‘Hungry’ Baby Stars Are Messy Eaters

The process of star birth isn’t as slow and steady as once thought; rather, it can get very messy indeed.

AsianScientist (Mar. 3, 2016) – An international team of astronomers have observed very active baby stars ‘feeding’ voraciously on surrounding material, even as they remain hidden inside their birth clouds. These dramatic moments in star formation, published in Science Advances, shed new light on how stars are born.

The team, led by researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan used the HiCIAO (High Contrast Instrument for the Subaru Next-Generation Adaptive Optics) camera on the eight-meter Subaru Telescope in Hawaii to make their observations.

Stars are born when giant clouds of dust and gas collapse under the pull of their own gravity. Over time, gravity in the densest regions of these clouds gathers in the surrounding gas and dust, via a process called accretion.

It is often assumed that accretion is a smooth and continuous process. However, this steady infall explains only a small fraction of the final mass of each star that is born in the cloud. Astronomers are still working to understand when and how the remaining material is gathered in during the process of star and planet birth.

A few stars are known to be associated with a sudden and violent ‘feeding frenzy’ from inside their molecular cloud nursery. When they gluttonize on the surrounding material, their visible light increases very suddenly and dramatically, by a factor of about a hundred. These sudden flareups in brightness are called FU Orionis outbursts, because they were first discovered toward the star FU Orionis.

Only a dozen out of thousands of stars are found to be associated with such outbursts; however, astronomers speculate that all baby stars may experience such outbursts as part of their growth. The reason we only see FU Ori outbursts in a few newborn stars is simply because they are relatively quiet most of the time.

To observe the target stars, which are located 1,500-3,500 light-years from our solar system, the research team used a novel imaging technique, imaging polarimetry with coronagraphy. This produced images of outbursting newborns which were surprising and fascinating, and nothing like anything previously observed around young stars.

Circumstellar structures revealed by Subaru-HiCIAO. The scale bars are shown in AUs (astronomical units). One AU is the average distance from the sun to the earth. The gas and dust surrounding baby stars (their food) are significantly more extended than our solar system. Here we show the first observations of such complex structures around active young stars. Credit: Science Advances, H. B. Liu
Circumstellar structures revealed by Subaru-HiCIAO. The scale bars are shown in AUs (astronomical units). One AU is the average distance from the sun to the earth. The gas and dust surrounding baby stars (their food) are significantly more extended than our solar system. Here we show the first observations of such complex structures around active young stars. Credit: Science Advances, H. B. Liu

Three of the outbursts had unusual tails. One showed an ‘arm,’ a feature created by the motion of material around the star. Another showed odd spiky features, which may result from an optical outburst blowing away circumstellar gas and dust. None of them match the picture of steady growth; rather, they show a messy and chaotic environment, much like a human baby eating food.

Left: Simulations of the motion of circumstellar material falling onto a baby star. Middle and right: Models of how we would observe the structure in scattered light, seen from two different angles. Credit: Science Advances, H. B. Liu.
Left: Simulations of the motion of circumstellar material falling onto a baby star. Middle and right: Models of how we would observe the structure in scattered light, seen from two different angles. Credit: Science Advances, H. B. Liu.

To understand the structures observed around these newborn stars, theorists on the team extensively studied one of several mechanisms proposed to explain FU Ori outbursts. It suggests that gravity in circumstellar gas and dust clouds creates complicated structures that look like cream stirred into coffee (above left). These oddly shaped collections of material fall onto the star at irregular intervals.

The team also conducted further computer simulations for scattered light from the outburst. Although more simulations are required to match the simulations to the observed images, these images show that this is a promising explanation for the nature of FU Ori outbursts.

In spite of these exciting new results, there is a still great deal more work to do to understand the mechanisms of star birth. More detailed comparisons between observation and theory are needed.

Astronomers are also planning to construct telescopes significantly larger than Subaru in the coming decades, including the Thirty Meter Telescope and the European Extremely Large Telescope. These should allow detailed studies of regions very close to newborn stars.


The article can be found at: Liu et al. (2016) Circumstellar Disks of the Most Vigorously Accreting Young Stars.

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Source: Subaru Telescope; Photo: Shutterstock.
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