Massive stars - Formation
Massive stars are an important component in the evolution of galaxies. However, little is known about their formation process. Low-mass stars are known to form through accretion of material from circumstellar protoplanetary disks. This scenario cannot be extrapolated to higher masses, because the material will then accrete so rapidly, that the radiation from the growing star will already blow away the accretion disk at an early stage. In this way, stars can never become more massive than a few solar masses. Yet, more massive stars do exist, so somehow they must overcome this `radiation pressure' problem. Current lines of research are focused on modeling massive accretion disks in order to understand how nature circumvents the radiation pressure problem, and by exploring alternatives to the disk formation scenario.
Definitive progress can be made with actual observations of massive star formation. Hoewever, observations are difficult for three reasons: the relative rarity of massive stars (less than 10% of the observed stars are massive), their short lifetime, and the obscuration of their birth places by dust clouds. Because of the last aspect, observations have hitherto mainly been of radiation that can pierce through these clouds, namely infrared and radio observations.
At the API, we use data from various state-of-the-art observatories to clue us in on this mysterious formation process. In particular, the newly made optical-to-infrared spectrograph X-shooter on the Very Large Telescope in Chile is being used to observe newly born massive stars. These infant stars sometimes still have not shed their accretion disks, allowing us to peek in on their recent history. In a few cases, we even witness the spectacular event of accretion through disks and jets in progress.
Fig. 1: A star in the process of formation: the central star accretes material from a rotating circumstellar disk, while the system sheds its angular momentum through a very powerful bipolar outflow, or 'jet'.
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The following people are working on this topic: