Binary orbits and masses (Söderhjelm):
Re-analysis of visual and speckle binary data, in combination with Hipparcos
measurements, has yielded improved orbits, mass sums and individual masses
for over 200 systems. The relative uncertainties in the individual
masses are < 7.5% for 42 main-sequence stars, and < 15% for 146 stars.
Compared with mass determinations from eclipsing binaries, this sample
has the advantage that the components are widely separated, which excludes
evolutionary interaction and permits strict comparison with theoretical
models of isolated stars.
Statistical modelling of the observed characteristics of binaries
in the Hipparcos Catalogue has allowed to obtain new information about
their true distributions, in particular concerning the mass ratios and
The prospects for binary observations by
Gaia are spectacular. Statistical modelling shows that some 50,000
mass values will be determined, of which some 10,000 could have relative
errors less than 1%.
Söderhjelm (A&A 341, 121, 1999),
Söderhjelm (ASPC 318, 413, 2004),
Söderhjelm (A&A 463, 683, 2007).
Hipparcos Transit Data (Quist, Lindegren):
Analysis techniques using the intermediate Hipparcos Transit Data (published
on CD-ROM as part of the Hipparcos and Tycho Catalogues; ESA SP-1200, 1997)
were further developed. The Transit Data are intermediate astrometric
and photometric results permitting re-analysis of double and multiple stars,
and which can combined in a rigorous manner with independent (ground-based)
information, such as speckle data. Programs to facilitate the use
of the Transit Data have been made available on the Web.
Quist & Lindegren (A&AS 138, 327, 1999).
Hipparcos binary statistics (Quist, Lindegren):
The statistics of binaries in the Hipparcos Catalogue
were investigated by means of a model describing the spatial and orbital
distributions of the objects as well as the observation process and Hipparcos
double-star reductions. In a sample of 13,000 bright main-sequence
stars (the typical member being an A star at 100 pc distance from the Sun),
the observed number of acceleration solutions leads to an estimated 0.25
± 0.05 companion per star in the semi-major axis interval a = 1
to 10 AU. This is roughly a factor two higher than the companion
frequency among nearby solar-type stars. The results suggest a higher
frequency of binaries among early main-sequence field stars, and a more
peaked distribution in log(a), than among late-type field stars.
Quist & Lindegren (A&A 361, 770, 2000).
Brown-dwarf detection from Gaia (Quist):
We simulated Gaia observations of brown-dwarf companions (masses 0.015
to 0.08 MSun) to main-sequence stars within 100 pc from the
Sun. Such companions can be detected mainly from the non-linear proper
motion that they induce in the parent star, but sometimes also by direct
imaging. Gaia will be able to detect most such companions with periods
from 0.03 to 200 years. For periods below 10 years, the complete orbits
of the stars around the mass centre can be determined, giving a good handle
of the companion masses without the sin(i) ambiguity in spectroscopic
Quist (A&A 370, 672, 2001).