Hubble sees cosmic flutter ‘Bat Shadow’

Nicknames sometimes turn out to be closer to reality than you can imagine.

NASA’s Hubble Space Telescope captured a striking image of a fledgling star’s invisible, planet-forming disk casting a huge shadow through a more distant cloud in a star-forming region, like a fly wandering in the beam of a flashlight shining on a wall.

The young star is named HBC 672, and the shadow feature was nicknamed “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 battling!

“The shadow moves. It flaps 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 can be caused by a planet that pulls the disk and deforms it. The team witnessed the flapping for 404 days.

But what created Bat Shadow in the first place?

“You have a star that is surrounded by a disk, and the disk is not like Saturn’s rings, it is not flat. It is swollen. And that means that if the light from the star is directed upward, it can continue in a straight line. it’s not blocked by anything. But if you try to go along the plane of the disk, it doesn’t come out and cast a shadow, “Pontoppidan explained.

He suggests imagining a lamp with a shade that casts a shadow on the wall. In this case, the light bulb is the star, the lamp shade is the disk, and the cloud is the wall. Depending on the shape of the shadow, the disk should be widened, with an angle that increases with distance, such as flared pants or a trumpet.

The disk, a circular structure of gas, dust and rock, could be saddle-shaped, with two peaks and two falls, which would explain the “flapping” of the shadow. The team speculates that a planet is embedded in the disk, with an orbit tilted toward the plane of the disk. This planet would be the cause of the doubly deformed shape of the orbiting disk and the resulting motion in its shadow.

“If there was a simple bump on the disc, we would expect both sides of the shadow to tilt in opposite directions, like the plane’s wings during a turn,” said team member Colette Salyk of Vassar College in Poughkeepsie, New York.

The shadow, which stretches from the star through the surrounding cloud, is so large, about 200 times the length of our solar system, that light does not instantly travel through it. In fact, the time it takes for light to travel from the star to the perceptible edge of the shadow is approximately 40 to 45 days. Pontoppidan and his team estimate that a planet that deforms the disk will orbit its star in no less than 180 days. They calculate that this planet would be approximately the same distance from its star as the Earth from the Sun.

If it’s not a planet, an alternative explanation for shadow motion is a lower-mass stellar companion orbiting HBC 672 out of the disk plane, causing HBC 672 to wobble relative to its shadow disk. But Pontoppidan and his team doubt that this is the case, depending on the thickness of the disc. There is also no current evidence for a binary partner.

The disk is too small and 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. She is only one or two million years old, which is young in cosmic terms.

This finding was fortuitous. The first image of Bat Shadow was taken by another team. Later, the image was programmed for use in the NASA Learning Universe, a program that creates materials and experiences to allow students to explore the universe for themselves. The goal was to illustrate how shadows can transmit information about phenomena invisible to us. However, the original team only observed the Bat Shadow in a light filter, which did not provide enough data for the desired color image by NASA’s Universe of Learning.

To get the color image, Pontoppidan and his team had to observe the shadow in additional filters. When they combined the old and new images, the shadow seemed to have moved. At first, they thought that the problem was in image processing, but they quickly realized that the images were correctly aligned and that the phenomenon was real.

The team article will appear in a future issue of The astrophysical journal.