Annual Report 2011 Max Planck Institute for Astronomy

dM (A v )
1
0.1
0.01
p (ln (A v / mag)) (normalized)
1
0.1
0.01
0.001
0.0001
2
California
exp (–0.49 A v )
A B C D F G H I J
exp (–0.12 Av )
10
A B C D F G H I J
filamentary IRDC called “The Snake”. The shadowing
is caused by dust particles in the cloud that absorb the
background radiation. From the amount of this absorption
it is possible to estimate the mass of intervening
dust, and hence, the mass distribution of the cloud.
Kainulainen & Tan (submitted) presented a scheme
for combining these two types of data, Nir extinction
mapping with background stars and Mir extinction mapping
with surface brightness data, into mass distribution
data with unprecedented high sensitivity and spatial
Av [mag]
10 100
2.5
Orion B
exp (–0.22 A v )
Orion A
exp (–0.18 A v )
15
Av [mag]
20 25
3 3.5
ln (A v / mag)
III.2 Formation of star-forming structures in the interstellar medium 53
4 4.5 5
Fig. III.2.5.: Top: Column density PDFs of ten IRDCs (colored
lines) and the nearby Ophiuchus star-forming cloud (black
dashed line). The shapes of the PDFs of the IRDCs are, on
average, compatible with a log-normal function (shown with
a dotted black line). Left: The cumulative forms of the PDFs
for the IRDCs (colored lines) and for three nearby molecular
clouds (black dotted lines). The IRDCs contain somewhat
more high-column density material than nearby clouds, which
might reflect their location at smaller Galacto-centric radii, and
hence, higher pressures.
resolution. The bottom panels of Figure III.2.4 show as
an example the mass distribution map derived for “The
Snake”. The data reaches the resolution of 2, which
at the distance of “The Snake” corresponds to about
0.04 Pc.
The high-dynamic-range data resulting from the new
technique makes it possible not only to examine the
column density PDFs in potential high-mass star-forming
sites, but also to examine the PDFs of molecular
clouds over a greatly wider dynamic range than the data
discussed in the context of low-mass clouds (see Fig.
II.2.2). Figure III.2.5 shows the column density PDFs
of 10 IRDCs and that of Ophiuchus, which is a typical
nearby star-forming cloud. The PDFs of IRDCs are
quite similar to that of Ophiuchus (they extend to higher
column densities because the dynamic range of their data
is wider). This, first of all, suggests that star-formation
may have already started in the IRDCs. Indeed, some
of the IRDCs included in Fig. III.2.5 show some typical
star-forming signs in the near-and mid-infrared, such
as bubbles and reflection nebulae. However, finding out
exactly how many young stars are currently forming in
Credit: J. Kainulainen