The star EC 82 (right of center) casts two long shadows onto the nebulosity around it in this Hubble image of a young star-forming region. Credit: NASA, ESA, and STScI
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The star EC 82 (right of center) casts two long shadows onto the nebulosity around it in this Hubble image of a young star-forming region. Credit: NASA, ESA, and STScI

Hubble sees a star flapping its wings (video)

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Jun 26, 2020

In the constellation of Serpens, about 1400 light years from Earth, a fledgling star flaps its wings.

OK, I'm being slightly poetic there. It's more like a forming planet gravitationally warped its circumstellar disk into a quadrupolar shape such that the shadow cast by the star onto nearby nebulosity looks like flapping wings.

OK, I'm being slightly too prosaic there. How about we compromise with an image and video of this bizarre and extremely cool object by Hubble Space Telescope, shall we?

The star EC 82 (upper right) casts two long shadows onto the nebulosity around it in this Hubble image of a young star-forming region. Credit: NASA, ESA, and STScI

This image, taken in near-infrared light, shows a star-forming region in Serpens. Many of the stars you see here are quite young, some just a couple of million years old. You can also see quite a lot of gas and dust, common where stars are born (that's what they use as raw material after all).

To the upper right of center is a brightish star called EC 82, situated in the middle of the brighter area of the nebula. In fact, the star is likely massive (2.5 to 3 times the Sun's mass) and quite luminous (blasting out 30 times the Sun's energy), and is the source of illumination in that part of the nebula, casting it a cool blue.

If you look carefully you'll also see it's sitting in the very center of two diverging cones of darkness, one to the upper left and the other to the lower right. Those, it turns out are shadows! Surrounding the young star is a disk of gas and dust, swirling around it and likely forming planets. The disk is thick enough to block the light from the star… so it's actually casting a shadow onto the material surrounding the star farther out!

That's nifty, but we've stuff like that before. What makes this particular case so very, very cool is that two observations taken using Hubble a little over a year apart show the shadow moving!

The star EC 82 has a disk of material around it, and that disk is casting an enormous shadow into space and onto nebulosity around it. But images taken over time show the shadow... flapping! The disk may be warped like a Pringles potato chip, which is making the shadow look like a bird flapping its wings. This video alternates several times between two Hubble images taken about a year apart. The warp may be caused by a planet orbiting in the disk.

Original video: https://hubblesite.org/contents/media/videos/2020/22/1281-Video?news=true
Credit: NASA, ESA, and K. Pontoppidan and J. DePasquale (STScI)

That video switches between two images taken 404 days apart, repeating many times so you can see the motion. It's not often you see such rapid and obvious changes in the sky. And it really does look like a bird or bat flapping its wings; the two sides of the shadow move up and down together. That, it turns out, is important.

What we're seeing is a change in the inner disk around the star, something happening to change the shadows being cast. The best guess by the astronomers who took the data is that the disk of material around EC 82 isn't flat, but warped. That can happen in galaxy disks, where one side is bent up and the other down.

But that doesn't explain this motion; you'd expect the two sides to move in opposite directions in that case. Instead, the astronomers envision the disk to have four flaps to it, two bent down and two up, giving it a saddle shape (or, if you prefer, like a Pringles chip).

A schematic diagram of the possible warped disk around the star EC 82 that’s casting shadows in space. As the disk rotates the warps moves the shadows up and down in a flapping motion. Credit: NASA, ESA, and A. James and G. Bacon (STScI)

While it's not clear how this shape would arise, exactly, they speculate a planet orbiting the star at a slight angle could gravitationally tug on the disk might due the trick. The disk itself then flaps up and down, changing the shadows cast into space. It's like an anti-lighthouse!

Coooool.

They also note that since a shape like this has never been seen before, there might be a different cause. One idea is that the star is actually a binary, and the two stars orbit each other perpendicular to a relatively flat disk. Furthermore, one of the stars is much brighter than the other, so it's the one that's the light source for the shadows. As the star bobs up and down relative to the disk, the shadow changes. This also explains what we see here, but a binary orbiting that way is unlikely — the physics makes it far more likely that the orbit is in the plane of the disk. Also, only one star is detected there, so they think the warped disk is the better explanation.

A schematic showing a possible explanation of the flapping star shadow: The star is actually binary, orbiting “up and down” relative to the disk, causing the shadows to move. Credit: Pontoppidan et al.

If there's a planet there warping the disk, it probably orbits EC 82 about once every 180 days or so, and would be about the same distance from the star as Earth from the Sun (EC 82 is more massive than the Sun, so the planet would orbit more rapidly).

The scale of this thing is quite large. The best defined part of the shadow extends about 45 light days (over a trillion kilometers!) on either side, though they can trace the shadow out to about twice that distance. This means the finite speed of light makes a difference! If, say, the star were to flare briefly, getting much brighter, you'd see that bright flash moving out along the shadow (probably looking like a glow along the top and bottom of it like a Jacob's Ladder), taking 45 days to move across it. The fact that the two sides of the shadow move together means that any motion is slow compared to that time, which is how they got that 180-day period of the planet (if there are four warps to the disk, then the shortest period for the planet is 4 x 45 days = 180 days).

Unfortunately no other observations are available to nail down the period of this, or to see what other changes might be occurring on shorter timescales. Hubble is far too busy to revisit this object often enough. That's too bad. The shadow magnifies the disk in a sense; a bump in the disk far too small to see directly would cast a shadow large enough to be detected, so this would be a way to see small-scale structure in it, and see them change over time! What an opportunity!

Ah well. We'll just have to launch more space telescopes to peer at it. That may take a while though. Hopefully, in the meantime, this peculiar object will continue to flap away, going nowhere.

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