University of Heidelberg

Star Formation

The study of Star Formation addresses the question of how stars are build up in galaxies and the early universe. Present day star formation - which is the main focus of our research group - takes place in molecular clouds and giant molecular clouds (GMSs) like the Orion nebula. Within these cloulds overdense cores might collapse under their own weight until stars or brown dwarfs are formed. Observed stars have a large range in masses (from a few hundred solar masses down to the hydrogen burning limit of 0.07 solar masses). One of the outstanding problems in star formation is the origin of the Initial Mass Function (IMF), which links the number of stars of a certain mass range to their mass itself. The IMF can be parametrised by a broken power law where the high mass range (above one solar mass) follows closely a Salpeter type power law (Salpeter 55). Large scale turbulent motions (which is an active research topic by its own), for instance, could be the driving force for the assembly of the spectrum of self-gravitating cores which mass function (CMF) resembles the IMF (e.g. Mac Low and Klessen, 2004).


Many stars are accompanied by one or more companion stars. These multiple star systems could be the result of fragmentation during the collapse of turbulent cloud cores (Dobbs, Bonnell and Clark, 2005) and/or the fragmentation of the proto-stellar disk (Banerjee and Pudritz 2004). For fragmentation to occur it is crucial that the collapsing cloud core is able to cool efficiently. Therefore the chemical composition and reactions influence strongly the fate of collapsing objects. We are studying the evolution of condensed cloud cores with numerical simulations accounting for radiative processes and using different equations of state (EOS).

Massive Stars

Unlike low mass, stars massive stars (> 8 solar masses ) face a major obstacle during their assembly: Massive stars are still accreting a large fraction of gas of their final mass while they are already into their hydrogen buring phase. The subsequently released radiation might be strong enough to halt the accretion process and limit the final mass of the young star. Spherical (Wofire and Cassinelli 1987) and two dimensional models (Yorke and Sonnhalter 2002) showed that the mass limit should be around 40 solar masses. On the other hand observers find stars much more massive than this theoretical predicted limit (e.g. Eta Carinae > 100 solar masses). We are planing to address this problem with three dimensional numerical simulations which will include the necessary radiative transfer to calculate the radiation feedback.

Jets and Outflows

Observations indicate that young stellar objects (YSOs) are accompanied by outflows and highly collimated fast moving jets. These objects are referred to as Herbig-Haro objects and are ubiquitously found in star forming regions. It is still under debate how these outflows and jets are launched. One possibility for the jet launching mechanism could be the interplay of magnetic fields and the rotating disk and/or proto-star. Numerical simulations including magnetic fields during the collapse of cloud cores confirm that outflows are generated during the star formation stage (Banerjee and Pudritz 2006).
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