826 research outputs found
Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs
Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia).Comment: Accepted by MNRAS on 13 February 2011. 15 pages, 7 figures and 5
table
Space-borne global astrometric surveys: the hunt for extra-solar planets
The proposed global astrometry mission {\it GAIA}, recently recommended
within the context of ESA's Horizon 2000 Plus long-term scientific program,
appears capable of surveying the solar neighborhood within 200 pc for
the astrometric signatures of planets around stars down to the magnitude limit
of =17 mag, which includes late M dwarfs at 100 pc. Realistic end-to-end
simulations of the GAIA global astrometric measurements have yielded first
quantitative estimates of the sensitivity to planetary perturbations and of the
ability to measure their orbital parameters. Single Jupiter-mass planets around
normal solar-type stars appear detectable up to 150 pc (12 mag) with
probabilities 50 per cent for orbital periods between 2.5 and
8 years, and their orbital parameters measured with better than 30 per
cent accuracy to about 100 pc. Jupiter-like objects (same mass and period as
our giant planet) are found with similar probabilities up to 100 pc.These first
experiments indicate that the {\it GAIA} results would constitute an important
addition to those which will come from the other ongoing and planned
planet-search programs. These data combined would provide a formidable testing
ground on which to confront theories of planetary formation and evolution.Comment: 13 pages, 10 figures, uses mn.sty, accepted by MNRA
Fourteen New Companions from the Keck & Lick Radial Velocity Survey Including Five Brown Dwarf Candidates
We present radial velocities for 14 stars on the California & Carnegie Planet
Search target list that reveal new companions. One star, HD 167665, was fit
with a definitive Keplerian orbit leading to a minimum mass for the companion
of 50.3 Mjup at a separation from its host of ~5.5 AU. Incomplete or limited
phase coverage for the remaining 13 stars prevents us from assigning to them
unique orbital parameters. Instead, we fit their radial velocities with
Keplerian orbits across a grid of fixed values for Msini and period, P, and use
the resulting reduced chi-square surface to place constraints on Msini, P, and
semimajor axis, a. This technique allowed us to restrict Msini below the brown
dwarf -- stellar mass boundary for an additional 4 companions (HD 150554, HD
8765, HD 72780, HD 74014). If the combined 5 companions are confirmed as brown
dwarfs, these results would comprise the first major catch of such objects from
our survey beyond ~3 AU.Comment: 29 pages, 14 figures, accepted to Ap
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
Intrinsic definitions of "relative velocity" in general relativity
Given two observers, we define the "relative velocity" of one observer with
respect to the other in four different ways. All four definitions are given
intrinsically, i.e. independently of any coordinate system. Two of them are
given in the framework of spacelike simultaneity and, analogously, the other
two are given in the framework of observed (lightlike) simultaneity. Properties
and physical interpretations are discussed. Finally, we study relations between
them in special relativity, and we give some examples in Schwarzschild and
Robertson-Walker spacetimes.Comment: 29 pages, 12 figures. New proofs in special relativity and a new open
problem in general relativity (see Remark 5.2). An Appendix has been added,
studying the relative velocities in Schwarzschild, with new figures. Some
spelling erros fixe
The PRIMA fringe sensor unit
The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced
Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides
fringe sensing for all observation modes, comprising off-axis fringe tracking,
phase referenced imaging, and high-accuracy narrow-angle astrometry. It is
installed at the Very Large Telescope Interferometer (VLTI) and successfully
servoed the fringe tracking loop during the initial commissioning phase. Unique
among interferometric beam combiners, the FSU uses spatial phase modulation in
bulk optics to retrieve real-time estimates of fringe phase after spatial
filtering. A R=20 spectrometer across the K-band makes the retrieval of the
group delay signal possible. The FSU was integrated and aligned at the VLTI in
summer 2008. It yields phase and group delay measurements at sampling rates up
to 2 kHz, which are used to drive the fringe tracking control loop. During the
first commissioning runs, the FSU was used to track the fringes of stars with
K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT)
and baselines of up to 96 m. Fringe tracking using two Very Large Telescope
(VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and
combining stellar light with two ATs, the FSU showed its ability to improve the
VLTI sensitivity in K-band by more than one magnitude towards fainter objects,
