Max Planck Institutes and Experts

Several hundred Brown Dwarfs have been identified in the solar neighborhood and seem to be as numerous as main sequence stars. But the models for their structure and evolution are not as reliable as the models for stars. Spatially resolved binary systems offer a unique opportunity to determine the masses without using models, but such cases are rare. A group at the MPIA has now succeeded in determining the parameters of the Brown Dwarfs Kelu-1A and B. Conclusion: Existing models yield masses which are too low. The spectra also suggest the presence of an invisible third Brown Dwarf in Kelu-1.

A group of astronomers headed by the Max Planck Institute for Astronomy has measured the velocity of the stars in the galactic halo and thereby derived the most accurate value to date for the total mass of the galaxy: The region within a radius of 200 000 light years contains 4×10¹¹ solar masses. An extrapolation to 800 000 light years leads to 10¹² solar masses. This result shows that the mass of the Milky Way has previously been significantly over-estimated. It also proves that our Milky Way has been extraordinarily efficient at forming stars.

A team of astronomers, including members of the Max Planck Institute for Astrophysics, have combined the observational power provided by the Hubble Space Telescope with the predictive power of the Millennium Run cosmological simulations to investigate an intriguing cosmic puzzle. If luminous quasars in the early Universe mark the regions that were the first to collapse and form massive clusters of galaxies as predicted by theory, then why is the observational evidence for such cosmic "cities-under-construction" currently so scarce?

The investigation of possibilities to influence electrons in solids and at surfaces on the femtosecond (10^-15 s) time scale is an important area of research. This is also relevant for the steering of magnetic switching processes by ultrashort laser pulses and for the control of the spin of excited electrons. At the MPI of Microstructure Physics, the control of the spin of optically excited photoelectrons is investigated by the absorption of multiple photons at metal surfaces.

Spin systems are mostly known for collective magnetic ordering behavior at low temperature. In this brief review two exotic possibilities for novel ground states driven by the interplay of magnetic frustration and quantum fluctuations are discussed together with prospects to experimentally observe these phases.