From dark Clouds to bright Clusters

Although most stars in galaxies were likely formed in stellar clusters, the physical origin of these clusters remains largely a mystery. We propose here to tackle this problem observationally by analyzing recently acquired state-of-the-art data on a unique survey of massive star forming regions at several wavelengths and resolutions. The innovative aspect of this proposal is that it does not only concentrate on the physical properties of embedded clusters, nor on the molecular cloud structure and properties, but on how the two connect. Our sample was designed to include, for spatial resolution, the closest massive star forming region to Earth (the Orion A cloud) and, for statistics, the largest AO survey of the youngest and most massive star forming regions in the Milky Way. The core data of this project comprises our recently finished ESO VISTA P90 survey of the Orion A cloud (21 square degrees) and our ESO VLT- NACO/VLT-ISAAC survey on about 100 of the most massive star forming regions known in the Milky Way. Our sample of young massive clusters is about an order of magnitude larger than any previous study of this kind and will therefore provide the ultimate data base of very young clusters where models can be tested and constrained. We are requesting in this proposal the salaries for two PhD students from the University of Vienna already involved in the project (Josefa Großschedl and Stefan Meingast).

The Orion constellation hosts the famous Orion Nebula that is part of the giant molecular cloud Orion A. This is the closest massive star-forming region to earth (within 1600 ly), making it a bench- mark region to study the process of star formation. The region was already subject of numerous studies in the community, including our group in Vienna. With near-infrared observations con- ducted with the ESO-VISTA survey telescope we started the so called Vienna Survey in Orion (VI S I O N). Infrared observations allow us to look trough the cloud and detect embedded stars which are otherwise invisible in the optical. The Ph.D. student Josefa Großschedl used the VI S I O N data, within the FWF project From dark clouds to bright clusters, to construct the most complete catalog of young stars in Orion A up in existence, which enabled us to use robust statistics and test theories. A mystery about the Orion A cloud are its different star-forming environments. The part around the Orion Nebula produced both low-mass and the less common high-mass stars at a relatively high-rate, resulting in the rich Orion Nebula Cluster (ONC). The low-rate star- forming regions, to the South of the ONC, are forming mostly low-mass stars, more comparable to other nearby star- forming regions. Using our new catalog of young stars we find that, in fact, the star formation rate near the ONC is about ten times higher compared to the star formation quieter parts. At first guess, one might assume that this higher star formation rate is fuelled by a larger amount of gas available in this region. However, when investigating the gas distribution across the cloud, by using recent observations of the Herschel space observatory, we find that it is distributed rather evenly. This statement seems to be contradictory, and it raises the question of the formation mechanisms of the cloud and its stellar content. On an attempt to understand the previous counter-intuitive result, and to learn more about this important star-forming region, we seized the immense opportunities opened by the recently published Gaia all-sky survey, which provides us with the most accurate distance measurements up to date. Now, we are able to investigate the 3D structure of the cloud, by using its young stars as proxy to cloud distance. We find that the stars in Orion A near the ONC are at about 1300 ly, confirming previous estimates, but the stars to the South show that this part of the cloud must be highly inclined away from the plane of the sky, with the far end reaching almost until 1600 ly from the Sun. Thus, the Orion A cloud is about twice as long as previously assumed. By studying the 3D shape of the cloud, we find that the cloud seems to have a bent head (the ONC region) which points towards the Galactic mid-plain. This is different to the previously assumed picture, of a shorter cloud which lies almost parallel to the mid-plane. In addition, the cloud is located much farther from the Galactic mid- plane compared to other similar structures in the Milky-Way. This striking new view of this important star-forming region shows the new window, that is opening up with the VI S I O N and Gaia missions to study star-forming clouds. New scientific questions are introduced: How did the cloud reach its current position? How did it evolve to its current shape and orientation? Answering these and other new questions, that arise with Gaia, will keep astronomers busy for the next decades, and let us look forward to numerous exciting results.