Monday, January 9, 2012

Astronomy: Debris Disks

I figured I'd make a post about some cool things in astronomy, particularly those relevant to my own research. Today's topic: debris disks.

While writing this I realize the topic is far too broad to give it justice, but hopefully you'll get a flavor for what these are and why they are so cool.

Artists conception of HD 98800
Credit: NASA/JPL-Caltech/T. Pyle (SSC)

So what are debris disks? 
The short definition is that these are rings or disks of dust grains surrounding a star. Think of them like Saturn's rings, except on a much larger scale. Read on to learn more.

How can we find these disks and rings?
The trick to finding them, is to look at systems in infrared light. What we see with our eyes is, aptly named, visible light. When you go outside and see your friends, you are seeing visible light from the Sun, the Moon, or nearby artificial sources reflecting off of your friends. In complete darkness you would not be able to see them without some equipment, like heat-vision goggles or infrared cameras.
We humans have a body temperature of about 310 Kelvin (98.6 Farenheit), which means we glow at a wavelength of 12 microns. Visible light, in comparison, has wavelengths of 0.4-0.7 microns or so. Hence we cannot see our own glow without specialized instruments. The peak wavelength an object emits depends on its temperature. A hot object, like a lightbulb's filament or the surface of the Sun and other stars, will emit light near the visible range. Colder objects, like our bodies, emit at longer wavelengths and thus in the infrared and submillimeter range. The dust grains constituting debris disks can be as warm as 300 K, but are generally colder than 100 K. To observe these systems we need to look at light in the 20-500 micron range.

Here is what the sky looks like at 100 microns:
Credit: R. Hurt/IRAS/DIRBE, see here
As you can probably tell, the sky looks very different from what you are used to. What we see is the cool dust spread throughout the galaxy; we barely see any stars at all. The Galactic plane runs horizontal across the image and we can see a few nearby star forming regions- Ophiucus is near the center and above the plane, Taurus is at the left, Orion at the right, and some galaxies- the two blobs below the plane midway to the right are the Large and Small Magellanic Clouds, nearby galaxies to our own Milky Way.

Stars barely produce any light at these wavelengths; their emission peaks in the visible, remember? Hence, we can look at where the stars are supposed to be and see if there is any light at 100 microns (or any other such long wavelength). Sometimes we see that there is indeed light coming at those wavelengths. By looking at multiple long wavelengths (say 12, 25, 60, 100, etc, microns) we can reconstruct what's going on and say 'something of temperature X is present around this star'. As we've learned, this is a disk or ring of dust surrounding the system.

Where does the dust come from?
This is an interesting question as there are two possible answers. The first is that the dust is primordial material, that is, it is left over from the star formation process. This will generally be the case for the very youngest stars. However, stars are very effective at clearing out the system of dust. In only a few thousand years the dust grains should be cleared out, though the timescale depends greatly on the properties of the star, the removal mechanism, and the dust grain sizes. Most stars, however, are hundreds of millions of years old and are not expected to have any material leftover from the star formation process. In this case, the dust we see has been recently (or continuously) produced by collisions of rocky objects. For example, asteroids or Kuiper belt objects, when they collide, will produce dust. If enough of these collisions occur, then a detectable debris disk emerges in the system.

Can we actually see these disks?
In many cases, the resolving power of the far-infrared instruments we've used to find these disks is not sufficient to actually produce an image. What we see is just a blob and can't tell if there is any structure there. However, for the systems closest to the Earth, or those with the largest disks, we've been able to actually produce an image of the disk. In some cases, we can also see the disk in visible or shorter-wavelength infrared light, when the grains reflect the light of the star. This tends to produce some of the nicest images we have.
Here are a couple of debris disks seen with the Hubble Space Telescope:

Does our own solar system have a debris disk?
Our solar system has tons of small objects (asteroids, comets, and Kuiper belt objects). When these collide they produce dust; however, collisions are much less frequent today than they were billions of years ago when the solar system was young. Hence, the amount of dust is minute. Our telescopes are capable of detecting dust in distant star systems if it these contain substantial quantities of dust, say at least 100 times that of our own solar system (though if I recall correctly, the Herschel spacecraft does get close enough to the expected contribution from the Kuiper belt). So detecting an analog to our own solar system from afar is not feasible with our current technology. However, we are embedded in our system and thus have a much closer view. Collisions and comets have produced dust in our system that we can see as the zodiacal light:
Credit: Bob King / Duluth News Tribune
You need to be in a very dark sky to see this triangular wedge of light. You'll spot it right after sunset or right before sunrise. I must admit to never having seen the zodiacal light, though I've been in dark observation sites.

What about planets?
Planets are one of the hottest things in astronomy right now. Everyone is talking about them, especially the public. Debris disks indirectly suggest planets, or at least planetesimals like asteroids, exist or have existed in distant stars. This is encouraging as it's easier to spot debris disks than it is to search for planets. Furthermore, the properties of the disk can suggest a nearby planet. For example, gaps, warps, or offsets in imaged disks or rings can suggest an unseen object (ie, a planet) is tugging the material to produce these features. This was the case for the beta Pictoris star system, where a secondary dust disk was observed in the central regions. A close examination revealed a massive planet in the system:
Credit: ESO/A.-M. Lagrange et al.

My own research:
One particular line of study I've done is to explore how often binary or multiple stars, that is star systems with two stars or more, possess debris disks. It should not come as a surprise that a second (or third, etc) star will disrupt the system. While planets are known to exist in these multiple star systems, they are mostly found around either very widely separated stars, so that the gravitational influence of the second star is minimal; or around very tightly spaced stars, so that the planet effectively sees them as just one object. For intermediate separations, say a second star located at Saturn's orbit in our own solar system, the second star would completely disrupt the disk preventing a solar system-analog from forming.

Finishing thoughts:
Debris disks are cool. They are a by-product of planet formation and their study allows us to explore how planets like our own are formed. As a bonus, they are also really neat when imaged. For now, all we can do for planets is get a tiny point of light, but for nearby disks we can get the whole structure.

I hope to do more of these astronomy posts from time to time, and perhaps to revisit debris disks in a more focused post. Stay tuned for a brief summary of my impressions of the American Astronomical Meeting #219 in Austin, TX.

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