478 research outputs found
Modelling electron distributions within ESA's Gaia satellite CCD pixels to mitigate radiation damage
The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012.
Its primary science drivers are the composition, formation and evolution of the
Galaxy. Gaia will achieve its unprecedented positional accuracy requirements
with detailed calibration and correction for radiation damage. At L2, protons
cause displacement damage in the silicon of CCDs. The resulting traps capture
and emit electrons from passing charge packets in the CCD pixel, distorting the
image PSF and biasing its centroid. Microscopic models of Gaia's CCDs are being
developed to simulate this effect. The key to calculating the probability of an
electron being captured by a trap is the 3D electron density within each CCD
pixel. However, this has not been physically modelled for the Gaia CCD pixels.
In Seabroke, Holland & Cropper (2008), the first paper of this series, we
motivated the need for such specialised 3D device modelling and outlined how
its future results will fit into Gaia's overall radiation calibration strategy.
In this paper, the second of the series, we present our first results using
Silvaco's physics-based, engineering software: the ATLAS device simulation
framework. Inputting a doping profile, pixel geometry and materials into ATLAS
and comparing the results to other simulations reveals that ATLAS has a free
parameter, fixed oxide charge, that needs to be calibrated. ATLAS is
successfully benchmarked against other simulations and measurements of a test
device, identifying how to use it to model Gaia pixels and highlighting the
effect of different doping approximations.Comment: 12 pages, 6 figures, appearing in Proc. of SPIE Optics and Photonics
Conference (Focal Plane Arrays for Space telescopes IV), 2-6 August 2009, San
Diego, US
Modelling Gaia CCD pixels with Silvaco 3D engineering software
Gaia will only achieve its unprecedented measurement accuracy requirements
with detailed calibration and correction for radiation damage. We present our
Silvaco 3D engineering software model of the Gaia CCD pixel and two of its
applications for Gaia: (1) physically interpreting supplementary buried channel
(SBC) capacity measurements (pocket-pumping and first pixel response) in terms
of e2v manufacturing doping alignment tolerances; and (2) deriving electron
densities within a charge packet as a function of the number of constituent
electrons and 3D position within the charge packet as input to microscopic
models being developed to simulate radiation damage.Comment: 4 pages, 3 figures, contributed poster, appearing in proceedings of
the ELSA conference: Gaia, at the frontiers of astrometry, 7-11 June 2010,
S\`evres, Pari
Risk of death and cardiovascular outcomes with thiazolidinediones: a study with the general practice research database and secondary care data.
OBJECTIVE: To describe the likely extent of confounding in evaluating the risks of cardiovascular (CV) events and mortality in patients using diabetes medication. METHODS: The General Practice Research Database was used to identify inception cohorts of insulin and different oral antidiabetics. An analysis of bias and incidence of mortality, acute coronary syndrome, stroke and heart failure were analysed in GPRD, Hospital Episode Statistics and death certificates. RESULTS: 206,940 patients were identified. The bias analysis showed that past thiazolidinedione users had a lower mortality risk compared to past metformin users. There were no differences between past users of rosiglitazone and pioglitazone (adjusted RR of 1.04; 95% CI 0.93-1.18). Current rosiglitazone users had an increased risk of death (adjusted RR 1.20; 95% CI 1.08-1.34) and of hospitalisation for heart failure (adjusted RR of 1.73; 95% CI 1.19-2.51) compared to current pioglitazone users. Risk of mortality was increased two-fold shortly after starting rosiglitazone. Excess risk of death over 3 years with rosiglitazone was 0.3 per 100 in those aged 50-64 years, 2.0 aged 65-74, 3.0 aged 75-84, and 7.0 aged 85+. The cause of death with rosiglitazone was more likely to be due to a disease of the circulatory system. CONCLUSIONS: Higher risks for death (overall and due to cardiovascular disease) and heart failure were found for rosiglitazone compared to pioglitazone. These excess risks were largest in patients aged 65 years or older. The European regulatory decision to suspend rosiglitazone is supported by this study
Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite
