Showing papers by "Anthony G. A. Brown published in 2010"

On the identification of merger debris in the Gaia era

Abstract: We model the formation of the Galactic stellar halo via the accretion of satellite galaxies on to a time-dependent semicosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close manner to what may be possible with the Gaia mission. We have created a synthetic Gaia solar neighbourhood catalogue by convolving the six-dimensional phase-space coordinates of stellar particles from our disrupted satellites with the latest estimates of the Gaia measurement errors, and included realistic background contamination due to the Galactic disc(s) and bulge. We find that, even after accounting for the expected observational errors, the resulting phase space is full of substructure. We are able to successfully isolate roughly 50 per cent of the different satellites contributing to the 'solar neighbourhood' by applying the mean shift clustering algorithm in energy-angular momentum space. Furthermore, a Fourier analysis of the space of orbital frequencies allows us to obtain accurate estimates of the time since accretion for approximately 30 per cent of the recovered satellites.

Distance determination for RAVE stars using stellar models . II. Most likely values assuming a standard stellar evolution scenario

Abstract: The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way which already collected over 400000 spectra of ∼330000 different stars. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235064 high quality spectra which show no peculiarities and belong to 210872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al. (2010, A&A, 511, A16). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a larger dataset. The derived distances of both dwarfs and giants match within ∼21% to the astrometric distances of Hipparcos stars and to the distances of observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with half of the objects showing a distance scatter of <11%. RAVE dwarfs are ∼300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.

On the identification of merger debris in the {\it Gaia} Era

Abstract: We model the formation of the Galactic stellar halo via the accretion of satellite galaxies onto a time-dependent semi-cosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close manner to what may be possible with the {\it Gaia} mission. We have created a synthetic {\it Gaia} Solar Neighbourhood catalogue by convolving the 6D phase-space coordinates of stellar particles from our disrupted satellites with the latest estimates of the {\it Gaia} measurement errors, and included realistic background contamination due to the Galactic disc(s) and bulge. We find that, even after accounting for the expected observational errors, the resulting phase-space is full of substructure. We are able to successfully isolate roughly 50% of the different satellites contributing to the `Solar Neighbourhood' by applying the Mean-Shift clustering algorithm in energy-angular momentum space. Furthermore, a Fourier analysis of the space of orbital frequencies allows us to obtain accurate estimates of time since accretion for approximately 30% of the recovered satellites.

The quest for the Sun's siblings: an exploratory search in the Hipparcos Catalogue

Abstract: We describe the results of a search for the remnants of the Sun’s birth cluster among stars in the Hipparcos Catalogue. This search is based on the predicted phase-space distribution of the Sun’s siblings from simple simulations of the orbits of the cluster stars in a smooth Galactic potential. For stars within 100 pc, the simulations show that it is interesting to examine those that have small space motions relative to the Sun. From amongst the candidate siblings thus selected, there are six stars with ages consistent with that of the Sun. Considering their radial velocities and abundances only one potential candidate, HIP 21158, remains, but essentially the result of the search is negative. This is consistent with predictions by Portegies Zwart on the number of siblings near the Sun. We discuss the steps that should be taken in anticipation of the data from the Gaia mission in order to conduct fruitful searches for the Sun’s siblings in the future.

A fast model of radiation-induced electron trapping in CCDs for implementation in the Gaia data processing

Abstract: The European Space Agency's Gaia mission1 is scheduled for launch in 2012. It will operate at L2 for 5 years, rotating slowly so that its two optical telescopes will repeatedly observe more than one billion stars. The resulting data set will be iteratively reduced to solve for the relative position, parallax-distance and proper motion of every observed star, yielding a three dimensional dynamical model of our galaxy. The focal plane contains 106 large area silicon CCDs continuously operating in TDI mode at a line rate synchronised with the satellite rotation.2 One of the greatest challenges facing the mission is radiation damage in the CCDs which will cause charge loss and image distortion. This is particularly severe because the large focal plane is difficult to shield and because the launch will coincide with solar maximum. Despite steps taken to minimize the effects of radiation (e.g. regular use of charge injection), the residual distortion will need to be calibrated during the pipeline data processing. Due to the volume of data involved, this requires a trapping model which is physically realistic, yet fast enough and simple enough to implement in the pipeline. The current prototype Charge Distortion Model will be presented. This model was developed specifically for Gaia in TDI mode. However, an imaging mode version has already been applied to other missions, for example, to indicate the potential impact of radiation damage on the proposed Euclid mission.

Design drivers for a wide-field multi-object spectrograph for the William Herschel Telescope

Abstract: Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector (Agocs et al, this conf.) that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT.