which is of fundamental importance to achieve the scientific objectives of
PRIMA.Comment: 19 pages, 23 figures. minor changes and language editing. this
version equals the published articl
Finite-source and finite-lens effects in astrometric microlensing
The aim of this paper is to study the astrometric trajectory of microlensing
events with an extended lens and/or source. We consider not only a dark lens
but also a luminous lens as well. We find that the discontinuous finite-lens
trajectories given by Takahashi (2003) will become continuous in the
finite-source regime. The point lens (source) approximation alone gives an
under (over)estimation of the astrometric signal when the size of the lens and
source are not negligible. While the finiteness of the source is revealed when
the lens transits the surface of the source, the finite-lens signal is most
prominent when the lens is very close to the source. Astrometric microlensing
towards the Galactic bulge, Small Magellanic Cloud and M31 are discussed, which
indicate that the finite-lens effect is beyond the detection limit of current
instruments. Nevertheless, it is possible to distinguish between self-lensing
and halo lensing through a (non-)detection of the astrometric ellipse. We also
consider the case where the lens is luminous itself, as has been observed where
a lensing event was followed up with the Hubble Space Telescope. We show that
the astrometric signal will be reduced in a luminous-lens scenario. The
physical properties of the event, such as the lens-source flux ratio, the size
of the lens and source nevertheless can be derived by fitting the astrometric
trajectory.Comment: 12 pages, 12 figures, 1 table, published in MNRA
Spatio-temporal variation in marine fish traits reveals community-wide responses to environmental change
Marine ecosystems are exposed to a range of environmental and anthropogenic stressors, including climate change and overexploitation. A promising way towards understanding the impacts of such stressors on community composition is by considering species traits rather than species identity. Here, we describe the spatio-temporal dynamics in fish community traits using >30 yr of species abundance data from the North Sea combined with trait information on body size, life history, growth rate, reproduction and trophic level for demersal fish species in the area. We assessed whether the derived patterns and trends in community-weighted mean traits could be explained by a range of environmental stressors and fishing. Our results revealed strong spatial structuring and long-term changes in the trait composition of North Sea fish, with temporal changes not being uniformly distributed in space. Among the environmental drivers investigated, depth was one of the best predictors, primarily explaining the spatial variation in lifespan, growth rate, trophic level and fecundity. This can be explained by variables that co-vary with depth, e.g. temperature, seasonality, salinity and productivity. Finally, we found only weak relationships between fishing and the spatial variation of traits, suggesting that the spatial trait composition of the community is mostly determined by the environment. Yet, long-term changes in trait composition, primarily in body size, have previously been shown to be affected by size-selective fishing. Our study exemplifies how traits can be used to summarize complex community dynamics and responses to environmental and anthropogenic stressors as well as their usefulness for ecosystem-based management
High-precision astrometry on the VLT/FORS1 at time scales of few days
We investigate the accuracy of astrometric measurements with the VLT/FORS1
camera and consider potential applications. The study is based on two-epoch
(2000 and 2002/2003) frame series of observations of a selected Galactic Bulge
sky region that were obtained with FORS1 during four consecutive nights each.
Reductions were carried out with a novel technique that eliminates atmospheric
image motion and does not require a distinction between targets and reference
objects. The positional astrometric precision was found to be limited only by
the accuracy of the determination of the star photocentre, which is typically
200-300 microarcsec per single measurement for bright unsaturated stars
B=18-19. Several statistical tests have shown that at time-scales of 1-4 nights
the residual noise in measured positions is essentially a white noise with no
systematic instrumental signature and no significant deviation from a Gaussian
distribution. Some evidence of a good astrometric quality of the VLT for frames
separated by two years has also been found. Our data show that the VLT with
FORS1/2 cameras can be effectively used for astrometric observations of
planetary microlensing events and other applications where a high accuracy is
required, that is expected to reach 30-40 microarcsec for a series of 50 frames
(one hours with R filter).Comment: 11 pages, 9 figures, accepted for publication in A&
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