We summarize the progress to date of our Legacy Science Program entitled The Formation and Evolution of Planetary Systems (FEPS) based on observations obtained with the Spitzer Space Telescope during its first year of operation. In addition to results obtained from our ground–based preparatory program and our early validation program, we describe new results from a survey for near–infrared exce...

We study the evolution and observability of young, compact star clusters near the Galactic center, such as the Arches and Quintuplet systems. The clusters are modeled by integrating the equations of motion of all stars while accounting for the internal evolution of the stars and binaries and the effect of the tidal field of the Galaxy. We find that clusters within 150pc of the Galactic center d...

Stellar clusters are thought to be the simplest stellar systems and the closest observational counterparts to theoretical models for single stellar populations. Progress in our understanding of the atmospheres and evolution of massive stars has led to generally reliable synthesis models. The future release of new evolution models with rotation, however, will require non-trivial updates to previ...

We derive age-metallicity relations (AMRs) and orbital parameters for the 1658 solar neighbourhood stars to which accurate distances are measured by the HIPPARCOS satellite. The sample stars comprise 1382 thin disc stars, 229 thick disc stars, and 47 halo stars according to their orbital parameters. We find a considerable scatter for thin disc AMR along the one-zone Galactic chemical evolution ...

We discuss recent models on the evolution of massive stars at very low metallicity including the effects of rotation, magnetic fields and binarity. Very metal poor stars lose very little mass and angular momentum during the main sequence evolution, and rotation plays a dominant role in their evolution. In rapidly rotating massive stars, the rotationally induced mixing time scale can be even sho...

The similarity between the mass and spatial distributions of prestellar gas cores in star-forming clouds and young stars in clusters provides strong circumstantial evidence that these gas cores are the direct progenitors of individual stars. Here I describe a physical model for the evolution of massive cores into stars, starting with the intial phases of collapse and fragmentation, through disk...

We summarize the present status of the predictions of massive star models for the evolution of their surface properties. After discussing luminosity, temperature and chemical composition, we focus on the question whether massive stars may arrive at critical rotation during their evolution, either on the main sequence or in later stages. We find both cases to be possible and briefly discuss obse...

Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium coreburning objects with very thin hydrogen-rich envelopes. In recent years it has become increasingly clear from observational surveys that a large fraction of these objects are members of binary systems. To better understand their formation, we here present the results of a detailed investigation of the three main b...

We present two-dimensional simulations for the cooling of neutron stars with strong magnetic fields (B ≥ 1013 G). We study how the cooling curves are influenced by magnetic field decay. We show that the Joule heating effects are very large and in some cases control the thermal evolution. We characterize the temperature anisotropy induced by the magnetic field and predict the surface temperature...

Models of galactic chemical evolution (CEMs) show that the shape of the stellar initial mass function (IMF) and other assumptions regarding star formation affect the resultant abundance gradients in models of late-type galaxies. Furthermore, intermediate mass (IM) stars undeniably play an important role in the buildup of nitrogen abundances in galaxies. Here I specifically discuss the nitrogen ...