Glowing dust doughnut circles white dwarf

Astroboffins grab first image of pulverised asteroid disks

Extremely patient astroboffins have put together the first image of debris rings around a white dwarf, obtained over 12 years of Very Large Telescope observations.

Researchers led by Christopher Manser of the University of Warwick’s Astronomy and Astrophysics Group used Doppler tomography* to snap the rings of SDSS1228+1040, likely comprised of the dust particle and debris remains of an asteroid captured and ripped apart by the star's gravity.

Manser said: "We knew about these debris disks around white dwarfs for over twenty years, but have only now been able to obtain the first image of one of these disks."

The Doppler Tomography image of the white dwarf and rings

The picture features a couple of solar radii fractions for scale, "corresponding to material in circular orbits at two different distances from the star", according to the Astrophysics Group's professor Boris Gänsicke.

It's "an unusual type of image", Gänsicke admitted, because it shows "the velocities of the gas in the disc around the white dwarf rather than its position".

The result is an "inside out" snapshot. Manser explained to The Register: "Just like in the solar system, planets that are further out will go around the Sun slower [See Kepler's second law of planetary motion]. So in the case of the image, the material that is near the edge of the image with higher velocities, is actually closer to the white dwarf."

Helpfully, Warwick Uni has provided an artist's impression of what the rings look like the right way round, glowing a deep red as gas within them is illuminated by ultraviolet light from the white dwarf. Saturn is included to demonstrate how the system measures up (click for a bigger version):

The white dwarf rings and Saturn compared

Manser explained: "The diameter of the gap inside of the debris ring is 700,000 kilometres, approximately half the size of the Sun and the same space could fit both Saturn and its rings, which are only around 270,000 km across. At the same time, the white dwarf is seven times smaller than Saturn but weighs 2,500 times more."

Regarding the rings' structure, professor Gänsicke said: "When we discovered this debris disk back in 2006, we thought we saw some signs of an asymmetric shape. However, we could not have imagined the exquisite details that are now visible in this image constructed from twelve years of data - it was definitely worth the wait."

He concluded: "Over the past decade, we have learned that remnants of planetary systems around white dwarfs are ubiquitous, and over thirty debris disks have been found by now. While most of them are in a stable state, just like Saturn's rings, a handful are seen to change, and it is those systems that can tell us something about how these rings are formed.”

The findings - published imminently as Doppler-imaging of the planetary debris disc at the white dwarf SDSS J122859.93+104032.9 in the Monthly Notices of the Royal Astronomical Society - might offer an insight into the ultimate fate of our own solar system.

Our Sun is destined to one day become a white dwarf, so it's possible we might all end up as part of a glowing dust doughnut, gently bathed in ultraviolet light.** ®

Bootnotes

*As we understand it, the extended time taken to obtain the Doppler tomography data is because of the need to observe the disks as they slowly rotate in relation to the observer (on Earth, in this case).

Handily, Low Orbit Helium Assisted Navigator (LOHAN) fan and astrophysist Dr Joni Johnson, of New Mexico State University, was able to provide us with an overview of how this Doppler malarkey works:

I'll presume that readers are familiar with the Doppler effect. Any given emission or absorption line that we see from a star or an accretion disk is going to be the sum of all the bits in the detector's field of view.

In a single star, this leads to rotational broadening of the spectral line due to one side of the star rotating towards us and getting blue-shifted with respect to the central (meaning at the star's average velocity) wavelength, and the other side that's rotating away from us producing the red-shifted portion of the line. A normal line has a Gaussian (bell curve) profile with the left/blue/high energy side coming towards us, the red side going away.

In a system with an accretion disk, it's obviously more complicated. However, we can take advantage of this and obtain observations at different orbital phases. The line profile will change as the components of the system orbit around, with a component changing from producing a blue-shifted line to a red-shifted line.

The idea is to build up a time-series of one or more spectral lines over the orbit. The spectral line has to have high enough resolution to show good velocity separation.

So what we get are a series of spectra with different components at different velocities that change with time. From this, with a few assumptions, we can build up a picture of what bits are stationary, what bits are moving, and how fast the various bits are moving. The lines will also show how much material is doing the emitting (density) and an estimate of the temperature if they have the right lines. The end result will be a map showing the distribution of material in the disk.

The "normal" pattern is that the material orbits like good ol' Kepler said: the square of the period of the orbit is proportional to the cube of the semimajor axis of the orbit.

Thanks very much to Joni for her input, and there's more on Doppler tomography in this 2004 paper (PDF) by T.R. Marsh of the University of Warwick.

**Yes, we know - the Sun's red giant phase will probably have done for poor old Earth long before that. Thankfully, that's not for another 5 to 6 billion years, so we've got plenty of time to get a few pints in before final closing time.


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