Stellar Magnetism at the Main Sequence

Stellar magnetism is an indispensable element for a full understanding of stellar structure and evolution and hence has immediate implications for many astrophysical problems. The evolution of our Sun and its influence on planetary atmospheres - of which our terrestrial atmosphere is most prominent - is only one example. But modelling of magnetic field effects frequently is extremely difficult and therefore limited in use. Theory and observations both tell us, for instance, that magnetic fields influence microscopic diffusion of chemical elements within the stellar envelopes, they may also redistribute angular momentum or give rise to enhanced hydrodynamic instabilities and thus modify mixing properties in stellar interior. However, more realistic modelling of such processes has become only recently possible thanks to a deeper understanding of magnetohydrodynamics and a dramatic increase in computing power. The component of a star which usually is accessible to direct observations in the visual part of the electromagnetic spectrum is the stellar envelope. It comprises the stellar atmosphere and the region below until - for unevolved stars - convection becomes important for radiative energy transfer. All the information we process within the current projects will be obtained from spectroscopic and photometric observations of stellar atmospheres. Our project aims at a major breakthrough in revealing the geometry of stellar magnetic fields and related 3-D atmospheric structures by applying modern observing and modelling techniques. We hope to shed more light on the origin and evolution of magnetic fields of early type stars and on the transition to activity phenomena of cooler stars which are comparable to our Sun. New information about the interior structure of stars is expected to come also from asteroseismological data, in particular from those obtained in space. The Canadian space telescope MOST, operated with Austrian participation, will provide such observations and probably will lead to suprises similar to what has been found, e.g., for Procyon (Nature 430, 51 (2004)), the first observed science target of MOST.

To date very little is known with certainty about the origin and structure of magnetic fields and their connection and interaction with surface abundance patches, pulsation, and stratification of stars. Magnetic field geometries and abundance distributions have been studied independently by various groups but provided frequently inconsistent results. During the course of our project, a wealth of magnetic stars of different chemical peculiarity types were observed and analyzed, called magnetic Ap stars. Some of which were newly discovered by the team, hence contributing to a better understanding of the interaction of magnetic fields with diffusing atoms and ions in their atmospheres. These stratification effects can be best studied in the atmospheres of magnetic stars, but they play also a significant role in apparently `non-magnetic` counterparts. With the improvements of magnetic field measuring techniques it appears now that (weak) disk-averaged magnetic fields are much more common than was assumed a decade ago. A detected weak surface magnetic field does not necessarily imply that it is also weak inside the star, because at present we only can measure magnetic field effects integrated over the visible stellar hemisphere. Obviously, the net magnetic field measurable for us becomes significantly reduced, if a (strong) field pattern at the surface is complex. We can report on a first successful modelling of a pulsating magnetic Ap star using space photometry from MOST and contemporaneous spectroscopy from ground, and we obtained new insights in the 3-dimensional atmospheric structure of these unique stellar laboratories. We could derive for the first time in a consistent way the surface magnetic field structure of a pulsating star and at the same time the surface distribution of chemical elements. Furthermore, we determined the atmospheric structure and the abundance of chemical elements as a function of depth. In order to investigate evolutionary effects we observed 10 open star clusters of different age. Such clusters are ensembles of several hundred stars born at about the same time and originating from the same cloud of interstellar matter. The main difference for cluster members, hence, is their stellar mass. The magnetic field of several chemically peculiar stars in these clusters was measured and a correlation between the magnetic field strength with age and mass of the stars investigated. We have found a strong correlation between peculiarities of metallic line (Am) stars and rotational velocity, and a good agreement of diffusion models for Am stars and the actually observed abundances. This is the largest and most complete investigation of this kind yet available.