Showing papers by "Anthony G. A. Brown published in 2005"

The primordial binary population. I. A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2

Abstract: We present the results of a near-infrared adaptive optics survey with the aim to detect close companions to Hipparcos members in the three subgroups of the nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type stars in the K S band, and a subset also in the J and H band. We find 151 stellar components other than the target stars. A brightness criterion is used to separate these components into 77 background stars and 74 candidate physical companion stars. Out of these 74 candidate companions, 41 have not been reported before (14 in US; 13 in UCL; 14 in LCC). The angular separation between primaries and observed companion stars ranges from to . At the mean distance of Sco OB2 (130 pc) this corresponds to a projected separation of 28.6 AU to 1612 AU. Absolute magnitudes are derived for all primaries and observed companions using the parallax and interstellar extinction for each star individually. For each object we derive the mass from K S , assuming an age of 5 Myr for the US subgroup, and 20 Myr for the UCL and LCC subgroups. Companion star masses range from to . The mass ratio distribution follows with , which excludes random pairing. No close () companion stars or background stars are found in the magnitude range . The lack of stars with these properties cannot be explained by low-number statistics, and may imply a lower limit on the companion mass of ~. Close stellar components with are observed. If these components are very low-mass companion stars, a gap in the companion mass distribution might be present. The small number of close low-mass companion stars could support the embryo-ejection formation scenario for brown dwarfs. Our findings are compared with and complementary to visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an overall companion star fraction of 0.52 in this association. This is a lower limit since the data from the observations and from literature are hampered by observational biases and selection effects. This paper is the first step toward our goal to derive the primordial binary population in Sco OB2.

Detection of satellite remnants in the Galactic Halo with Gaia — I. The effect of the Galactic background, observational errors and sampling

Abstract: We address the problem of identifying remnants of satellite galaxies in the halo of our galaxy with Gaia data The Gaia astrometric mission offers a unique opportunity to search for and study these remnants using full phase-space information for the halo of our Galaxy However, the remnants have to be extracted from a very large data set (of the order of 109 stars) in the presence of observational errors and against a background population of Galactic stars We address this issue through numerical simulations with a view towards timely preparations for the scientific exploitation of the Gaia data We present a Monte Carlo simulation of the Gaia catalogue with a realistic number of entries We use a model of the galaxy that includes separate light distributions and kinematics for the bulge, disc and stellar halo components For practical reasons we exclude the region within Galactic coordinates: −90°≤l≤ 90° and −5°≤b≤ 5°, close to the Galactic plane and centre Nevertheless, our catalogue contains 35 × 108 stars No interstellar absorption has been modelled, as we limit our study to high Galactic latitudes We perform tree code 106–body simulations of satellite dwarf galaxies in orbit around a rigid mass model of the Galaxy We follow the simulations for 1010 yr The resulting shrinking satellite cores and tidal tails are then added to the Monte Carlo simulation of the Gaia catalogue To assign photometric properties to the particles we use a Hess diagram for the Solar neighbourhood for Galactic particles, while for the dwarf galaxy particles we use isochrones from the Padova group When combining the Milky Way and dwarf galaxy models we include the complication that the luminosity function of the satellite is probed at various depths as a function of position along the tidal tails The combined Galaxy and satellites model is converted to a synthetic Gaia catalogue using a detailed model for the expected astrometric and radial velocity errors, depending on magnitude, colour and sky position of the stars We explore the feasibility of detecting tidal streams in the halo using the energy versus angular momentum plane We find that a straightforward search in this plane will be very challenging The combination of the background population and the observational errors will make it difficult to detect tidal streams as discrete structures in the E–Lz plane In addition the propagation of observational errors leads to apparent caustic structures in the integrals of motion space that may be mistaken for physical entities Any practical search strategy will have to use a combination of pre-selection of high-quality data and complementary searches using the photometric data that will be provided by Gaia

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 ESA's Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20th magnitude. Through its massive measurement of stellar distances, motions and multi-colour 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 nineteen selected passbands (extending from the ultraviolet 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 to deal with the main Gaia science case is outlined.

From Detailed Galaxy Simulations to a Realistic End-of-Mission Gaia Catalogue

Abstract: We address the problem of identifying remnants of satellite galaxies in the halo of our galaxy with Gaia data. We make use of N-body simulations of dwarf galaxies being disrupted in the halo of our galaxy combined with a Monte Carlo model of the Milky Way galaxy. The models are converted to a simulated Gaia catalogue containing a realistic number ($\sim10^8$--$10^9$) of stars. The simulated catalogue can be used to study how to handle the large data set that Gaia will provide and to study issues such as how to best retrieve information on substructure in the Galactic halo. The techniques described are applicable to any set of N-body simulations of (parts of) the Galaxy.