93 research outputs found
Silvaco ATLAS model of ESA's Gaia satellite e2v CCD91-72 pixels
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 accuracy requirements with detailed calibration and correction for CCD radiation damage and CCD geometric distortion. In this paper, the third of the series, we present our 3D Silvaco ATLAS model of the Gaia e2v CCD91-72 pixel. We publish e2v's design model predictions for the capacities of one of Gaia's pixel features, the supplementary buried channel (SBC), for the first time. Kohley et al. (2009) measured the SBC capacities of a Gaia CCD to be an order of magnitude smaller than e2v's design. We have found the SBC doping widths that yield these measured SBC capacities. The widths are systematically 2 ?m offset to the nominal widths. These offsets appear to be uncalibrated systematic offsets in e2v photolithography, which could either be due to systematic stitch alignment offsets or lateral ABD shield doping diffusion. The range of SBC capacities were used to derive the worst-case random stitch error between two pixel features within a stitch block to be ±0.25 ?m, which cannot explain the systematic offsets. It is beyond the scope of our pixel model to provide the manufacturing reason for the range of SBC capacities, so it does not allow us to predict how representative the tested CCD is. This open question has implications for Gaia's radiation damage and geometric calibration models
Radial Distribution of Stellar Motions in Gaia DR2
By taking advantage of the superb measurements of position and velocity for
an unprecedented large number of stars provided in Gaia DR2, we have generated
the first maps of the rotation velocity, , and vertical velocity,
, distributions as a function of the Galactocentric radius, , across a radial range of ~kpc. In the
map, we have identified many diagonal ridge features, which are compared with
the location of the spiral arms and the expected outer Lindblad resonance of
the Galactic bar. We have detected also radial wave-like oscillations of the
peak of the vertical velocity distribution.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Lette
Constraining Ultra Light Dark Matter with the Galactic Nuclear Star Cluster
We use the Milky Way’s nuclear star cluster (NSC) to test the existence of a dark matter ‘soliton core’, as predicted in ultra-light dark matter (ULDM) models. Since the soliton core size is proportional to m−1DM, while the core density grows as m2DM, the NSC (dominant stellar component within ∼3 pc) is sensitive to a specific window in the dark matter particle mass, mDM. We apply a spherical isotropic Jeans model to fit the NSC line-of-sight velocity dispersion data, assuming priors on the precisely measured Milky Way’s supermassive black hole (SMBH) mass and the well-measured NSC density profile. We find that the current observational data reject the existence of a soliton core for a single ULDM particle with mass in the range 10−20.4 eV ≲ mDM ≲ 10−18.5 eV, assuming that the soliton core structure is not affected by the Milky Way’s SMBH. We test our methodology on mock data, confirming that we are sensitive to the same range in ULDM mass as for the real data. Dynamical modelling of a larger region of the Galactic centre, including the nuclear stellar disc, promises tighter constraints over a broader range of mDM. We will consider this in future work
Diving deep into the milky way using anti-reflection coatings for astronomical CCDs
We report two anti-reflection (AR) coatings that give better quantum efficiency (QE) than the existing AR coating on the Gaia astrometric field (AF) charged coupled devices (CCDs). Light being the core of optical astronomy is extremely important for such missions, therefore, the QE of the devices that are used to capture it should be substantially high. To reduce the losses due to the reflection of light from the surface of the CCDs, AR coatings can be applied. Currently, the main component of the Gaia satellite, the AF CCDs use hafnium dioxide (HfO2) AR coating. In this paper, the ATLAS module of the SILVACO software has been employed for simulating and studying the AF CCD pixel structure and several AR coatings. Our findings suggest that zirconium dioxide (ZrO2) and tantalum pentoxide (Ta2O5) will prove to be better AR coatings for broadband astronomical CCDs in the future and will open new avenues to understand the evolution of the milky way
Impacts of the Local arm on the local circular velocity inferred from the Gaia DR3 young stars in the Milky Way
A simple one-dimensional axisymmetric disc model is applied to the kinematics
of OB stars near the Sun obtained from Gaia DR3 catalogue. The model determines
the 'local centrifugal speed' - defined as the circular
velocity in the Galactocentric rest frame, where the star would move in a
near-circular orbit if the potential is axisymmetric with the local potential
of the Galaxy. We find that the values and their gradient
vary across the selected region of stars within the solar neighbourhood. By
comparing with an N-body/hydrodynamic simulation of a Milky Way-like galaxy, we
find that the kinematics of the young stars in the solar neighbourhood is
affected by the Local arm, which makes it difficult to measure
. However, from the resemblance between the observational
data and the simulation, we suggest that the known rotational velocity gap
