Max Planck Institute for Astronomy - Annual Report 2005

50
III. Selected Research Areas
III.1 Star Formation in the Magellanic Clouds
The Magellanic Clouds, two of the closest galaxies to
our own, provide a unique opportunity to study the recent
star formation process on full-galaxy scales. Although
they are much smaller than the Milky Way Galaxy, both
the Small and the Large Magellanic Cloud exhibit extraordinary
star formation activity, which is demonstrated
by super-giant expanding shells of interstellar medium,
active star-forming regions associated with ionized
hydrogen and a large variety of young systems of stars,
such as stellar clusters and associations. Young stellar
systems are excellent targets for probing the star formation
processes on various scales in a galaxy.
Young stellar associations in the Magellanic Clouds
form a complete sample of targets with a variety of
characteristics suitable for investigating questions concerning
the clustered formation of stars. The information
that has been provided from ground-based observations
so far concerns the massive young stars in associations
of the Magellanic Clouds and the distribution of their
masses, which is called the Initial Mass Function. There
is a lack of information, though, on the low-mass stellar
content of these systems, which is currently being filled
in with results from observations with the Hubble Space
Telescope. The combination of studies for both the high-
and low-mass stellar content of young associations in the
Magellanic Clouds provides a complete picture of recent
clustered star formation in an environment quite different
from our Milky Way.
Introduction
The Large and Small Magellanic Cloud (LMC, SMC)
are two dwarf galaxies which are satellites of our own
Galaxy, orbiting it about every 1.5 billion years. They
are conspicuous objects in the southern hemisphere, and
they can be seen by the naked eye like separated pieces
of the Milky Way. They are named after the Portuguese
maritime explorer, Ferdinand Magellan, who made them
known to Europe during his first circumnavigation of
the Earth in 1519. The Large Magellanic Cloud (LMC),
at its distance of 179 000 light years, is the largest and
nearest external, still undisrupted galaxy. The Small
Magellanic Cloud (SMC) orbits our Milky Way galaxy
at about 210 000 light years distance, which makes it
the second most nearby external galaxy known. Both
Magellanic Clouds (MCs) are irregular dwarf galaxies
and they are close enough to be resolved into individual
stars. Deep photographs reveal them to be highly complex
systems with large numbers of interesting objects,
including diffuse bright nebulae and dust clouds, globular
and open clusters, supernova remnants and planetary
nebulae apparently scattered at random across the face of
the galaxies (Fig. III.1.1). Consequently, they are ideal
laboratories for studying different stellar populations.
The very low interstellar absorption from our Galaxy
toward their direction certainly offers a great advantage
for these studies, because the light from the MCs is not
reduced by the foreground dusty disk of the Milky Way.
In addition, their depth seems to be very small and so,
all observed stars are located more or less at the same
distance with very small foreground contamination by
Milky Way stars. Both the MCs, having lower abundances
in elements heavier than hydrogen (metals) than
the Milky Way, also have low intrinsic absorption from
the interstellar medium (ISM).
The MCs provide a unique laboratory for the study
of star formation in an environment different from that
of the Milky Way. Various investigations have provided
valuable information on them due to their proximity to
the Earth. For example, the metal abundance (content of
elements heavier than hydrogen) in the LMC is almost 4
times lower than that of the Galaxy, while its star formation
rate (newly-formed stellar mass as a function of time)
is high, almost on par with the Galaxy's (Westerlund
1997, The Magellanic Clouds, Cambridge Univ. Press).
This results in a large number of early-type stars establishing
intense radiation fields, causing the star formation
in the LMC to be different than that in the Milky Way.
The MCs show very active star formation with the
impressive star-burst region of the Tarantula nebula (30
Doradus) in the LMC, giant and super-giant shells of atomic
hydrogen (HI shells) and regions of ionized-emitting
hydrogen (HII regions). The glowing gas of the ISM in
the latter is the breeding ground for the formation of new
stars (Fig. III.1.2). All these features are linked to recent
star formation, while most of them are related to newlyformed
young stellar systems. Specifically, both the MCs
are characterized by a unique sample of HII regions, HI
shells, molecular CO and H 2 clouds and young star clusters
and stellar associations located in regions of recent
star formation.
The concept of stellar associations was originally introduced
by Ambartsumian in 1947, who showed that they
are star-forming regions of our Galaxy. Extragalactic