Title:
The Formation of the Most Massive Stars

Abstract:
Importance of massive stars:
Stars with more than 100 times the mass of our sun are observed in the present-day universe. These massive stars are of great importance for a wide range of astrophysical problems. In spite of the fact that they are both rare and short-living, they represent the major source of radiation energy in their stellar clusters. Therefore, they act as valuable tracers of star formation in distant galaxies. Additionally, the radiation emitted during a massive star's lifetime influences the surroundings through various interactions such as heating and ionization of gas, evaporation of dust, and radiative forces leading to powerful stellar winds and outflows. Finally, when a massive star dies, it enriches its neighborhood with heavy elements. In this sense, massive stars are the main drivers of the morphological, dynamical, and chemical evolution of their complex environments.

Observational difficulties of massive star formation:
However, knowledge about their formation is rather poor compared to the case of low-mass star formation. Observationally, this is mostly due to their larger average distances to us and the fact that they are deeply embedded in dense, opaque cores of gas and dust especially during their early evolutionary phases. Also, their short lifetime, rareness, and complex environment pose difficulties for detailed observations. Nevertheless, current observational results support the assumption that the basic concepts of star formation, including the collapse of an unstable gas and dust core, the formation of both bipolar outflows and jets, as well as accretion disks, are possibly valid throughout the whole range of stellar masses.

Theoretical problems of massive star formation:
From the theoretical point of view, assuming that the formation of high-mass stars is basically a scaled-up version of low-mass star formation, implies its own challenges: Their rapid evolution inside the collapsing core leads to the interaction of the in-falling accreting flow of gas and dust with the emitted radiation from the newborn star. The radiative force of the emitted photons onto the stellar environment thereby quickly exceeds the gravitational attraction of the nearby dust grains by the star. How is such a star able to sustain accretion despite its superior radiative force to allow the formation of the most massive stars observed?

Our methods:
In this talk I present the results of several series of self-gravitating radiation-hydrodynamics simulations of core collapse towards high-mass star formation. To be able to study in especially the radiation pressure problem in high detail, we have developed a specially adapted hybrid radiation transport scheme for problems of dominating radiative point sources.

Our findings:
The classical accretion disk scenario of star formation self-consistently allows to circumvent the radiation pressure problem: Due to angular momentum conservation, the forming high-mass protostar is surrounded by an high-mass accretion disk. The irradiated and heated inner rim of such an accretion disk re-emits the radiative flux preferentially perpendicular to the disk's midplane (due to the fact that the optical depth in this direction is much smaller than in any other). This anisotropy of the thermal radiation field allows the accretion to continue in the shadow behind the inner disk rim, while the strong radiative force in the direction perpendicular to the disk yields the launch of wide-angle radiation-pressure-dominated outflows. By this process, roughly half of the initial core mass is blown away from the evolving system. The angular momentum transport - required for ongoing accretion through the circumstellar disk - is provided self-consistently by the strong self-g!
ravity of the disk, which yields the formation of spiral arms in the disk.

Summary:
I will demonstrate a straight-forward and self-consistent mechanism, which allows the formation of the most massive stars known in the present-day universe despite of their strong radiative feedback. The basic theory of star formation herein is just a scaled-up version of low-mass star formation.