Max Planck Institute for Astronomy - Annual Report 2005

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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
Fig. III.1.1: This two-colour image shows an overview of the full Small Magellanic Cloud and was composed from two images from the Digitized Sky Survey 2 (DSS2). The field of view is slightly larger than 3°. 5. Credit: Esa/HubblE, Digitized Sky Survey 2, and Davide De Martin. associations are easily outlined by their massive stars of »early spectral type«, while the stellar members of the associations in the Milky Way are confused with fore- and background stars of the spiral arms of the Galactic disk, where they are located. In general, stellar associations are considered to be single, loose concentrations of early type luminous stars, which are embedded in the most recent star-formating regions in a galaxy (Kontizas et al. III.1 Star Formation in the Magellanic Clouds 51 1999). Their dimensions range from those of ordinary galactic clusters with diameters of a few parsecs to several tens of parsecs, and the stellar mass density in these systems is defined to be less than one tenth of a solar mass per cubic parsec. Photometric and spectroscopic investigations of young stellar associations in the MCs have previously been based on ground-based observations, which are appropriate for the study of their massive stellar content only down to about 2 times the solar mass (M 0 ). The upper mass limit of the massive stars of these systems is found to be of the order of 100 M 0 . The latter stars are characteristic of very recent star formation, due to their very short lifetime. Their existence clearly suggests that stellar associations of the MCs are very young, with estimated ages between 1 and 30 million years. The
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Page 5: Contents Preface ..................
Page 8 and 9: 6 I. General I.1 Scientific Goals U
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