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Less than 1 % of all new-born stars have a mass of more than 8 solar
masses by the time nuclear burning in their centres is initiated and
they start to shine. Both, the life and death of a massive star are
intriguingly different and much more exciting than those of a low mass
star, such as our Sun. During its lifetime, a massive star heavily
affects its parental gas cloud -- mostly consisting of cold (10
Kelvin), molecular gas -- by strong UV radiation and a fast stellar
wind. Due to the emitted UV radiation, the surrounding molecular cloud
is quickly ionised and heated to about 10,000 Kelvin, and a so-called
HII region is formed. The hot, ionised bubble expands into the cold,
turbulent environment, thereby sweeping up more and more gas and
possibly triggering new star formation. The massive star exhausts its
fuel fairly rapidly, burning for only a few million years. In death,
it explodes as a Supernova Type II and releases an enormous amount of
energy, which further accelerates and heats the surrounding gas to up
to 100 million Kelvin.
Even though massive stars are rare, they are most important for galaxy
formation and evolution. They represent the main source of stellar
feedback energy and are able to destroy molecular clouds from within,
thus regulating the star formation efficiency in the galaxy. Moreover,
their feedback, and in particular their death in form of a Supernova
explosion, may even drive large-scale galactic winds and outflows. Gas
which is driven out of the galaxy by this process might ultimately be
unavailable for new star formation.
Scientists at MPA study the dispersal of molecular clouds by UV
radiation (see Fig. 1) and Supernova explosions of massive stars (see
Fig. 2) in highly resolved, three-dimensional computer
simulations. They show that relatively small molecular clouds with a
mass of 10,000 solar masses may easily be disrupted by ionising
radiation alone long before the star explodes as a Supernova. However,
the disruption of more massive molecular clouds requires more drastic
measures. While the initial ionising feedback due to radiation is
still an essential ingredient when modelling the disruption of
high-mass clouds, only Supernovae are actually able to disrupt clouds
with 100,000 to 1 million solar masses. Modelling the stellar feedback
involves complex, non-linear cooling processes in the interstellar
medium. This means that the Supernova explosion is much more efficient
when taking place in a pre-ionised, low-density HII region. By
performing simulations of Supernova explosions in clouds with and
without previous ionisation, the scientists are able to quantify how
much the efficiency improves. In fact, for simulations with previous
ionisation and cooling the results are remarkably close to the ideal
and well-studied Sedov explosion case, in which the cloud is not at
all allowed to cool radiatively. This result is very important for
correctly estimating the impact of feedback in the interstellar
medium.
Understanding how this feedback propagates over more than six orders
of magnitude in spatial scale, from milli-parsec scales, at the sites
of massive star formation, to galactic kilo-parsec scales, is a
computationally challenging quest. The team is now ready to set the
next milestone in performing deeply resolved SImulations of the whole
LifeCycle of a molecular Cloud (SILCC-project). They have been awarded
more than 40 million CPU hours on SuperMUC, the new 3 petaflop
supercomputer, which has just been launched at the
Leibniz-Rechenzentrum Garching. Currently,
SuperMUC
is Europe's
fastest super-computer and number four in the known universe. The
SILCC project will shed light into the intricate impact of massive
stars, from molecular cloud assembly, over star formation and
feedback, to gas ejection from the galactic disk (see Fig. 3). These
complex three-dimensional simulations will involve a multitude of
physical effects that, to date, have not yet been included in a single
computation. Investigating all of these processes at the same time
will give detailed insight about how feedback from massive stars can
regulate the star formation efficiency in the galaxies.
Stefanie Walch, Thorsten Naab
Original publications:
Walch, S.K.; Whitworth, A.P.; Bisbas, T.; Wünsch, R., Hubber, D.,
"Dispersal of molecular clouds by ionising radiation",
accepted for publication in MNRAS (2012);
arXiv1206.6492
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