Blink and You'll Miss it: The Brief Life of a Gasous Disc around a White Dwarf

Artist's impression of a debris disc in orbit around a white dwarf. Over the past eight years we have seen such a disc suddenly form, then rapidly disappear around the white dwarf SDSS J1617+1620. Image Credit: NASA, ESA, STScI, and G. Bacon. 

 Today I published my first research paper, graced with the snappy name of "Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf". The tone of the title is a hint to the tone of the paper, so I'll attempt to provide a much more readable post about what we've done here.

(Note: Where I use "we " below I'm generally referring to all of the authors of the paper. I didn't actually start working on this project until January, so other people have done a lot of work on this before me.)

History of a dying star

Around four hundred million years ago, SDSS J1670+1620 was a star similar to our Sun (although with 2-3 times the mass). As it reached the end of the hydrogen fuel supply in its core, it swelled up into a red giant, then blew away its outer layers to become the tiny white dwarf that we see today.

Just like the Sun, SDSS J1670+1620 hosted a system of planets, asteroids and other objects. As the star underwent the turbulent transformation into a white dwarf its inner planets were destroyed, tumbling into the red giant.

The rest of the system may have survived relatively unscathed. The remaining planets would have started moving out into wider orbits as SDSS J1617+1620 shrank down to a tiny ball of carbon and oxygen the size of the Earth, with only around a third of its original mass.

Systems like SDSS J1617+1620 are referred to as evolved planetary systems. Over the past couple of decades astronomers have discovered dozens of systems like this, and we now know that they exist at around at least half of all white dwarfs.

Around 33 (and counting) white dwarfs we've detected rings of dust. These structures, as wide across as the Sun but only a few millimetres thick, are formed by asteroids thrown in towards the white dwarf by distant planets. When they get too close the gravity of the dead star overcomes the forces bonding the rock of the asteroids together, ripping them apart into a dusty disc.

Such a disc was observed around SDSS J1617+1620 in 2012. But as well as a dust disc, we also saw signs of a much rarer phenomenon: Another disc, this one made of gas.

How to find a gas disc

The way we find these gaseous discs is, in my opinion, rather cool- another one of those places where a seemingly complicated plot clearly shows what's actually there.

"Diagram" showing how we observe gas discs. Instead of a single colour in the spectrum, the light emitted from calcium is spread out into a double peak. The red- and blue-shifted light is from the parts of the disc moving away from and towards us respectively. Image Credit: Me.

The diagram above, as well as demonstrating my amazing Paint skills, shows how we find the gas discs. First the light from the white dwarf is split into a spectrum separating it out into all the different colours coming from the Star. A rainbow is the result of this happening to the Sun; in effect we are making a star rainbow.

Each element in the object emitting the light leaves its mark on the spectrum, emitting or absorbing light at very specific wavelengths. When we look at the emission from calcium, we see something odd. Instead of a sharp line, the emission is smeared out into as distinctive double-peak, with one peak on each side of where the line should be.

This is a result of the Doppler Effect. If an object emitting light is moving towards us, the light waves get bunched up, resulting in a lower wavelength. The light appears to be slightly more blue than expected. As it moves away, the light is stretched out and the wavelength increases, becoming more red. The Doppler Effect has many applications, most famously being used by Edwin Hubble to show that the universe is expanding.  

With this in mind we can see that the double-peak emission must be coming from a disc around the white dwarf. As the disc rotates, the side of it moving away from us emits red-shifted light, and the side moving towards us emits blue-shifted light.

Not only can we tell that there's a disc, but we can also measure its size and location. The amount that the light is shifted by depends on the speed of the material in the disc, which depends in turn on how far away the gas is from the white dwarf. Measuring the wavelengths of the inner and outer extent of the peaks can therefore tell us the exact position of the disc.

Many mysteries

Doing this measurement has had a surprising result. Including this one, we've seen gaseous discs around just seven white dwarfs. They should have been easy to explain: As the dust gets closer into the white dwarf, the heat should sublimate it into gas.

But when the positions of the gas discs were measured, they were all found to be too far away from the white dwarf for this to happen. The question is then: if it's not the dust sublimating, how did they form? And why do we see them at only a small number of white dwarfs?

A key missing piece of evidence is variability. As the discs are formed by asteroids, which are regularly scattered in towards the white dwarf, we know that these are highly active systems. But despite being observed over several years or even decades, none of the gas or dust discs have shown any changes with time.

