Showing papers by "Anthony G. A. Brown published in 2007"

The primordial binary population. II. Recovering the binary population for intermediate mass stars in Scorpius OB2

Abstract: We characterize the binary population in the young and nearby OB association Scorpius OB2 (Sco OB2) using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (>70% with 3σ confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form fq(q) ∝ q γq , with γq ≈− 0.4. Sco OB2 has a semi-major axis distribution of the form fa(a) ∝ a γ a with γa ≈− 1.0 (Opik’s law), in the range 5R� < a < 5 × 10 6 R� . The log-normal period distribution of Duquennoy & Mayor (1991) results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars.

The primordial binary population II: Recovering the binary population for intermediate mass stars in Sco OB2

Abstract: We characterize the binary population in the young and nearby OB association Scorpius OB2 using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (> 70% with 3sigma confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form f(q) ~ q^-0.4. Sco OB2 has a semi-major axis distribution of the form f(log a) ~ constant (Opik's law), in the range 5-5e6 Rsun. The log-normal period distribution of Duquennoy & Mayor results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars.

A brown dwarf desert for intermediate mass stars in Scorpius OB2

Abstract: We present JHK S observations of 22 intermediate-mass stars in the Scorpius-Centaurus OB association, obtained with the NAOS/CONICA system at the ESO Very Large Telescope. This survey was performed to determine the status of (sub)stellar candidate companions of Sco OB2 member stars of spectral type A and late-B. The distinction between companions and background stars is made on the basis of a comparison to isochrones and additional statistical arguments. We are sensitive to companions with an angular separation of $0.1''{-}11''$ ($13{-}1430$ AU) and the detection limit is $K_{\rm S}=17$ mag. We detect 62 stellar components of which 18 turn out to be physical companions, 11 candidate companions, and 33 background stars. Three of the 18 confirmed companions were previously undocumented as such. The companion masses are in the range $0.03~{M}_\odot \leq M \leq 1.19~{M}_\odot$, corresponding to mass ratios $0.06 \leq q \leq 0.55$. We include in our sample a subset of 9 targets with multi-color ADONIS observations from Kouwenhoven et al. (2005, AA those with $K_{\rm S} > 12$ mag as background stars. The multi-color analysis in this paper demonstrates that the simple $K_{\rm S}=12$ mag criterion correctly classifies the secondaries in ~80% of the cases. We reanalyse the total sample (i.e. NAOS/CONICA and ADONIS) and conclude that of the 176 secondaries, 25 are physical companions, 55 are candidate companions, and 96 are background stars. Although we are sensitive (and complete) to brown dwarf companions as faint as $K_{\rm S}=14$ mag in the semi-major axis range $130{-}520$ AU, we detect only one, corresponding to a brown dwarf companion fraction of $0.5 \pm 0.5\%$ ($M \ga 30~{M_{\rm J}}$). However, the number of brown dwarfs is consistent with an extrapolation of the (stellar) companion mass distribution into the brown dwarf regime. This indicates that the physical mechanism for the formation of brown dwarf companions 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 intermediate mass stars in the Sco OB2 association.

Getting ready for the micro-arcsecond era

Abstract: As the title of this symposium implies, one of the aims is to examine the future of astrometry as we move from an era in which thanks to the Hipparcos Catalogue everyone has become familiar with milliarcsecond astrometry to an era in which microarcsecond astrometry will become the norm I will take this look into the future by first providing an overview of present astrometric programmes and how they fit together and then I will attempt to identify the most promising future directions In addition I discuss the important conditions for the maximization of the scientific return of future large and highly accurate astrometric catalogues; catalogue access and analysis tools, the availability of sufficient auxiliary data and theoretical knowledge, and the education of the future generation of astrometrists