Witnessing the wild phase of star formation

February 05, 2016

Have you ever seen a stellar embryo? A research team that includes an MPIA astronomer has inspected the birth places of stars: the FU Orionis objects, a class of very young stellar objects. These protostars are surrounded by large asymmetrical disks, indicating a short, violent episode during the early formation of the star. By employing the 8m Subaru Telescope on the summit of Mauna Kea on Hawaii, four of those disks have now been imaged with extreme resolution, thereby allowing the astronomers to identify their structure. The results are being published in the journal 'Science Advances'.

Stars are born in a very wild and violent process. Before a star begins to 'live' and to release energy by nuclear fusion, it accretes mass from its surrounding gas, grows and heats up. This gas is not directly falling onto the central object, but instead the gas forms a disk around it and feeds the star. Among the youngest stellar objects are the FU Orionis objects, which are stellar embryos undergoing a growth spurt. In the early phase, the disks surrounding these protostars are largely asymmetrical and chaotic, exhibiting spiral structures, arms and gaseous clumps. These gaseous clumps may be ejected from the disks or remain there and may later form a planetary or stellar companion. The chaotic disk phase lasts for a few hundred thousand years before the disk finally stabilizes.

During the chaotic disk phase, FU Orionis objects show dramatic changes of their brightness, typically on the order of a factor of 1000 within a year. These cause for these extreme brightness is unclear; current models suggest that from time to time the disks heat up by infalling material and produce these brightness changes. During such an accretion outburst, which typically lasts for many decades, large quantities of material fall from the disks onto the stellar embryos.

To investigate these chaotic disks and to resolve their structures, observations need to be carried out with the largest telescopes available. A team that includes MPIA astronomer Thomas Henning observed four disks surrounding FU Orionis objects with the 8 m Subaru Telescope, which is located on the summit of the Mauna Kea volcano on Hawaii. Due to the extreme resolution of the HiCiao camera, astronomers have identified sub-structures in the chaotic disks around the growing stellar embryos. These observations reveal complex structures in those disks, exhibiting arms and gas streams. These images finally provide a much clearer and robust picture of the birth places of stars. We are now able to witness the formation of new stars and new worlds at the earliest stage.

Background information

The results described here have been published as H. B. Liu et al., "Circumstellar Disks of the Most Vigorously Accreting Young Stars" in Science Advances on February 5, 2016.

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