that the best current facilities are incapable of detecting water emission at the exceptionally low levels required to produce the observed activity in Main Belt Comets
X THEN X: Manipulation of Same-System Runoff Elections
Do runoff elections, using the same voting rule as the initial election but
just on the winning candidates, increase or decrease the complexity of
manipulation? Does allowing revoting in the runoff increase or decrease the
complexity relative to just having a runoff without revoting? For both weighted
and unweighted voting, we show that even for election systems with simple
winner problems the complexity of manipulation, manipulation with runoffs, and
manipulation with revoting runoffs are independent, in the abstract. On the
other hand, for some important, well-known election systems we determine what
holds for each of these cases. For no such systems do we find runoffs lowering
complexity, and for some we find that runoffs raise complexity. Ours is the
first paper to show that for natural, unweighted election systems, runoffs can
increase the manipulation complexity
Effects of hydrogen/deuterium absorption on the magnetic properties of Co/Pd multilayers
The effects of hydrogen (H2) and deuterium (D2) absorption were studied in
two Co/Pd multilayers with perpendicular magnetic anisotropy (PMA) using
polarized neutron reflectivity (PNR). PNR was measured in an external magnetic
field H applied in the plane of the sample with the magnetization M confined in
the plane for {\mu}_o H= 6.0 T and partially out of plane at 0.65 T. Nominal
thicknesses of the Co and Pd layers were 2.5 {\AA} and 21 {\AA}, respectively.
Because of these small values, the actual layer chemical composition,
thickness, and interface roughness parameters were determined from the nuclear
scattering length density profile ({\rho}_n) and its derivative obtained from
both x-ray reflectivity and PNR, and uncertainties were determined using Monte
Carlo analysis. The PNR {\rho}_n showed that although D2 absorption occurred
throughout the samples, absorption in the multilayer stack was modest (0.02 D
per Pd atom) and thus did not expand. Direct magnetometry showed that H2
absorption decreased the total M at saturation and increased the component of M
in the plane of the sample when not at saturation. The PNR magnetic scattering
length density ({\rho}_m) revealed that the Pd layers in the multilayer stack
were magnetized and that their magnetization was preferentially modified upon
D2 absorption. In one sample, a modulation of M with twice the multilayer
period was observed at {\mu}_o H= 0.65 T, which increased upon D2 absorption.
These results indicate that H2 or D2 absorption decreases both the PMA and
total magnetization of the samples. The lack of measurable expansion during
absorption indicates that these changes are primarily governed by modification
of the electronic structure of the material.Comment: to appear in Physics review B, 201
Mind the gap: Can a professional development programme build a university’s public engagement community?
A number of ‘gaps’ may be present within public engagement with research –
disparity of skills, priorities and knowledge between research staff and
engagement practitioners, as well as differences between institutional ambition
and departmental reality. Such gaps are often perceived as problems – deficits
to be addressed through training and culture change initiatives. The design and
delivery of Imperial College London’s Engagement Academy with 12 members
of research, teaching and professional services staff sought to explore and work
across such gaps. We propose that these areas of disconnect within and across
universities may be challenging, but they may also be structurally necessary, and
potentially even a source of rich public engagement
Flight and tunnel test results of the MDC mechanical jet noise suppressor nozzle
The flight and wind tunnel tests to determine the acoustic and performance effects of a mechanical jet noise suppressor nozzle mounted on a Viper engine of an HS-125 airplane are discussed. Flyover noise measurements were made with microphones mounted on top of a 137.5 m bridge tower. Seven nozzle configurations including two references nozzles, two suppressors, and three ejector inlets were tested. The suppressor nozzle of interest for an advanced supersonic transport, the suppressor/treated ejector, achieved a measured noise reduction of 14 EPNdB relative to a conventional conical reference nozzle at the highest pressure ratio tested (approximately 2.5). The unique engine nacelle, flight hardware, and nacelles from the HS-125 flight test program, combined with a simulated HS-125 fuselage were windtunnel tested. Both propulsion and acoustic data were recorded. Preliminary thrust data results from the wind tunnel tests are summarized and compared to other mechanical suppressor test results. The test results indicate that a noise reduction of at least 16 EPNdB would be possible for the suppressor/ejector nozzle scaled to typical AST engine size with a 5% thrust loss at a typical takeoff climb speed
Physical and dynamical characterisation of low Delta-V NEA (190491) 2000 FJ10
We investigated the physical properties and dynamical evolution of Near Earth
Asteroid (NEA) (190491) 2000 FJ10 in order to assess the suitability of this
accessible NEA as a space mission target. Photometry and colour determination
were carried out with the 1.54 m Kuiper Telescope and the 10 m Southern African
Large Telescope during the object's recent favourable apparition in 2011-12.
During the earlier 2008 apparition, a spectrum of the object in the 6000-9000
Angstrom region was obtained with the 4.2 m William Herschel Telescope.
Interpretation of the observational results was aided by numerical simulations
of 1000 dynamical clones of 2000 FJ10 up to 10^6 yr in the past and in the
future. The asteroid's spectrum and colours determined by our observations
suggest a taxonomic classification within the S-complex although other
classifications (V, D, E, M, P) cannot be ruled out. On this evidence, it is
unlikely to be a primitive, relatively unaltered remnant from the early history
of the solar system and thus a low priority target for robotic sample return.
Our photometry placed a lower bound of 2 hrs to the asteroid's rotation period.
Its absolute magnitude was estimated to be 21.54+-0.1 which, for a typical
S-complex albedo, translates into a diameter of 130+-20 m. Our dynamical
simulations show that it has likely been an Amor for the past 10^5 yr. Although
currently not Earth-crossing, it will likely become so during the period 50 -
100 kyr in the future. It may have arrived from the inner or central Main Belt
> 1 Myr ago as a former member of a low-inclination S-class asteroid family.
Its relatively slow rotation and large size make it a suitable destination for
a human mission. We show that ballistic Earth-190491-Earth transfer
trajectories with Delta-V < 2 km s^-1 at the asteroid exist between 2052 and
2061.Comment: 2 Tables, 11 Figures, accepted for publication in Astronomy &
Astrophysic
Sputtering of Oxygen Ice by Low Energy Ions
Naturally occurring ices lie on both interstellar dust grains and on
celestial objects, such as those in the outer solar system. These ices are
continu- ously subjected to irradiation by ions from the solar wind and/or
cosmic rays, which modify their surfaces. As a result, new molecular species
may form which can be sputtered off into space or planetary atmospheres. We
determined the experimental values of sputtering yields for irradiation of
oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ and
O2+) charged ions with 4 keV kinetic energy. In these laboratory experiments,
oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber
at low temperature to simulate the environment of space. The number of
molecules removed by sputtering was observed by measurement of the ice
thickness using laser interferometry. Preliminary mass spectra were taken of
sputtered species and of molecules formed in the ice by temperature programmed
desorption (TPD). We find that the experimental sputtering yields increase
approximately linearly with the projectile ion mass (or momentum squared) for
all ions studied. No difference was found between the sputtering yield for
singly and doubly charged ions of the same atom within the experimental
uncertainty, as expected for a process dominated by momentum transfer. The
experimental sputter yields are in good agreement with values calculated using
a theoretical model except in the case of oxygen ions. Preliminary studies have
shown molecular oxygen as the dominant species sputtered and TPD measurements
indicate ozone formation.Comment: to be published in Surface Science (2015
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