Thread by @mattkenworthy

Okay, I've had to keep my mouth shut for several months but now I can talk about this amazing object in orbit around CS Cha! Here's the paper led by Christian Ginski at @SterrewachtNL : arxiv.org/abs/1805.02261 (disclosure: I am a coauthor on the paper)

So: CS Cha is a very young binary star system, with a dust disk surrounding a binary star system. Starlight is unpolarised, but light that bounces from the dust in the surrounding disk is polarised, as seen in the image below 2/n

He saw the disk clearly, but wait! There's another dim companion off to the right, beyond the edge of the disk. It looks very faint, but it's about the right brightness to be a planet or low mass brown dwarf. Lovely!

But now it gets weird.

3/n

Here's the unpolarised image blinked with the polarised image - you can see the starlight is removed to show the dust disk, and the companion.

But wait.... the companion is polarised too.

And that's very, very odd. 4/n

This caused Christian several headaches. Giant planets and brown dwarfs can be polarised very slightly, at about the 1% level, and several astronomers have searched hard for this size of signal.

The companion to CS Cha?

That sucker has 19% polarisation!!!

5/n

That's mind blowing. Really.

When Christian showed me the data, I was pretty much went:

6/n

Very, very few objects in astronomy have such a high degree of polarisation - one type of object is called an Active Galactic Nucleus (AGN), which you find in very distant galaxies.

So - maybe Christian had just taken a picture of an unrelated, far in the distance AGN?

7/n

As a scientist, if you get a weird result, you need to be skeptical and carry out further tests. So, that's what Christian did.

All stars appear to move slowly across the sky - it's called proper motion. This star is relatively nearby to us, so we can measure it by....

8/n

...looking at images of CS Cha from several years ago, try to find the companion in those images, and then see if they are moving together across the sky, or the source stayed in the same place whilst CS Cha wandered past.

Turns out that CS Cha was observed by...

9/n

... the Hubble Space Telescope! But nobody had noticed the companion before! Why?

Because the little devil was hiding behind diffraction spikes of the primary stars! Christian knew where the companion *should* be, and he found it - peekaboo!

10/n

It's not a background galaxy, it's moving along with the CS Cha binary and disk, and it's highly polarised.

Now, it could be that Nature is pulling a very cruel trick, and it's an unrelated object just floating by in space. But now that's very, very unlikely.

11/n

Christian worked with other collaborators to measure the brightness of the companion at many different wavelengths. His collaborators built toy models of what the object could be. Definitely low mass, sort of looks like an exoplanet, but not quite... lots of challenges.

12/n

But, it most closely resembles a brown dwarf surrounded by a dust disk that is nearly edge on to us!

What a wonderful and weird object.

Very, very, very cool indeed.

13/n

Needless to say, follow up observations are already underway. Does the polarisation change? Does the brightness change? What's going on in there? Go see the press release!

14/n

astronomie.nl/#!/index/_deta…

...and as @dalcashdvinsky points out, the circumbinary disk is very interesting in its own right too.

This is thorough work by Christian Ginski and his collaborators.

It will be fun to see what we find out next about this curious companion!