The shadow cast by a protoplanetary disk takes the shape of a bat — and over time, flaps like one, too, yielding clues to the planet-forming material in the disk.
A "bat wing" in the Serpens Nebula is providing a unique, indirect picture of planet formation. Astronomers are using new images of the young star and the flapping shadow it casts to reveal the system’s protoplanetary disk.
In 2018, the Hubble Space Telescope snapped an image of the young, Sun-like star HBC 672 in Serpens Cauda, the Serpent’s Tail. Like a lampshade blocking the light from the bulb inside it, the disk casts a bat-like shadow. Now, follow-up images of the same star show that the shadow has shifted its angle slightly, most likely due to the rotation of the disk where young planets could currently be forming.
"This is a fascinating object with an interesting story and a mystery," says Klaus Pontoppidan (Space Telescope Science Institute). Pontoppidan and his colleagues captured the original photo of the flapping wings, which suggests that the planet-forming disk is nearly edge-on to the Sun. The disk itself remains unseen, as it is too far from Earth to resolve.
When the team again used Hubble to snap follow-up images of the young star they had found, they found — to their surprise — that the shadow had shifted. Further probing revealed that small changes in the protoplanetary disk had been imprinted on the wing-like shadows themselves, which will allow researchers to trace the disk’s orientation back in time. Pontoppidan presented the new results at a semiannual meeting of the American Astronomical Society in Honolulu, Hawai‘i.
A Unique Opportunity
The Serpens Nebula is a stellar nursery, a region where young stars are born. After stars form, the leftover dust and gas around them collapses into a protoplanetary disk, with some of that material eventually building planets. Observing disks like the one surrounding HBC 672 can show planet formation in action.
As light streams through the material around the star, the material affects the depth of the shadow, just as imperfections in the lampshade may brighten or darken the lamp’s shadow on the wall. Each side of the shadow around HBC 672 stretches for nearly 15,000 astronomical units, roughly 100 times the width of the solar system.
The original image already revealed important insights, as the shadow’s puffy shape suggested a gas-rich environment in the disk. The shadow also reveals the disk’s clumpiness. As planetary cores come together, they drift inward through the disk until they are large enough to clear out a lane of dust and gas, which stops their travel toward the star. Changes along the shadow's edge can provide evidence for clumps that hint toward growing worlds and the paths they clear.
The disk’s shifting shadow, as seen in the new images, is now giving scientists the chance to watch the disk as it evolves. Since light takes roughly 88 days to travel the length of the shadow, every snapshot shows a record of the previous three months of disk rotation. But only the inner half of the shadow is distinct enough to reveal details about the disk — at larger distances, background stars make it more difficult to detect relative changes in the shadow’s depth. Nevertheless, with enough images, astronomers can piece together the fingerprints of the disk over a full rotation.
"This is a unique opportunity," Pontoppidan says.
Monitoring the Bat Signal
In the future, Pontoppian and his colleagues would like to turn Hubble toward the “bat wing” every 40 days or so, to watch the shadow — and thus the disk — as it rotates. If the shadow continues to flap in a cyclic way, the length of the cycle could tell how quickly the disk is rotating, shedding light on what changes a disk experiences over that timescale. But even if the shadow doesn't match up between rotations, it can still provide information on what’s happening inside young protoplanetary disks.
Alternatively, continued observations could reveal that a periodically shifting shadow isn't caused by a disk at all but by a second star orbiting the first — an unexpected companion. Pontoppidan says this is unlikely, however, as there have been no other suggestions of a second star.
The upcoming James Webb Space Telescope will be able to take a closer look at the shadow after its 2021 launch. "Observing the shadow with longer wavelengths with Webb may give us unique insights into the nature of the absorbing dust in the disk," Pontoppidan says.