Until now.

Slide show of some of the spectra of SDSS J1617+1620. In the space of two years double-peaked emission lines revealing a gaseous disc appeared out of nowhere. A couple of years later the emission had decreased dramatically, and by last year the disc had completely disappeared.

The disappearing disc

After discovering the first gaseous disc around a white dwarf in 2006, a search was made in the Sloan Digital Sky Survey for more white dwarfs with double-peaked calcium emission lines.

Two observations had been made of  SDSS J1617+1620 one in 2006 and later in 2008. The later spectrum showed clear evidence for a gaseous disc, a huge structure as wide across as the Sun. However, in the earlier spectrum the disc was nowhere to be seen.

Intrigued by this clear, unprecedented evidence for variability, we started observing the white dwarf with ever more powerful telescopes. To our surprise, the gaseous disc dispersed almost as quickly as it has formed. Just a year after the observations first showing the disc the emission from the disc had fallen by more than half. The gas disc continued to dissipate, until by 2013 there was nothing left.

How did this happen? The video above shows one possibility. We know that SDSS J1617+1620 is orbited by a dust debris disc, formed form the shattered remains of an asteroid. A second asteroid, thrown in from the outer reaches of the system by an orbiting planet, could have impacted in this disc, creating a short-lived burst of gas.

A more intriguing possibility is that we're actually seeing the formation of the dust disc itself. Depending on how close it gets to the white dwarf, an asteroid being pulled apart by gravity might not turn entirely to dust in one go. It might take several orbits, passing close to the white dwarf and loosing a little more mass to the forming disc each time before moving away again.

Each time the debris of the doomed asteroid returned it would interact with the growing dust disc, forming a gaseous layer like the one we're seen. The intriguing possibility here is that the asteroid may return again in a few years time, perhaps creating another gas disc. This could provide a key insight into the dynamics of evolved planetary systems, and we plan to keep observing this star to find out if the gas disc returns.

However it formed, we also need to try and explain how the gas disc disappeared so quickly. Here we have a helping hand, as there has been plenty of theory work done studying the behaviour of gaseous and dusty discs.

Each particle of dust orbits the white dwarf independently, interacting very rarely with the rest of the dust in the disc. The gas, however, behaves more like one big object, with each particle mixing and colliding with those around it. This has the effect of slowing the gas (and dust) down, causing it to spiral inwards and fall onto the white dwarf. The more gas, the faster the accretion.

This accretion is easily visible as metal-pollution in the otherwise pure hydrogen atmosphere of SDSS J1617+1620. We should be able to test this theory, seeing if the accretion rate changes in time with the variability of the gas disc

However when we measured the accretion rates, they stayed the same. This could mean that our ideas about the behaviour of the gas discs are wrong, but its more likely that there was too little gas to cause a noticeable effect- this would also explain the short lifetime of the disc. Alternatively there could be a delay between the formation of the disc and the change in accretion rate as the disturbed material moves inward. More reason the keep watching!

The future

 As we were finishing off this paper, another paper was published showing variability at another white dwarf- but this time it was the dusty part of the disc changing, not the gas. With clear evidence that both types of disc at white dwarfs can change, we may be able to begin to answer some of the outstanding questions about these intriguing systems.

Studying these systems also provides an insight into the future of our own Solar System. Five billion years from now, the Sun will begin the transformation into a white dwarf. The more we learn about evolved planetary systems, the better we'll understand what happens next.

The full paper is available here. I haven't gone into all of the details about what we've discovered at SDSS J1617+1620 as that would be a really long post, but if you want to find out more feel free to ask me any questions in the comments or on Twitter.

How Dead Stars are Teaching Us What You Need to Build a Planet

What I actually do for living...

Repost alert! This first appeared as a guest post on Andrew Rushby's excellent II-I blog. I'm reposting it here as I'm off to a conference at Cambridge today, so will hopefully have quite a lot to write about soon. This post was written as an introduction to the research I'm doing for my PhD, so may be useful in providing some context for what I talk about next time. If you've already read it, feel free to stop here. If not, read on!

An asteroid plummets to its doom around the white dwarf GD 29-38. Studying the debris left from these asteroids can reveal the chemical composition of exoplanets. Image Credit: NASA

Twenty seven years ago astronomers noticed something strange about the white dwarf star GD29-38.

