Akkretion und Rückkopplungsmechanismen in der Entstehung massereicher Sterne

01/01/2015 to 31/12/2017

Context: Massive stars dominate the radiation energy budget of star forming regions and their host galaxies. But due to their large distances and the fact that they are born embedded in dense, opaque cores, knowledge about their formation remains rather poor. However, upcoming ALMA observations might indicate, if their formation is qualitatively similar to their lower-mass siblings, by first confirmations of disks around O-type stars, and delivering detailed images from the launching region of protostellar outflows up to their interaction region with the natal cloud.
From a theoretical point of view, feedback effects greatly impact these disk – outflow systems around massive protostars. Feedback potentially controls the observed outflow broadening in time, might limit the star formation efficiency to the low values predicted by observations, and could be responsible for the upper mass limit of stars. The epochs of successive feedback can be chronologically classified in 1) magneto-centrifugal acceleration 2) radiation pressure, and 3) ionization.
Methods: We propose to study the impact of these feedback effects and their interplay onto the forming accretion disk – outflow system in multi-physics numerical models of the collapse of massive pre-stellar cores from the sub-AU regime up to the most distinctive features of high-mass star formation, namely protostellar outflows and HII regions. In addition, chemical evolution and purposeful synthetic observations will be used to check the reliability of molecular tracers and the origin of organics in shocked outflow regions.
Aims: We will compute the star formation efficiency of the pre-stellar core collapse, taking into account the different feedback effects as well as their interplay. We will test various hypotheses, that are proposed to explain the upper mass limit of stars, namely the combined feedback of magnetic and radiation pressure onto the disk-to-star accretion flow, photo-evaporation of disks, and the decay of accretion due to fragmentation of massive disks. Additionally, observations indicate a clear trend of increasing multiplicity with increasing mass of the primary star. We will test, if disk fragmentation behind the dust sublimation front represents the origin of spectroscopic massive binaries.
Résumé: The proposed research group will investigate various feedback effects onto the disk – outflow system of a forming massive protostar. Special focus is given on, first, ordering the feedback effects by their strength and epochs to derive their corresponding impact on the star formation efficiency, and secondly, testing a number of hypotheses for explaining the upper mass limit of stars.