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Stellar & Planetary Systems

In a synthesis of our stellar and planetary research we plan to extend the studies of extrasolar planet systems beyond the mere measurement of orbital and spectroscopic parameters. The main topics are: planet-induced stellar activity, spin-orbit alignment and angular momentum history, and the various correlations between stellar activity and planetary system parameters.

Planet-induced stellar activity

Since many of the newly discovered extrasolar planets are located very close to their host stars, they will tidally interact with and could induce activity on the host stars. Similar to the Earth-Moon system, a nearby (giant) planet would induce two tidal bulges on a star, which would very likely be the seats of the hypothesized enhanced activity. If planet-induced activity is confined to a narrow range of stellar longitudes, all activity signatures should vary on (half) the planetary orbital time scale rather than the stellar rotational time scale. If, conversely, the induced activity is of magnetic character, it will be modulated with the planetary period and its strength should vary according to the product of the planetary and stellar magnetic field. In either case, the interaction varies with inverse distance so the greatest effects should be observed for the nearest, i.e., shortest period, planets. The hitherto best case of presumably planet-induced stellar activity is the star HD 179949, where Shkolnik et al. (2003) found a variation in Ca II HK emission level at the planet orbital time scale of 3.093 days, such that the Ca II enhancement coincides with the sub-planetary point consistent with a magnetic rather than tidal interaction scenario. So far, with the exception of the limited Shkolnik et al. sample, no systematic study of Ca II temporal variation on stars has been done. The long-term steller monitoring program planned for our own HRT and OLT telescopes will be adjusted to the needs of this RTG project. We will record time series of optical spectra including the Ca II HK signatures of all accessible planet host stars and reference stars. Correlations with all other known planet system parameters will be investigated and, if found, interpreted. This program primarily makes use of our own observing facilities. Data available from public archives will be included.

Stellar activity and planetary system properties

Systematic chromospheric and coronal studies of exoplanet bearing host stars have not been carried out yet. RV planet searches prefer older stars, whose line profiles do not show activity-related variations and deformations. As a consequence, the X-ray emission of known planet host stars tends to be relatively weak. Nevertheless, out of over hundred stars known to harbor extrasolar planets, about 35 are known to emit X-rays. Studies of volumelimited samples of field stars yield detection rates approaching 100 percent, yet this comparison is misleading, since the population of planet hosting stars is not volume-limited. Whether or not the X-ray luminosities of the planet bearing stars are consistent with those of field stars and whether or not there is a dependence of the X-ray activity of these stars on planetary orbital parameters is an open question. Since, in the case of the closest-in exoplanets, the magnetosphere is estimated to lie within the atmosphere, exposing it directly to the stellar wind, activity might be expected to have a direct influence on the evolution of these hot Jupiters. The RTG research plan on stellar activity foresees systematic and deep X-ray observations of planet host stars and reference field stars. The data will be analyzed using the diagnostic and analytical tools developed by the stellar group in Hamburg over many years. The modeling and interpretation will be performed in collaboration with the theory team.

Spin-orbit alignment in extrasolar planet systems

The technique to measure the position angle of stellar rotation axes has only recently been conceived. One first set of observing data has been obtained at the VLT in excellent conditions. The research plan foresees the evaluation of this data-set and an assessment of the basic technique. The advantages and disadvantages of simultaneous I2 cell calibrations will be determined. The optimal measurement sequence and the need for simultaneous measurements must be assessed. After the generation of a robust data reduction package we will search the VLT and other archives for suitable stellar data, and initiate the generation of a general catalog of stellar spin position angles. Equally interesting will be the relative orientation of stellar spins and circumstellar disks. Main sequence stars will require the fine plate scale from spectrographs at large telescopes, while observations of the bright giant stars should be possible with our own smaller facilities. The catalog of stellar PAs will be of use to many programs. We can immediately compare the known orientation of the GL876 planet orbit, (probably Eri as well) to that of the star. Undoubtedly, astrometry will provide the orbital planes of known RV planets in the near future. The spin orientation data will be available to other investigations, for instance of binary systems. We will concentrate on exoplanet systems and, as a first application, provide observational constraints for the star-encounter-triggered planet formation theory. We plan to collaborate with the group of P. Kroupa (Bonn), who provides quantitative estimates for this mechanism, which is currently the only one known to result in discrepant stellar and planetary orientation.

Irradiated atmospheres

In close systems (e.g., a G-type star and an EGP) the effect of irradiation from one star on its companion has either been neglected entirely or only treated in a very limited way. In the near future we will likely be able to observe directly the color indices or spectra for many cool very low mass stars (VLMS), L dwarfs and EGPs that are less than 1 arcsec from a much hotter primary. In order to interpret the data, place these objects on an Hertzsprung-Russell diagram, and deduce the evolutionary status of the system, we have to calculate realistic flux distributions. In the RTG, it is one of our goals to expand the current understanding of the effects of irradiation on L dwarfs and EGPs by close-by hotter primaries and to calculate detailed model atmosphere and synthetic spectra of the irradiated components. We currently have the ability to treat the effects of external radiation accurately in 1-D models with our state-of-the-art atmosphere code PHOENIX (Barman et al. 2004, 2002, 2001), and it is our plan to extend this treatment to compute more realistic 3-D models and spectra of irradiated EGPs.