Annual Report 2011 Max Planck Institute for Astronomy

Credit: J. Kainulainen
Star-forming clouds deviate from this shape by having a
significantly higher relative amount of high-column density
fluctuations. The result is illustrated in Fig. III.2.2,
which shows the column density PDFs for a typical starforming
(Taurus) and a quiescent (Lupus V) cloud. In the
context of turbulent molecular clouds, the result suggests
that only the low-column density regions in the clouds
are dominantly shaped by turbulent motions, while higher-column
density regions are affected more crucially
by some other process. Interestingly, the PDFs of active
star-forming clouds have all rather similar PDFs (similar
to that of Taurus in Fig. III.2.2). This suggests that there
exists some fundamental state that the cloud structures
have to reach before significant star formation can occur.
The difference between the PDFs of quiescent and
star-forming clouds also means that star-forming clouds
contain higher relative amount of high-column density
material than quiescent clouds. This result is illustrated
in the lower left panel of Fig. III.2.2, which shows
the cumulative mass functions of a sample of molecular
clouds. The cumulative mass functions describe what
fraction of the clouds mass is at column densities higher
than the value at x-axis. The star-forming clouds that are
shown with blue curves in Fig. III.2.2 harbour a clearly
higher fraction of material at high column densities than
quiescent clouds that are shown with red curves.
An immediate question arising from the observed
PDFs is: What physics exactly cause the deviation from
the log-normal shape in the PDFs on star-forming clouds?
As described earlier, a log-normal column density PDF
Fig. III.2.3: Virial parameters of clumps that represent structures
in the “tail” part of the column density PDFs as a function of
the clumps’ mass. Virial parameter of α 1 reflects equipartition
between gravitational and kinetic energies of the structure.
The relation shows that the structures are not significantly supported
by their self-gravity, and it also suggests that external
pressure might play a significant role in supporting them.
log a
2
1.5
1
0.5
0
0.5
a = 1
1 1.5
2
log (M /M s )
13 CO: a M –0.640.13
2.5 3 3.5
III.2 Formation of star-forming structures in the interstellar medium 51
is expected for turbulent, isothermal media, with possible
deviations caused by various other processes. An
intuitive option would be to relate the deviation to selfgravitating
structures forming in the cloud. However,
the deviation from the log-normal form seems to occur
at column densities clearly lower (A V 2 – 5 mag) than
traditionally connected to self-gravitating dense cores.
This question was further looked into by Kainulainen et
al. (2011, A&A, 530, A64), and interestingly, it seems
that self-gravity does not provide significant support for
structures above the threshold of A V 2 − 5 mag. This
is illustrated in Fig. III.2.3 which shows the virial parameters
of structures above the threshold. Virial parameters
are clearly higher than unity, which indicates they
are not gravitationally bound. Instead of self-gravity, the
structures above that column density can be significantly
supported against dispersal by the external pressure that
is imposed to them by the massive, but more diffuse,
cloud material surrounding them. This inwards pressure
can roughly match the internal thermal pressure of the
structures, making them more long-lived, or even quasistable.
With this mechanism, structures formed in a turbulent,
atomic flow could survive for the time required
for molecule formation, cooling, and hence decreased
thermal support. This can be followed by further accretion
of material, structure formation, and eventually star
formation to take place.
Summarizing the above, star-forming clouds seem
to contain pressure-confined structures that also leave
their imprint to the column density PDFs. Non-starforming
clouds do not contain such pressure-confined
structures. This leads to a picture in which the formation
of pressure-confined structures introduces a pre-requisite
for star formation. This picture emphasizes the role
of the pressure from the large-scale, diffuse molecular
cloud for the stability of structures forming in the cloud.
Subsequent studies on the dynamic structure of molecular
clouds that cover the regime of both the dense cores
and diffuse material surrounding them are needed to affirm
the global nature of this picture. Such studies are
currently in progress at MPIA.
From low-mass to high-mass star-forming regions
The results discussed above are best suited to describe
structure formation in relatively low-mass clouds that
form low-mass stars, much like that of our own Sun. It
is an imminent question whether the same results hold
when the birth-places of high-mass stars or star-clusters
are considered.
In the case of massive stars, the need to reach the quiescent
stage of cloud evolution to study the natal cloud
structure is even more pressing than in the case of lowmass
clouds, because of the more violent nature of the
high-mass star formation. The best candidates for the future
high-mass star formation are the dense and massive