Abstract

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.