Comparison of a fast analytical model of radiation damage effects in CCDs with experimental tests

Abstract: ESA's Gaia mission aims to create a complete and highly accurate stereoscopic map of the Milky Way. The stellar parallaxes will be determined at the micro-arcsecond level, as a consequence the measurement of the stellar image location on the CCD must be highly accurate. The solar wind protons will create charge traps in the CCDs of Gaia, which will induce large charge loss and distort the stellar images causing a degradation of the location measurement accuracy. Accurate modelling of the stellar image distortion induced by radiation is required to mitigate these effects. We assess the capability of a fast physical analytical model of radiation damage effects called the charge distortion model (CDM) to reproduce experimental data. To realize this assessment we developed a rigorous procedure that compares at the sub-pixel level the model outcomes to damaged images extracted from the experimental tests. We show that CDM can reproduce accurately up to a certain level the test data acquired on a highly irradiated device operated in time delay integration mode for different signal levels and different illumination histories. We discuss the potential internal and external factors that contributed to limit the agreement between the data and the charge distortion model. To investigate these limiting factors further, we plan to apply our comparison procedure on a synthetic dataset generated through detailed Monte-Carlo simulations at the CCD electrode level.

Design drivers for a wide-field multi-object spectrograph for the William Herschel Telescope

Abstract: Wide-field multi-object spectroscopy is a high priority for European astronomy over the next decade. Most 8-10m telescopes have a small field of view, making 4-m class telescopes a particularly attractive option for wide-field instruments. We present a science case and design drivers for a wide-field multi-object spectrograph (MOS) with integral field units for the 4.2-m William Herschel Telescope (WHT) on La Palma. The instrument intends to take advantage of a future prime-focus corrector and atmospheric-dispersion corrector that will deliver a field of view 2 deg in diameter, with good throughput from 370 to 1,000 nm. The science programs cluster into three groups needing three different resolving powers R: (1) high-precision radial-velocities for Gaia-related Milky Way dynamics, cosmological redshift surveys, and galaxy evolution studies (R = 5,000), (2) galaxy disk velocity dispersions (R = 10,000) and (3) high-precision stellar element abundances for Milky Way archaeology (R = 20,000). The multiplex requirements of the different science cases range from a few hundred to a few thousand, and a range of fibre-positioner technologies are considered. Several options for the spectrograph are discussed, building in part on published design studies for E-ELT spectrographs. Indeed, a WHT MOS will not only efficiently deliver data for exploitation of important imaging surveys planned for the coming decade, but will also serve as a test-bed to optimize the design of MOS instruments for the future E-ELT.

Distance determination for RAVE stars using stellar models II: Most likely values assuming a standard stellar evolution scenario

Abstract: The RAdial Velocity Experiment (RAVE) is a spectroscopic survey of the Milky Way. We use the subsample of spectra with spectroscopically determined values of stellar parameters to determine the distances to these stars. The list currently contains 235,064 high quality spectra which show no peculiarities and belong to 210,872 different stars. The numbers will grow as the RAVE survey progresses. The public version of the catalog will be made available through the CDS services along with the ongoing RAVE public data releases. The distances are determined with a method based on the work by Breddels et al.~(2010). Here we assume that the star undergoes a standard stellar evolution and that its spectrum shows no peculiarities. The refinements include: the use of either of the three isochrone sets, a better account of the stellar ages and masses, use of more realistic errors of stellar parameter values, and application to a larger dataset. The derived distances of both dwarfs and giants match within ~21% to the astrometric distances of Hipparcos stars and to the distances of observed members of open and globular clusters. Multiple observations of a fraction of RAVE stars show that repeatability of the derived distances is even better, with half of the objects showing a distance scatter of \simlt 11%. RAVE dwarfs are ~300 pc from the Sun, and giants are at distances of 1 to 2 kpc, and up to 10 kpc. This places the RAVE dataset between the more local Geneva-Copenhagen survey and the more distant and fainter SDSS sample. As such it is ideal to address some of the fundamental questions of Galactic structure and evolution in the pre-Gaia era. Individual applications are left to separate papers, here we show that the full 6-dimensional information on position and velocity is accurate enough to discuss the vertical structure and kinematic properties of the thin and thick disks.

Accurate Modelling the Attitude of the Gaia Satellite

Abstract: The Gaia Attitude Model (GAM) is a simulation package that is developed to achieve a detailed understanding of the Gaia spacecraft attitude. It takes into account external physical effects and considers internal hardware components controlling the satellite. The main goal of the Gaia mission is to obtain extremely accurate astrometry, and this requires a good knowledge of Gaia’s behaviour as a spinning rigid body under the influence of various disturbances.