White dwarfs are dead stars, the burnt out carbon cores of stars like our Sun which have exhausted their hydrogen fuel; incredibly dense, incredibly hot balls of matter roughly the size of the Earth. Because of this high temperature, tens of thousands of degrees, all white dwarfs glow blue.

But the light from GD 29-38 wasn’t just blue. When it was split into a spectrum, separated into a rainbow of separate colours, there seemed to be something else there. Something shining with an infrared light, beyond the range of our eyesight.

Initially the discovers were excited, as the red light could have come from an orbiting brown dwarf, a mysterious object several times bigger than a planet but much smaller than a star. But both the white dwarf and the infrared source were pulsating slightly, periodically getting brighter and dimmer. If the red light was from a separate object, then it shouldn’t have pulsed in time with the white dwarf.

The spectrum also revealed metals in the white dwarf’s atmosphere, heavy elements like calcium, magnesium and iron. These were also out of place, as white dwarfs have such a strong gravity that anything heavier than hydrogen or helium should have sunk down into their cores long ago. The metals must be falling onto the white dwarf from the space around it- but how did they get there?

It took until 2003 for the origin of the mysterious infrared glow to be found, during which time many more white dwarfs with similar red spectra and metal polluted atmospheres were found. The explanation was that the infrared light is coming from a disc of dusty debris surrounding the white dwarf.

This debris was formed from the wreckage of an asteroid, leftover from when GD29-38 was a Sun-like star with its own system of planets. The dust in the disc rains down onto the white dwarf, explaining the metals we see in the atmosphere.    

The spectrum of GD 29-38. Along the bottom is its wavelength, or colour, going from blue on the left to invisible infrared on the right. The vertical axis shows how bright the white dwarf is at each wavelength. The difference between the blue white dwarf and red dust cloud can be clearly seen. Image Credit: NASA

The story of how the debris disc got there is a result of the turbulent formation of the white dwarf. As it runs out of fuel a star swells up to a huge red giant, then blows away roughly half of its mass in an immense stellar wind, leaving the tiny white dwarf core.

 With the gravitational force at its heart cut in two, the system of planets around the dying star is thrown into chaos. Planets begin to migrate outwards, trying to reach orbits twice as far away from the central star as before. As they do this, they risk coming into close contact with each other.

Some of the planets survive these encounters and carry on as they are. Others, especially when a big Jupiter sized planet is involved, are thrown out of the system into the depths of interstellar space. And some are scattered into the centre of the system towards the white dwarf. 

These unlucky asteroids and dwarf planets fall in towards the white dwarf until they reach a point known as the tidal disruption radius. There the tidal force, the difference in gravitational pull between the parts of the asteroid nearest the white dwarf and the areas further away, becomes so great that the asteroid is ripped apart, forming the dusty debris disc that we see as an infrared glow.

The discovery of this process lead to an important conclusion. As the dust rains down onto the white dwarf it becomes visible to our telescopes. If we can measure what metals there are, and how much of each there is, then we can reveal the chemical composition of the asteroid or planet that formed the disc. We can ask, and answer, the question: “What are planets made of?”

Two decades ago we only knew about the eight planets in our solar system (Pluto was never a planet, it was just mislabelled). Now we know of over a thousand planets, new worlds orbiting hundreds of stars. Through our telescopes we can measure the size of these planets, what their masses are, and even in some cases get a glimpse into their atmospheres.

But we can’t find out what they’re made of, what the geology of these newly discovered planets is like. This means that we don’t know for sure if the way that the rocky planets are built in our solar system, the particular mix of iron, oxygen, magnesium, silicon and other chemicals that make up the Earth and its neighbours, is the way all planets are built.

The metal polluted white dwarfs form a perfect laboratory, presenting us with rocky objects that have broken apart into their chemical components. By observing as many as we can, we can begin to explore the chemical diversity of planets and planetary systems. We can see if the way our planets are built is the normal way to construct a planet, or whether Earth is even more unique than we thought.

To date we’ve discovered around a dozen white dwarfs with enough chemicals to compare their systems in detail with our own. So far, they look fairly similar to the Earth, a hopeful sign. But we need many more to truly explore this area, and over the next few years myself and others will be scouring the sky, using the Hubble Space Telescope above us and an array of telescopes on the ground. We will find more metal polluted white dwarfs, measure the chemicals of the planetary debris around them, and begin to explore in detail what things you need to build a planet.

