A nearby star rings in the new year

The entire universe in blog form
Jan. 2 2012 7:00 AM

A nearby star rings in the new year

I am fascinated by junk floating around stars. And no, not paparazzi, har har. I mean circumstellar material, literally gas and dust orbiting other stars. We see it around stars that are dying, we see it around stars being born, and we see it even after stars are well into their youth.

One such young'un is the bright and shiny HR4796, a star 240 light years away, with about twice the mass of the Sun. It's known to be less than 10 million years old -- compare that to the Sun's age of 4.56 billion years; we're 450 times older! -- and has also been known for some time to have material around it in the shape of a ring. New observations by Japan's huge 8.2 meter Subaru telescope have provided some of the sharpest views of this ring ever taken, and revealed some surprises.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Isn't that lovely? [Click to enannulusenate.]

This picture is in the infrared, well outside what the human eye can see. The star itself is so bright it's saturated, overexposed. That part of the picture is blocked out to make it easier to see details around it, but the star's position is marked with a dot. The tendril-like structures radiating outward are not real, but are artifacts of the image processing techniques. You can ignore them.

The important thing is the ring itself, which is easy to spot. It's almost certainly a circle, but we're seeing it at an angle (about 13° from edge-on) so it looks like an ellipse. It's huge; 22 billion km (14 billion miles) across, more than twice as wide as our entire solar system.

Again, the ring has been known for some time; for example it was seen in Hubble observations back in 2009 [NOTE: as astronomer (and my friend) Glenn Scheider points out in the comments below, HR 4706's ring was seen long before 2009. I wasn't clear when I wrote the previous statement; I was only alluding to one particular earlier observation, but it wound up sounding like it was the earliest such observation. My apologies for any confusion.]. But there is some new stuff here. For one, if you look along the long axis of the ring, you can see it looks fuzzy. That's real! The ring is made of dust grains of various sizes, probably the result of bigger clumps colliding with each other and grinding themselves up into ever-smaller pieces (the authors of this reasearch (PDF) call this a "collisional cascade", my new favorite phrase for 2012). These grains of dust orbit the star, and the smaller ones get blown away from the star due to the pressure of its fierce light. Bigger grains are less affected, so they tend to stay in place.

So the main ring is made of bigger grains, while the smaller ones are blown back, forming a larger, extended ring. That fuzzier outer ring is fainter and harder to see, but we see it more easily along the long axis because of geometric effects (similar to why soap bubbles and giant shells of cosmic gas look like circles in space). So even though we only see a part of this outer ring, the fact that we only see it in those two spots is what makes it clear we're seeing a ring at all! Funny how that works.

But there's more. The ring isn't centered on the star like you'd expect. Very careful analysis shows it's offset by quite a bit, over 750 million kilometers! That's 5 times the distance of the Earth to the Sun, about as far as Jupiter orbits the Sun in fact. I drew a line by eye along the axis of the ring, and you can see the star's position is noticeably off.

How can that be? There are several possibilities, but these new observations rule many out, leaving one as being the most likely: the presence of a planet or planets closer in to the star, disturbing the orbits of the dust particles. As it happens, it's hard to make a giant ring around a star like this without planets; they shepherd the grains into that tight annulus in the first place. So this observation that the ring is offset just adds more grist to the mill for the existence of these unseen planets. And while no planets were seen in these images, that fact is useful too: you can put an upper limit to how bright the planets must be (if they were any brighter, they'd've been detected). At the age of the star, planets are still hot from their formation, and the more massive they are, the hotter they are, and the brighter they glow in the infrared. That means that by putting an upper limit on how bright they are, we can put an upper limit on how massive they are!

They found that farther out than about 22 billion km, there are no planets bigger than about 1.4 times Jupiter's mass, and closer in, at 2.7 billion km, there are no planets bigger than about 17 times Jupiter's mass. Lower mass planets can be detected farther out from the star, because they're not overwhelmed by the star's glare. Closer in, the planet has to be bigger and brighter to be seen. So while there must be planets circling this star, they have to be lower mass than those limits or else we'd see them.

Years ago, I worked on Hubble images of young stars with material like this around them (in fact I worked with some of the authors on this paper!) and it was one of my favorite projects to work on. The images were surreal and amazing, and it was always a thrill to get new data and see these gigantic systems for the first time, knowing no one had ever seen them as sharply and in such detail. But these new Subaru observation are as crisp as Hubble's, because the technology improves all the time. We'll be getting even better observations of these objects as time goes on, and that includes spying the planets currently invisible in all that muck. We've actually directly detected quite a few planets orbiting other stars, and that list will only get larger with time.

Tip o' the occulting mask to reddit for the news. Image credit: Subaru Telescope, National Astronomical Observatory of Japan

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