Hubble sees a cosmic flapping “Bat shadow”

NASA’s Hubble Space Telescope captured a striking image of a fledgling star’s unseen, planet-forming disk casting a huge shadow across a more distant cloud in a star-forming region. The young star is called HBC 672, and the shadow feature was nicknamed the “Bat Shadow” because it resembles a pair of wings. The nickname turned out to be surprisingly appropriate: Now, the team reports that they see the Bat Shadow flapping!

“Bat Shadow”. CREDITS:NASAESA, and STScI

“The shadow moves. It’s flapping like the wings of a bird!” described lead author Klaus Pontoppidan, an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, Maryland. The phenomenon may be caused by a planet pulling on the disk and warping it. The team witnessed the flapping over 404 days.

“Bat shadow” schematic. CREDITS:NASAESA, and A. James and G. Bacon (STScI)

The disk—a circling structure of gas, dust, and rock—might be roughly saddle-shaped, with two peaks and two dips, which would explain the “flapping” of the shadow. The team speculates that a planet is embedded in the disk, with an orbit inclined to the disk’s plane. This planet would be the cause of the doubly warped shape of the orbiting disk and the resulting movement in its shadow.

The shadow, extending from the star across the surrounding cloud, is so large—about 200 times the length of our solar system—that light doesn’t travel instantaneously across it. In fact, the time it takes for the light to travel from the star out to the perceivable edge of the shadow is about 40 to 45 days. Pontoppidan and his team calculate a planet warping the disk would orbit its star in no fewer than 180 days. They estimate that this planet would be about the same distance from its star as Earth is from the Sun.

If not a planet, an alternative explanation for the shadow motion is a lower-mass stellar companion orbiting HBC 672 outside the plane of the disk, causing HBC 672 to “wobble” relative to its shadowing disk. But Pontoppidan and his team doubt this is the case, based on the thickness of the disk. There is also no current evidence for a binary companion.

The disk is too small and too distant to be seen, even by Hubble. The star HBC 672 resides in a stellar nursery called the Serpens Nebula, about 1,400 light-years away. It is only one or two million years old, which is young in cosmic terms.

Source: “Hubble sees a cosmic flapping “Bat shadow” Hubble site, 25 June 2020. <https://hubblesite.org/contents/news-releases/2020/news-2020-22>

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