Stellar surface structures as a practical limitation to
ultra-high-precision optical astrometry
In the near future, high-precision astrometric observations may
become important for discovering exoplanets around
nearby stars, complementing other techniques such as Doppler
and transit observations. Brightness irregularities on the
surfaces of these stars (spots, plages, granulation, etc) will
however introduce noise in the observed positions,
possibly hiding the astrometric signatures of small
exoplanets. We use observed fluctuations of the magnitudes and
radial velocities for stars of different types to estimate the
likely size of their "astrometric jitter". It is
concluded that small (Earth-like) planets may indeed
be detected by the astrometric method, but only around stars
that are unusually stable, similar to our Sun.
Eriksson & Lindegren (A&A 476, 1389, 2007).
Bayesian estimation of stellar ages and star formation histories
The ages of stars can sometimes be determined from their positions in
the observational HR diagram, using theoretical isochrones. We have
developed a technique to derive probability density functions for the
ages of stars, based on their observed characteristics.
Critically, the method depends on the absolute magnitudes of the stars,
which are obtained by means of their trigonometric parallaxes.
The method is
essentially an application of Bayes' theorem to the classical isochrone
fitting problem. In an extension of the method, the age distribution
of a sample of stars can be derived, which in turn depends on the rate
of star formation as function of time (the star formation history).
Jørgensen & Lindegren (A&A 436, 127, 2005),
Jørgensen & Lindegren (ESA SP-576, p.171, 2005),
Nordström et al. (A&A 418, 989, 2004).
The local mass density (Holmberg, Flynn):
The Hipparcos data permit a detailed mapping of the spatial distribution
and velocities for well-defined samples of stars in the solar neighbourhood,
out to distances of a few hundred parsec for the more luminous stars.
These data can be used to trace the gravitational field, and hence the
dynamical mass density in our vicinity, and to identify and study kinematic
groups of stars with a common origin. Using a complete volume-limited
sample of A and F stars, an estimate of the local dynamical mass density
of 0.102 ± 0.010 MSun pc-3 was derived, to
be compared with 0.095 MSun pc-3 of visible disk
matter. Thus we find no significant amount of dark matter in the
Holmberg & Flynn (MNRAS 313, 209, 2000).
Old moving groups (Feltzing, Holmberg):
Stars scattered over the whole sky, but identified as a group by their
common space motion, provide an important evolutionary link between clusters
and field stars. The large number of accurate parallaxes provided
by Hipparcos has made it possible to reliably identify moving groups and
to study their ages, metallicities, and dynamical evolution. The
reality of the old (2 Gyr) metal rich ([Fe/H] = 0.2) moving group HR1614
was proved by combining Hipparcos data with metallicities derived from
Strömgren photometry. Supported by dynamical simulations, an
extended sample of probable member stars was derived.
Feltzing & Holmberg (A&A, 357, 153, 2000).
Cluster kinematics (Madsen):
The kinematics of classical moving clusters and associations were also
studied as part of the programme to determine astrometric radial velocities.
In particular, the internal structure and velocity dispersion of the Hyades
cluster has been studied in detail, including fine structure of the HR
diagram based on kinematically improved stellar distances.
Madsen (A&A 401, 565, 2003),
Madsen, Dravins & Lindegren (A&A 381, 446, 2002),
Lindegren, Madsen & Dravins (A&A 356, 1119, 2000).