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Jes Jørgensen
Right: movie showing the evolution in submillimeter (blue) and infrared (red/green) emission from a collapsing protostar during its first million years.

Studies of these early stages of protostars are particularly important for our understanding of the origins of stars and planets. How massive the young star can become depends on how much material it accretes during these stages. At the same time, whether or not planets are formed around a given young star, depends on the chemistry and physics of its disk. Our basic picture of star formation suggests that low-mass stars are a result of gravitational collapse of dense molecular cloud cores. During the first several 100,000 years of its evolution, the young stellar object is embedded in a large envelope of cold gas and dust that is gradually dispersed and thereby reveals the young, newly formed star and its surrounding circumstellar disk. So far, most of this scenario has been based on data with poor spatial resolution, encompassing the entire disk-envelope system (15"; a few thousand AU) and many aspects, for example when and how protoplanetary disks form, are poorly understood.


Tracing our origins

Penetrating the darkness

Over the next decade the Atacama Large Millimeter Array (ALMA) is therefore going to be the key facility for star formation research. ALMA will revolutionize studies of deeply embedded protostars by probing scales as small as 0.01" (a few AU at the distances of nearby star forming regions), thereby resolving the chemical and dynamical structure of disks at  the time when planets start to form. Its high sensitivity will furthermore allow us to explore the formation of solar-type stars as a function of environment by studying the properties of individual low-mass protostars in massive star forming regions and even nearby galaxies. This will lead to a new understanding of the importance of environment for the properties of emerging stars, disks, and planets.

For a popular Danish introduction to the ALMA array see this article that I wrote for the 2011 edition of the Copenhagen University Almanak.

The deeply embedded stages are particularly important for our understanding of the star formation process. The evolution of a protostar through these embedded stages is likely to affect the properties of the emerging star; its final mass depends, e.g., on the amount of material accreted in this phase. It is in these deeply embedded stages that the seeds for planets are planted: circumstellar disks are formed early in the evolution of protostars because material falling in from the larger scale envelope, due to its angular momentum, cannot accrete directly to the central star and piles up in a circumstellar disk. The properties of the emerging disks reflect the physical and chemical structure of the innermost regions of the centrally condensed envelopes that surround the protostars, but it remains an open question when they form and how rapidly they grow in size.

It is therefore important to zoom in and resolve the solar system scales of low-mass stars in these early stages of their evolution - and thus also trace the physical and chemical evolution of matter from the natal core to the protoplanetary disk. Our group tries to do this through observations and modeling of the physical and chemical properties of young stars and their disks in the earliest evolutionary stages. For this we in particular rely on observations at infrared and submillimeter wavelengths. At these wavelengths we can penetrate the molecular clouds and protostellar cores and probe the highly extincted inner regions where planets may be forming.

Early Protostars

Atacama Large Millimeter Array

Left: Movie showing the differing appearance of the young star forming region, NGC1333 when observed at optical, infrared and submillimeter wavelengths.
Deeply embedded protostars as a cartoon (left; from Th. Greene, 2001, American Scientist) and as observed in the mid-infrared and submillimeter (center and right; Jørgensen et al. 2006, 2007).
Above: This image shows an artist conception of the Atacama Large Millimeter Array (ALMA) currently being built in Chile. Image credit: ALMA (ESO/NAOJ/NRAO)