Flare stars in the solar vicinity

Young stars close to the sun (within 100 pc) yield an interesting
sample in many respects: They are relatively bright and because of
their close distance we can resolve the surroundings of these stars
by using adaptive optics on 8 to 10 m class telescopes (e.g. VLT or
Keck). In the K-band the achievable angular resolution is about 50
mas which corresponds to 5 AU at a star closer than 100 pc; 5 AU is
about the distance between the Sun and Jupiter. This possibility
can be used to study surrounding material such as disks made up of
gas and dust as well as stellar and substellar companions. A sample
consisting of young stars in the solar vicinity and in an
evolutionary stage between the classical T Tauri phase with a disk
and the zero-age main sequence can be provided by the catalog of
flare stars and related objects compiled by Gershberg et al. (1999)
because young stars are often variable and exhibit large eruptions
(flares). In a first step we need to verify that these stars are
indeed young and did not come to lie above the main sequence in a
Hertzsprung-Russell diagram because they are old or unresolved
binaries or multiples. Therefore, we have taken spectra of 223
stars lying above the main sequence (of the 463 stars of the
sample). The distances to these stars were measured (in most cases
by Hipparcos) and they are located at a few to 100 pc. The goal was
to detect lithium absorption at 6708 A which all young stars have
in common. In addition to the detection of lithium, we want to
identify other age indicators such as filling in or emission of the
Halpha -, the magnesium Ib- and the calcium lines. The G- and
K-type stars of the northern hemisphere were also observed with
high resolution, and high signal-to-noise ratio spectroscopy to
study these objects with methods of spectral synthesis analysis to
determine the surface gravity, the chemical composition, and the
temperature. The age determination of these 223 stars lead to a
value between 10 Myr and the zero-age main sequence, they are
indeed nearby and 17 stars are clearly pre-main sequence. In the
course of this work, we discovered the closest pre-main sequence
star (HIP 108405 A, 10+-10 Myrs at a distance of 16.1pc). The star
is younger than GJ 182 (27 pc, 20+-10 Myrs) which held the record
up to now. A planet with a mass of 5 MJup in orbit of a (for this
sample) typical M-star, would have an apparent magnitude in the
K-band of 14.5 to 17.5 mag at a distance of 16 pc. This would lead
to a magnitude difference DeltaK of 8 to 11 mag between the star
and the companion, which could be detected with 8 to 10 m class
telescopes at a separation of 1" or a projected separation of 16
AU. All newly discovered young flare stars were imaged using
NAOS/CONICA to search for distant companions. Depending on the
space motion of the stars, they have to be reobserved in one or
more years to distinguish comoving companions from stagnant
background stars. In this work we have measured radial velocity
variations of young stars for the first time using the échelle
spectrograph of the Thüringer Landessternwarte. In these
measurements one can see the problems of such an investigation,
such as variability caused by activity and stellar spots. But one
can also see that it is in principle possible to detect planets
around active young stars. To verify the results and to measure
longer rotation periods, we have to observe these stars for another
season.