Showing papers by "Anthony G. A. Brown published in 2006"

The design and performance of the Gaia photometric system

Abstract: The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.

A brown dwarf desert for intermediate mass stars in Sco OB2

Abstract: We present JHK observations of 22 intermediate-mass stars in Sco OB2, obtained with VLT/NACO. The survey was performed to determine the status of (sub)stellar candidate companions of A and late-B members. The distinction between companions and background stars is by a comparison with isochrones and statistical arguments. We are sensitive to companions in the separation range 0.1''-11'' (13-1430 AU) and K 12 as background stars. Our multi-color analysis demonstrates that the simple K=12 criterion correctly classifies the secondaries in ~80% of the cases. We reanalyse the total ADONIS/NACO sample and conclude that of the 176 secondaries, 25 are physical companions, 55 are candidates, and 96 are background stars. Although we are sensitive and complete to brown dwarfs as faint as K=14 in the separation range 130-520 AU, we detect only one, giving a brown dwarf companion fraction of 0.5% (M>30 MJ). However, the number of brown dwarfs is consistent with an extrapolation of the stellar companion mass distribution. This indicates that the physical mechanism for the formation of brown dwarfs around intermediate mass stars is similar to that of stellar companions, and that the embryo ejection mechanism does not need to be invoked in order to explain the small number of brown dwarf companions among these stars.

Gaia Data Processing Architecture

Abstract: Gaia is ESA's ambitious space astrometry mission the main objective of which is to astrometrically and spectro-photometrically map 1000 Million celestial objects (mostly in our galaxy) with unprecedented accuracy. The announcement of opportunity for the data processing will be issued by ESA late in 2006. The Gaia Data Processing and Analysis Consortium (DPAC) has been formed recently and is preparing an answer. The satellite will downlink close to 100 TB of raw telemetry data over 5 years. To achieve its required accuracy of a few 10s of Microarcsecond astrometry, a highly involved processing of this data is required. In addition to the main astrometric instrument Gaia will host a Radial Velocity instrument, two low-resolution dispersers for multi-color photometry and two Star Mappers. Gaia is a flying Giga Pixel camera. The various instruments each require relatively complex processing while at the same time being interdependent. We describe the overall composition of the DPAC and the envisaged overall architecture of the Gaia data processing system. We shall delve further into the core processing - one of the nine, so-called, coordination units comprising the Gaia processing system.

Searching for ghosts in the galactic halo: what can we learn about the formation of the galaxy from the stellar halo?

Abstract: We study the feasibility of recovering information of remnants of tidally disrupted satellite galaxies in the halo of our Galaxy, using space astrometry from Gaia. This mission will provide a very large data set ( 10 9 stars) with an unprecedent level of detail in phase space (tens of s). However, before recovering useful information, sampling biases, observational errors and the stellar galactic background must be taken into account. We present a Monte Carlo simulation of the Gaia catalogue that excludes the galactic disk (jbj < 5 ) within the rst and fourth galactic quadrants. The simulated catalogue contains a realistic number of stars (3 10 8 ), and for each we have simulated kinematic and photometric information. No interstellar extinction is included. We simulate the destruction of satellite galaxies in the Galactic halo, using 10 6 {particle satellites moving within a xed potential that represents the Galaxy. Photometric information is added assuming particular isochrones and simulation snapshots are added to the simulated catalogue. All phase-space data is converted to astrometric observables. We then add observational errors to all simulated Gaia observables according to a realistic model that includes dependence on apparent magnitude, color and ecliptic latitude. We test the feasibility of recovering past merger signatures in the energy vs. angular momentum plane. We conclude that recovery is possible, but a pre-selection of high signal-to-noise data as well as complementary photometric criteria must be used. The numerical tool here developed can be employed to test the feasibility of other search criteria.