Two Rings Discovered Around the Centaur Chariklo

The tiny world of Chariklo and its two mysterious, icy rings.

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Last Wednesday was an interesting day for astronomy, as the discovery of not one, but two strange new worlds right here in the Solar System were announced. A new dwarf planet, a tiny world orbiting the Sun far beyond Pluto, seems to have got most of the headlines. But I'm more interested in the other discovery, published here in Nature.

Most of the asteroids and other smaller worlds in the Solar System tend to be found in two areas: The Asteroid Belt  in between Mars and Jupiter, and the icy Kuiper Belt (of which Pluto is a member) out beyond the orbit of Neptune. The space in between is far from empty however.

In the late 1970s astronomers started noticing small objects orbiting in amongst the giant outer planets. Called Centaurs, these small rocks raised a lot of questions. Their orbits are inherently unstable, crossing over the orbits of  the huge gas giants. It's only a matter of time before they run smack into Jupiter, Saturn, Uranus or Neptune, colliding with them or being thrown out of the Solar System. This means they can't have been there long, so where did they come from?   

We also don't know what they really are. Are they comets, thrown in from the Oort Cloud in the depths of space? Maybe they're more like asteroids, scattered out of the Kuiper Belt by Neptune's gravity? Perhaps a mix of both? Whatever they are, they don't fit a neat explanation, and that makes them interesting.

In June 2013 several telescopes in South America were trained on the largest Centaur, a flattened sphere of rock 127 kilometers across called Chariklo. They were watching for a event called a stellar occultation, waiting for Chariklo to pass in between Earth and a background star. By looking at the way the Centaur blocked out the light coming from the star, the astronomers behind the telescope were hoping to find out things like what shape Chariklo is and how it rotates.

Right on time, the star disappeared as Chariklo moved across it, reappearing six seconds later as the Centaur moved on.

But just before it disappeared , the star dimmed very slightly for a split second, and then nearly vanished completely for a second more. As is came out from behind Chariklo it did it again, but this time in the opposite order: Flashing off, then dimming slightly.

The occultation of a background star by Chariklo, reveling the rings.

This image is one of my new favorite pictures. It shows the actual data from one of the telescopes. Time is along the bottom, going from left to right. Going up the graph shows how bright the background star was, going from 0 to 1, where zero means no light and one is the normal amount of light coming from the star. 

Going across you can clearly see the light dim a bit, dim a bit more, then go away completely as the star passes behind Chariklo. Then it comes back again and does the opposite.

This means that there must be something else blocking out the star for a moment as it gets close to Chariklo, causing two perfectly symmetrical dips in the star's light, either side of the Centaur. Only one thing would do that: Chariklo must have a pair of rings. 

Up until now we've only found four objects with rings: Jupiter, Uranus, Neptune and the stunning band of ice and dust around Saturn. These planets have one main thing in common: They're huge, many times the size of the Earth. That the fifth known ring system would belong to a rock smaller than Wales (to use a standard unit of measurement) was completely unexpected. 

The reason I like the plot so much is that is so clearly shows what's actually there: A big thing in the middle with two rings around it. Most plots need a long time and lots of extra information to work out what they show, but here it's nice and clear. 

The rings around such a small object raise a lot of questions. For a start, look at how much of the star's light the inner ring blocks. Over three-quarters doesn't get through. These rings aren't wispy clouds of dust, they're bits of ice and rock just as thick as Saturn's rings. How did rings like that form?

Chariklo's gravity is so small that is shouldn't be able to hold on to the rings for very long at all. As the Sun warms up the particles they radiate the heat away in all directions. But the Sun's heat is only coming from one direction, an imbalance that over time will give them a little kick, enough to escape Chariklo's tiny gravity. Either the rings are very young and won't be there much longer, or Chariklo has moons (that we haven't spotted) holding the rings in place. 

The discoverers suggest a few ideas about how the rings may have formed. Maybe something bumped into Chariklo, throwing up material off its surface but not so fast as to give it enough force to fly off entirely. Alternatively Chariklo may be slightly active, blowing stuff off itself in weak volcanic eruptions. Perhaps it could once have had two moons, which collided to form the ring.

We may never know the answers.  But even if the rest of Chariklo's secrets remain hidden, its intriguing and mysterious rings show us just how much we have to learn about space, even in our own back yard.

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A more traditional set of rings around Saturn. You're in this picture: Small dot, bottom right.