The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors
which will suffer from increased charge transfer inefficiency (CTI) as a result
of radiation damage. To aid the mitigation at low signal levels, the CCD design
includes Supplementary Buried Channels (SBCs, otherwise known as `notches')
within each CCD column. We present the largest published sample of Gaia CCD SBC
Full Well Capacity (FWC) laboratory measurements and simulations based on 13
devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in
the upper half of each CCD that are systematically smaller by two orders of
magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of
electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance
predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as
inputs to their simulations. However, all the CCDs already integrated onto the
satellite for the 2013 launch are post-2004. SBC FWC measurements are not
available for one of our five post-2004 CCDs but the fact it meets Gaia's image
location requirements suggests it has SBC FWCs similar to pre-2004. It is too
late to measure the SBC FWCs onboard the satellite and it is not possible to
theoretically predict them. Gaia's faint star astrometric performance
predictions depend on knowledge of the onboard SBC FWCs but as these are
currently unavailable, it is not known how representative of the whole focal
plane the current predictions are. Therefore, we suggest Gaia's initial
in-orbit calibrations should include measurement of the onboard SBC FWCs. We
present a potential method to do this. Faint star astrometric performance
predictions based on onboard SBC FWCs at the start of the mission would allow
satellite operating conditions or CTI software mitigation to be further
optimised to improve the scientific return of Gaia.Comment: Accepted for publication in MNRAS, 16 pages, 19 figure
Local Stellar Kinematics from RAVE data - VII. Metallicity Gradients from Red Clump Stars
We investigate the Milky Way Galaxy's radial and vertical metallicity
gradients using a sample of 47,406 red clump stars from the RAVE DR4. This
sample is more than twice the size of the largest sample in the literature
investigating radial and vertical metallicity gradients. The absolute magnitude
of Groenewegen (2008) is used to determine distances to our sample stars. The
resulting distances agree with the RAVE DR4 distances Binney et al. (2014) of
the same stars. Our photometric method also provides distances to 6185 stars
that are not assigned a distance in RAVE DR4. The metallicity gradients are
calculated with their current orbital positions ( and ) and with
their orbital properties (mean Galactocentric distance, and ),
as a function of the distance to the Galactic plane:
d[Fe/H]/d- dex/kpc for kpc and
d[Fe/H]/d- dex/kpc for kpc. This
reaffirms the radial metallicity gradient in the thin disc but highlights that
gradients are sensitive to the selection effects caused by the difference
between and . The radial gradient is flat in the distance
interval 0.5-1 kpc from the plane and then becomes positive greater than 1 kpc
from the plane. The radial metallicity gradients are also eccentricity
dependent. We showed that d[Fe/H]/d-, -,
- and - dex/kpc for , ,
and sub-samples, respectively, in the distance
interval kpc. Similar trend is found for vertical
metallicity gradients. Both the radial and vertical metallicity gradients are
found to become shallower as the eccentricity of the sample increases. These
findings can be used to constrain different formation scenarios of the thick
and thin discs.Comment: 18 pages, including 16 figures and 6 tables, accepted for publication
in PAS
The vertical metallicity gradients of mono-age stellar populations in the Milky Way with the RAVE and Gaia data
We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution and Radial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. The metallicity at disc plane remains almost constant between 2 and 8 Gyr, and it becomes significantly lower for the 8 < t ≤ 11 Gyr population. The current analysis also reveals that the intrinsic dispersion in metallicity increases steadily with age.We discuss that our results are consistent with a scenario that (thin) disc stars formed from a flaring (thin) star-forming disc
The vertical metallicity gradients of mono-age stellar populations in the Milky Way thin disk
We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. We also find that the intrinsic dispersion in metallicity increases steadily with age. Our results are consistent with a scenario that thin disk stars formed from a flaring thin star-forming disk
