Emmy Noether Application

drastically the star formation history. Present day star formation takes place in molecular
and giant molecular clouds (GMC) which consist mainly of molecular hydrogen enriched with
heavy element molecules and dust.
Our general picture of present day star formation is based on the collapse of overdense,
gravitationally unstable cores within GMCs. In the current paradigm of modern astrophysics
these protostellar cores are built up in shock regions of compressive, supersonic turbulence
which pervades the clouds (Klessen et al., 2000; Elmegreen & Scalo, 2004; Mac Low &
Klessen, 2004; Ballesteros-Paredes et al., 2007). Initially the contracting gas cools efficiently
via molecular excitations and gas-dust interactions and the collapse proceed essentially
isothermally. At densities of about n(H2) ∼ 10 10 cm −3 the gas becomes optically thick
and the core starts to heat up while the contraction slows down. If the temperature in this
first core approaches about 2000K (at about n(H2) ∼ 10 16 cm −3 ) molecular hydrogen starts
to dissociate. The dissociation of H2 efficiently cools the contracting gas and collapse follows
again an isothermal track. At the time when all of the hydrogen molecules are dissociated
the collapse comes to a halt. This hydrostatic object is called the second core (e.g., Larson,
1969, 2003). The core then gains mass by accreting gas from the surrounding envelope. At
this class 0 phase the envelope is still much more massive than the central object. During
this accretion phase, gas with large specific angular momentum settles closer to the central
core and builds up a protostellar disc. Subsequently the core gains mass and eventually
becomes more massive than the surrounding disc. This stage is usually accompanied by
powerful outflows which make up the class I phase of the protostellar evolution and are observed
as Herbig-Haro (HH) objects (see e.g., Reipurth & Bally, 2001). In the next stage,
i.e. the class II phase, the envelope is drained onto the central core and planets might form
in the disc while the hydrostatic core approaches the main-sequence, i.e. it starts to fuse
hydrogen. Subsequently, the remnant gas in the disc will be depleted and the remaining
configuration (e.g. star-planets) is a long-living stellar system.
The above description of star formation is fairly general and does not include a number of
complexities. In particular, massive stars (stars whose mass exceed M > 8M⊙ and go off
as type II, i.e. core collapse, supernovae at the end of their lifetime) accrete most of their
mass while the released radiation exerts a substantial pressure on the surrounding gas and
dust (e.g., Kahn, 1974; Wolfire & Cassinelli, 1987; Yorke, 2002; Yorke & Sonnhalter, 2002). In
principle, the radiation could be strong enough to stall accretion onto the central star. To keep
the standard paradigm of star formation where stars assemble through accretion, several
suggestions have been made to overcome this problem. For instance, Wolfire & Cassinelli
(1987) concluded that dust opacities must be modified to assemble massive stars. Norberg &
Maeder (2000) and McKee & Tan (2003) suggested that the accretion rates should be higher
than the standard values known from low-mass star formation which could then squeeze the
radiation pressure. Jijina & Adams (1996) and Yorke & Sonnhalter (2002) showed that nonspherical
accretion through the disk can help to assemble high-mass stars. Krumholz et al.
(2005b) showed that radiation will escape through cavities carved by outflows or through
radiation driven instabilities (Krumholz et al., 2005a), again relaxing the upper mass limits
set by radiation.
Changes to the standard paradigm where also suggested to solve the problem of radiation
limited accretion. Such scenarios assume that massive protostars assemble by coalescence
of small and intermediate size cores (Bonnell et al., 1998; Bally & Zinnecker, 2005). Massive
stars are mainly found in dense star clusters with star densities of about 10 8 pc −3 where
encounters are feasible (e.g., Beuther et al., 2007).
This latter property, that massive stars are predominately observed in clusters, adds another
level of complexity to the understanding of massive star formation (e.g., Tan, 2005). In such
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