261 research outputs found
Spin Asymmetry and Gerasimov-Drell-Hearn Sum Rule for the Deuteron
An explicit evaluation of the spin asymmetry of the deuteron and the
associated GDH sum rule is presented which includes photodisintegration, single
and double pion and eta production as well. Photodisintegration is treated with
a realistic retarded potential and a corresponding meson exchange current. For
single pion and eta production the elementary operator from MAID is employed
whereas for double pion production an effective Lagrangean approach is used. A
large cancellation between the disintegration and the meson production channels
yields for the explicit GDH integral a value of 27.31 b to be compared to
the sum rule value 0.65 b.Comment: 4 pages, 5 figures, revtex
On the ill-posed character of the Lorentz integral transform
An exact inversion formula for the Lorentz integral transform (LIT) is
provided together with the spectrum of the LIT kernel. The exponential increase
of the inverse Fourier transform of the LIT kernel entering the inversion
formula explains the ill-posed character of the LIT approach. Also the
continuous spectrum of the LIT kernel, which approaches zero points necessarily
to the same defect. A possible cure is discussed and numerically illustrated.Comment: 13 pages, 3 figure
Beyond Caustic Crossings: Properties of Binary Microlensing Light Curves
Binary microlensing light curves have a variety of morphologies. Many are
indistinguishable from point lens light curves. Of those that deviate from the
point lens form, caustic crossing light curves have tended to dominate
identified binary lens events. Other distinctive signatures of binary lenses
include significant asymmetry, multiple peaks, and repeating events. We have
quantified, using high resolution simulations, the theoretically expected
relative numbers of each type of binary lens event, based on its measurable
characteristics. We find that a microlensing survey with current levels of
photometric uncertainty and sampling should find at least as many non-caustic
crossing binary lens events as caustic crossing events; in future surveys with
more sensitive photometry, the contribution of distinctive non-caustic crossing
events will be even greater. To try to explain why caustic crossing light
curves appear to be so dominant among the published binary lensing events, we
investigate the influence of several physical effects, including blending,
sampling rate, and various binary populations.Comment: 17 pages, 17 figures, submitted to Ap
Unproceedings of the Fourth .Astronomy Conference (.Astronomy 4), Heidelberg, Germany, July 9-11 2012
The goal of the .Astronomy conference series is to bring together
astronomers, educators, developers and others interested in using the Internet
as a medium for astronomy. Attendance at the event is limited to approximately
50 participants, and days are split into mornings of scheduled talks, followed
by 'unconference' afternoons, where sessions are defined by participants during
the course of the event. Participants in unconference sessions are discouraged
from formal presentations, with discussion, workshop-style formats or informal
practical tutorials encouraged. The conference also designates one day as a
'hack day', in which attendees collaborate in groups on day-long projects for
presentation the following morning. These hacks are often a way of
concentrating effort, learning new skills, and exploring ideas in a practical
fashion. The emphasis on informal, focused interaction makes recording
proceedings more difficult than for a normal meeting. While the first
.Astronomy conference is preserved formally in a book, more recent iterations
are not documented. We therefore, in the spirit of .Astronomy, report
'unproceedings' from .Astronomy 4, which was held in Heidelberg in July 2012.Comment: 11 pages, 1 figure, .Astronomy 4, #dotastr
Planet Four: Terrains - Discovery of Araneiforms Outside of the South Polar Layered Deposits
We present the results of a systematic mapping of seasonally sculpted
terrains on the South Polar region of Mars with the Planet Four: Terrains (P4T)
online citizen science project. P4T enlists members of the general public to
visually identify features in the publicly released Mars Reconnaissance Orbiter
CTX images. In particular, P4T volunteers are asked to identify: 1) araneiforms
(including features with a central pit and radiating channels known as
'spiders'); 2) erosional depressions, troughs, mesas, ridges, and
quasi-circular pits characteristic of the South Polar Residual Cap (SPRC) which
we collectively refer to as 'Swiss cheese terrain', and 3) craters. In this
work we present the distributions of our high confidence classic spider
araneiforms and Swiss cheese terrain identifications. We find no locations
within our high confidence spider sample that also have confident Swiss cheese
terrain identifications. Previously spiders were reported as being confined to
the South Polar Layered Deposits (SPLD). Our work has provided the first
identification of spiders at locations outside of the SPLD, confirmed with high
resolution HiRISE imaging. We find araneiforms on the Amazonian and Hesperian
polar units and the Early Noachian highland units, with 75% of the identified
araneiform locations in our high confidence sample residing on the SPLD. With
our current coverage, we cannot confirm whether these are the only geologic
units conducive to araneiform formation on the Martian South Polar region. Our
results are consistent with the current CO2 jet formation scenario with the
process exploiting weaknesses in the surface below the seasonal CO2 ice sheet
to carve araneiform channels into the regolith over many seasons. These new
regions serve as additional probes of the conditions required for channel
creation in the CO2 jet process. (Abridged)Comment: accepted to Icarus - Supplemental data files are available at
https://www.zooniverse.org/projects/mschwamb/planet-four-terrains/about/results
- Icarus print version available at
http://www.sciencedirect.com/science/article/pii/S001910351730055
Fibre optic monitoring of a deep circular excavation