Single-lined Spectroscopic Binary Star Candidates in the RAVE Survey
Repeated spectroscopic observations of stars in the Radial Velocity
Experiment (RAVE) database are used to identify and examine single-lined binary
(SB1) candidates. The RAVE latest internal database (VDR3) includes radial
velocities, atmospheric and other parameters for approximately quarter million
of different stars with little less than 300,000 observations. In the sample of
~20,000 stars observed more than once, 1333 stars with variable radial
velocities were identified. Most of them are believed to be SB1 candidates. The
fraction of SB1 candidates among stars with several observations is between 10%
and 15% which is the lower limit for binarity among RAVE stars. Due to the
distribution of time spans between the re-observation that is biased towards
relatively short timescales (days to weeks), the periods of the identified SB1
candidates are most likely in the same range. Because of the RAVE's narrow
magnitude range most of the dwarf candidates belong to the thin Galactic disk
while the giants are part of the thick disk with distances extending to up to a
few kpc. The comparison of the list of SB1 candidates to the VSX catalog of
variable stars yielded several pulsating variables among the giant population
with the radial velocity variations of up to few tens of km/s. There are 26
matches between the catalog of spectroscopic binary orbits (SB9) and the whole
RAVE sample for which the given periastron time and the time of RAVE
observation were close enough to yield a reliable comparison. RAVE measurements
of radial velocities of known spectroscopic binaries are consistent with their
published radial velocity curves.Comment: 10 pages, 7 figures, accepted for publication in A
Chromospherically Active Stars in the RAVE Survey. I. The Catalogue
RAVE, the unbiased magnitude limited survey of the southern sky stars,
contained 456,676 medium-resolution spectra at the time of our analysis.
Spectra cover the CaII IRT range which is a known indicator of chromospheric
activity. Our previous work (Matijevi\v{c} et al. 2012) classified all spectra
using locally linear embedding. It identified 53,347 cases with a suggested
emission component in calcium lines. Here we use a spectral subtraction
technique to measure the properties of this emission. Synthetic templates are
replaced by the observed spectra of non-active stars to bypass the difficult
computations of non-LTE profiles of the line cores and stellar parameter
dependence. We derive both the equivalent width of the excess emission for each
calcium line on a 5\AA\ wide interval and their sum EW_IRT for ~44,000
candidate active dwarf stars with S/N>20 and with no respect to the source of
their emission flux. From these ~14,000 show a detectable chromospheric flux
with at least 2\sigma\ confidence level. Our set of active stars vastly
enlarges previously known samples. Atmospheric parameters and in some cases
radial velocities of active stars derived from automatic pipeline suffer from
systematic shifts due to their shallower calcium lines. We re-estimate the
effective temperature, metallicity and radial velocities for candidate active
stars. The overall distribution of activity levels shows a bimodal shape, with
the first peak coinciding with non-active stars and the second with the pre
main-sequence cases. The catalogue will be publicly available with the next
RAVE public data releases.Comment: 13 pages, 9 figure
RVS Spectra of Gaia Photometric Science Alerts
Gaia Photometric Science Alerts (GPSA) publishes Gaia G magnitudes and Blue Photometer (BP) and Red Photometer (RP) low-resolution epoch spectra of transient events. 27 high-resolution spectra from Gaia's Radial Velocity Spectrometer (RVS) of 12 GPSAs have also been published. These 27 RVS epoch spectra are presented next to their corresponding BP and RP epoch spectra in a single place for the first time. We also present one new RVS spectrum of a 13th GPSA that could not be published by the GPSA system. Of the 13 GPSA with RVS spectra, five are photometrically classified as unknown, five as supernovae (three as SN Ia, one as SN II, one as SN IIP), one as a cataclysmic variable, one as a binary microlensing event and one as a young stellar object. The five GPSAs classified as unknown are potential scientific opportunities, while all of them are a preview of the epoch RVS spectra that will be published in Gaia's fourth data release
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