Annual Report 2011 Max Planck Institute for Astronomy

48 III. Selected Research Areas
III.2 Formation of star-forming structures in the interstellar medium
New stars are born in the cold and dense molecular
clouds that are present “everywhere” in the Universe.
How these molecular clouds are formed and how they
evolve are key questions in the current efforts to establish
global laws that regulate the star formation process.
The fundamental physical processes that are responsible
for the evolution of molecular clouds can be studied
through accurate measurements of how the material in
them is distributed. Here we review our research on
this topic.
Molecular clouds of the interstellar medium (ISM) provide
the environments where new stars in the Universe
can be born. There is an intricate, two-way connection
between the physical structure of molecular clouds and
star formation inside them. On the one hand, star formation
is very concretely driven by the molecular cloud
structure: how much and how rapidly a cloud can form
stars depends crucially on how exactly the material in it
Fig. III.2.1: Dust extinction map of the Ophiuchus star-forming
region. The dust extinction can be used as a tracer of the gas
column density in molecular clouds, and hence, as a tracer of
their mass distributions. In Kainulainen et al. (2009, A&A,
508, L35) and Kainulainen et al. (2011, A&A, 530, A64), we
derived extinction maps for 23 nearby molecular clouds and
studied with the maps the probability distributions of column
densities in them. Based on our findings in these works, we
proposed a new evolutionary scheme for molecular clouds
in which the pressure from the diffuse medium surrounding
denser clumps in the clouds plays a significant role in the cloud
evolution.
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is organized. However on the other hand, the star-formation
process itself is a great sculptor of molecular clouds.
Once started in the cloud, active star formation will efficiently
restructure the cloud material with jets and
outflows from young stars, radiation pressure from massive
stars, and powerful shock waves from dying stars.
These processes rapidly erase the structural characteristics
of the clouds that were prevalent before the star formation
started in them.
How exactly the molecular clouds in the ISM come to
be and what are the physics driving their formation and
evolution are fundamental open questions in the field
of ISM- and star formation-research today. One way to
study these processes is borne out by the fact that different
physical mechanisms cause the material in the clouds
to be organized in different ways. Therefore, accurate
measurements of the clouds mass distribution lead to information
on what mechanisms are responsible for shaping
the clouds.
What regulates the structure-and star formation in molecular
clouds?
We know today that the molecular clouds are formed and
shaped primarily by the interplay of three forces: the turbulent
energy in the ISM, gravitational forces, and the
support of magnetic fields within them. Schematically,
the pathway of gas in galaxies towards star formation
begins from large-scale instabilities, e.g. from atomic
gas flows colliding at supersonic velocities, resulting
in shocks, and further, to strong density fluctuations in
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Credit: J. Kainulainen