between the Coma Bernices and Hyades-Pleiades moving groups could be driven by
the co-rotation resonance of the Local arm, which can be used to infer the
azimuthally averaged circular velocity. We find that obtained
from the kpc sample is well matched with this gap at the
position of the Local arm. Hence, we argue that our results from the
kpc sample, km
, is close to the azimuthally averaged circular velocity
rather than the local centrifugal speed, which is influenced by the presence of
the Local arm.Comment: 13 pages, 11 figure
New distances to RAVE stars
Probability density functions are determined from new stellar parameters for
the distance moduli of stars for which the RAdial Velocity Experiment (RAVE)
has obtained spectra with S/N>=10. Single-Gaussian fits to the pdf in distance
modulus suffice for roughly half the stars, with most of the other half having
satisfactory two-Gaussian representations. As expected, early-type stars rarely
require more than one Gaussian. The expectation value of distance is larger
than the distance implied by the expectation of distance modulus; the latter is
itself larger than the distance implied by the expectation value of the
parallax. Our parallaxes of Hipparcos stars agree well with the values measured
by Hipparcos, so the expectation of parallax is the most reliable distance
indicator. The latter are improved by taking extinction into account. The
effective temperature absolute-magnitude diagram of our stars is significantly
improved when these pdfs are used to make the diagram. We use the method of
kinematic corrections devised by Schoenrich, Binney & Asplund to check for
systematic errors for general stars and confirm that the most reliable distance
indicator is the expectation of parallax. For cool dwarfs and low-gravity
giants tends to be larger than the true distance by up to 30 percent. The
most satisfactory distances are for dwarfs hotter than 5500 K. We compare our
distances to stars in 13 open clusters with cluster distances from the
literature and find excellent agreement for the dwarfs and indications that we
are over-estimating distances to giants, especially in young clusters.Comment: 20 pages accepted by MNRAS. Minor changes to the submitted versio
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Understanding the evolution of radiation damage on the Gaia CCDs after 72 months at L2
The European Space Agency’s Gaia spacecraft has been operating in L2 ever since its launch in December 2013 with a payload that includes 106 scientific charge-coupled devices (CCDs). Due to the predicted radiation environment at the pre-flight testing stage in addition to the high level of accuracy demanded by the science objectives, the non-ionizing energy loss (NIEL) damage on the detectors was identified as a major factor that could affect the science goals of the mission. Here, we present the analysis of an extended set of charge calibration data, taken up to almost six years after launch. It is found that the rate of radiation damage accumulation by the CCDs has not differed significantly from previous results. While the parallel and serial CTI measure an increase in time, the trap defect landscape is still dominated by the preflight defects rather than the radiation-induced traps. CCD devices that were predicted to have a lower NIEL dose measure comparatively larger rates of CTI increase. In addition to this, thicker devices have been measured to have lower serial CTI values compared to thinner devices. The initial parallel CTI values have also been found to be dependent on manufacture year
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Gaia CCDs: charge transfer inefficiency measurements between five years of flight
The European Space Agency's Gaia spacecraft was launched in December 2013 and has been in orbit at the Earth-Sun Lagrange point 2 (L2) for over 6 years. The spacecraft measures the positions, distances, space motions and many other physical characteristics of around one billion stars in the Milky Way and beyond. It has a focal plane of 106 Charge-Coupled Devices (CCDs) which have all been performing well but have been measuring a small but quantifiable degradation in performance in time due to Non-Ionizing Energy Loss (NIEL) damage from interstellar radiation. This NIEL damage produces trap defects which can capture charge from signals and reduces the quality of the data. Gaia's original mission lifetime was planned to be around 5 years and the pre-flight testing and radiation damage analysis was tailored around those timescales as well as with the projected solar activity before launch. Closer to the time of launch and during Gaia's years of orbit, it has been noted that the solar activity was lower than what was initially predicted. From the previous analysis of in-flight data in 2016, it was calculated that Gaia was experiencing an order of magnitude less radiation damage than was predicted.
This paper describes the analysis of charge calibration data and corresponding Charge Transfer Inefficiency (CTI) measurements from the in-flight CCDs, both near the beginning of the mission and after more than 5 years in orbit to quantify the radiation damage impact. These sets of results can be compared with those from the pre-flight tests which can be used to evaluate and understand the differences between the on-ground and in-flight results
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