This paper describes part of the monitoring undertaken at Abbey Mills shaft F, one of the main shafts of Thames Water's Lee tunnel project in London, UK. This shaft, with an external diameter of 30 m and 73 m deep, is one of the largest ever constructed in the UK and consequently penetrates layered and challenging ground conditions (Terrace Gravel, London Clay, Lambeth Group, Thanet Sand Formation, Chalk Formation). Three out of the twenty 1·2 m thick and 84 m deep diaphragm wall panels were equipped with fibre optic instrumentation. Bending and circumferential hoop strains were measured using Brillouin optical time-domain reflectometry and analysis technologies. These measurements showed that the overall radial movement of the wall was very small. Prior to excavation during a dewatering trial, the shaft may have experienced three-dimensional deformation due to differential water pressures. During excavation, the measured hoop and bending strains of the wall in the chalk exceeded the predictions. This appears to be related to the verticality tolerances of the diaphragm wall and lower circumferential hoop stiffness of the diaphragm walls at deep depths. The findings from this case study provide valuable information for future deep shafts in London. The authors would like to thank Thames Water Ltd and the
Lee Tunnel Project Management Team for making the field
study possible in the first place.This is the accepted manuscript. The final version is available from ICE Publishing at http://www.icevirtuallibrary.com/content/article/10.1680/geng.13.00036
Col-OSSOS: Colors of the Interstellar Planetesimal 1I/`Oumuamua
The recent discovery by Pan-STARRS1 of 1I/2017 U1 (`Oumuamua), on an unbound
and hyperbolic orbit, offers a rare opportunity to explore the planetary
formation processes of other stars, and the effect of the interstellar
environment on a planetesimal surface. 1I/`Oumuamua's close encounter with the
inner Solar System in 2017 October was a unique chance to make observations
matching those used to characterize the small-body populations of our own Solar
System. We present near-simultaneous g, r, and J photometry
and colors of 1I/`Oumuamua from the 8.1-m Frederick C. Gillett Gemini North
Telescope, and photometry from the 4.2 m William Herschel Telescope. Our
grJ observations are directly comparable to those from the
high-precision Colours of the Outer Solar System Origins Survey (Col-OSSOS),
which offer unique diagnostic information for distinguishing between outer
Solar System surfaces. The J-band data also provide the highest signal-to-noise
measurements made of 1I/`Oumuamua in the near-infrared. Substantial, correlated
near-infrared and optical variability is present, with the same trend in both
near-infrared and optical. Our observations are consistent with 1I/`Oumuamua
rotating with a double-peaked period of hours and being a
highly elongated body with an axial ratio of at least 5.3:1, implying that it
has significant internal cohesion. The color of the first interstellar
planetesimal is at the neutral end of the range of Solar System and
solar-reflectance colors: it is like that of some dynamically excited objects
in the Kuiper belt and the less-red Jupiter Trojans.Comment: Accepted to ApJ
Reassessing the formation of the inner Oort cloud in an embedded star cluster
We re-examine the formation of the inner Oort comet cloud while the Sun was
in its birth cluster with the aid of numerical simulations. This work is a
continuation of an earlier study (Brasser et al., 2006) with several
substantial modifications. First, the system consisting of stars, planets and
comets is treated self-consistently in our N-body simulations, rather than
approximating the stellar encounters with the outer Solar System as hyperbolic
fly-bys. Second, we have included the expulsion of the cluster gas, a feature
that was absent previously. Third, we have used several models for the initial
conditions and density profile of the cluster -- either a Hernquist or Plummer
potential -- and chose other parameters based on the latest observations of
embedded clusters from the literature. {These other parameters result in the
stars being on radial orbits and the cluster collapses.} Similar to previous
studies, in our simulations the inner Oort cloud is formed from comets being
scattered by Jupiter and Saturn and having their pericentres decoupled from the
planets by perturbations from the cluster gas and other stars. We find that all
inner Oort clouds formed in these clusters have an inner edge ranging from 100
AU to a few hundred AU, and an outer edge at over 100\,000 AU, with little
variation in these values for all clusters. All inner Oort clouds formed are
consistent with the existence of (90377) Sedna, an inner Oort cloud dwarf
planetoid, at the inner edge of the cloud: Sedna tends to be at the innermost
2% for Plummer models, while it is 5% for Hernquist models. We emphasise that
the existence of Sedna is a generic outcome. We define a `concentration radius'
for the inner Oort cloud and find that its value increases with increasing
number of stars in the cluster, ranging from 600 AU to 1500 AU for Hernquist
clusters and from 1500 AU to 4000 AU for Plummer clusters...Comment: Accepted Icarus 201
Planet Hunters: New Kepler planet candidates from analysis of quarter 2
We present new planet candidates identified in NASA Kepler quarter two public
release data by volunteers engaged in the Planet Hunters citizen science
project. The two candidates presented here survive checks for false-positives,
including examination of the pixel offset to constrain the possibility of a
background eclipsing binary. The orbital periods of the planet candidates are
97.46 days (KIC 4552729) and 284.03 (KIC 10005758) days and the modeled planet
radii are 5.3 and 3.8 R_Earth. The latter star has an additional known planet
candidate with a radius of 5.05 R_Earth and a period of 134.49 which was
detected by the Kepler pipeline. The discovery of these candidates illustrates
the value of massively distributed volunteer review of the Kepler database to
recover candidates which were otherwise uncatalogued.Comment: Accepted to A
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