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Variable red giant stars

Saskia Hekker
Leiden University, The Netherlands

A spectroscopic survey of red giant stars is ongoing at Lick Observatory, California, USA. In 1999 this survey started with 180 K giant stars, all brighter than V = 6 mag. In 2003 the sample is extended with about 200 G and early K giants. Observations are taken on a monthly bases and are used to: (1) detect (periodic) radial velocity variations and (2) determine stellar parameters.

A large fraction of the initial 180 stars (~25%) show periodic radial velocity variations with long timescales. Due to the sinusoidal nature of these variations these can in principle be induced by either companions or pulsations. I will discuss a possible way to distinguish between these different phenomena and present possible implications.

Furthermore, stellar parameters are determined for all ~380 stars in the sample. We investigated the metallicity of red giant stars with known sub-stellar companions and found that companion hosting stars have on average a higher metallicity than the majority of the stars in our sample. This would be conform the metallicity enhancement known for companion hosting dwarfs, but in contradiction with other determinations for giants.

(Sub)giant stars also show variations on a much shorter timescale: solar-like oscillations. Only for a few stars these oscillations are observed at the moment, but with CoRoT, Kepler and Brite, many more oscillating giants will be discovered. This is very important to investigate the interior of red giants in more detail. Here, I will discuss some results from ground-based campaigns for solar-like oscillations in the red giant Epsilon Ophiuchi and in Procyon, as well as some preliminary results from CoRot.

2008 May 15, 13:30

Centro de Astrofísica da Universidade do Porto (Auditorium)
Rua das Estrelas, 4150-762 Porto

Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia