Optical image of accretion disk around baby star, taken by ground-based VLT

Stellar accretion disk
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Scientists today released an optical image of the accretion disk that surrounds a baby star about 5,000 light years away, taken by ground-based Very Large Telescope (VLT) in Chile and enhanced by data from the Atacama Large Millimeter/submillimeter Array (ALMA), also in Chile.

That image, reduced to post here, is to the right. The bright blue spot in the center is the main star, with the smaller dot to the lower left a companion star. From the press release:

The VLT observations probe the surface of the dusty material around the star, while ALMA can peer deeper into its structure. “With ALMA, it became apparent that the spiral arms are undergoing fragmentation, resulting in the formation of clumps with masses akin to those of planets,” says Zurlo.

Astronomers believe that giant planets form either by ‘core accretion’, when dust grains come together, or by ‘gravitational instability’, when large fragments of the material around a star contract and collapse. While researchers have previously found evidence for the first of these scenarios, support for the latter has been scant.

This data suggests that the latter is being observed, the first time gravitational instability has been identified as it is happening. You can read the scientist’s research paper here [pdf].

Hubble captures shadows on star’s outer accretion disk cast by inner accretion disk

Shadows cast on star's accretion disk
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Astronomers using the Hubble Space Telescope’s images taken five years apart have captured the changing shadows cast by a star’s inner accretion disk onto its outer accretion disk.

Those images are to the right, reduced and rearranged to post here. From the caption:

Comparison images from the NASA/ESA Hubble Space Telescope, taken several years apart, have uncovered two eerie shadows moving counterclockwise across a disc of gas and dust encircling the young star TW Hydrae. The discs are tilted face-on as seen from Earth and so give astronomers a bird’s-eye view of what’s happening around the star.

The [top] image, taken in 2016, shows just one shadow [A] at the 11 o’clock position. This shadow is cast by an inner disc that is slightly inclined to the outer disc and so blocks starlight. The picture on the [bottom] shows a second shadow that emerged from yet another nested disc at the 7 o’clock position, as photographed in 2021. What was originally the inner disc is marked [B] in this later view.

The shadows rotate around the star at different rates like the hand on a clock. They are evidence for two unseen planets that have pulled dust into their orbits. This makes them slightly inclined to each other. This is a visible-light photo taken with the Space Telescope Imaging Spectrograph. Artificial colour has been added to enhance details.

An artist’s conception of the system, as seen from an oblique angle, is available here. All told, this solar system of disks kind of resembles a spinning gyroscope, with its different rings tilted at different angles to conserve angular momentum.

Astronomers discover complex molecules in protostar accretion disk

Astronomers have discovered several complex molecules in the accretion disk surrounding a very young baby star about 1300 light years away in the constellation Orion.

The research team’s ALMA observations have clearly detected an atmosphere of complex organic molecules above and below the disk. These include methanol (CH3OH), deuterated methanol (CH2DOH), methanethiol (CH3SH), and formamide (NH2CHO). These molecules have been proposed to be the precursors for producing biomolecules such as amino acids and sugars. “They are likely formed on icy grains in the disk and then released into the gas phase because of heating from stellar radiation or some other means, such as shocks,” says co-author Zhi-Yun Li of the University of Virginia.

What is even most interesting about this discovery is that these complex molecules are not scattered throughout the disk, but are concentrated in regions above and below its central plane, what the astronomers are labeling “an atmosphere.” This suggests that differentiation — the same process that separates the heavier molecules from lighter ones both in centrifuges and in the cores of planets — occurs quickly in accretion disks as well.