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A Dwarfs and K Giants

A list of 102 interesting objects that you helped pick for follow up (let us call them Disk Detective Objects of Interest, or DDOIs) shows that many of the stars with disks we locate will be A dwarfs or K giant stars. We don’t yet know all the spectral types of the DDOI stars precisely, but you can see the distribution of the types we do know in the figure below. The peaks correspond to A dwarfs and K giants.

Distribution of DDOIs according to spectral type. The two peaks at AV and K show that most stars hosting disks in our list are stars of type A and K that are about twice as massive as our Sun.

Distribution of DDOIs according to spectral type. The two peaks at AV and K show that most stars hosting disks in our list are stars of type A and K that are about twice as massive as our Sun.

So what are A dwarfs and K giants? “A” dwarfs are very hot, fast spinning and blue stars that are younger and brighter than “G” stars such as our Sun. The bright stars Sirius and Vega are some well known A dwarfs. Many of the best studied debris disks are around A dwarfs.

What are these “K giants”? K giants and A dwarfs are two sides of the same coin.  Let’s talk a bit about the life cycle of a typical star.

Most ordinary stars like our Sun burn hydrogen fuel for many millions of years. Once all the hydrogen is used up however, the star balloons in size and becomes a red giant. In the far future when our own Sun becomes a red giant, it will become so big that it will swallow up Mercury, Venus and possibly the Earth. Giants also tend to steadily lose a lot of their own mass all the time. This is because hot winds are blowing off the gas that is part of the star. (This hot gas is tricky because it might be mistaken for a dusty disk)

K giants are former A stars that have evolved for hundreds of millions of years. Like the sun, they have burned through their hydrogen, and ballooned up in size. Both A and K stars are about twice as massive as our Sun.

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Left: Artist’s impression of Sirius, and A dwarf. Credit: NASA, ESA, G. Bacon

K giants are also really interesting because Jupiter-sized exoplanets orbiting these old, giant stars have been found to be more common than Jupiter-sized exoplanets orbiting less massive stars that are still on the main sequence. These exoplanets around K giants have been found by the popular radial velocity (Doppler shift) method.

Also, some of these K giants have debris disks, sometimes even dustier than their younger counterparts. This is surprising, because giants are very bright and light from the star exerts radiation pressure on small dust particles that ought to blow the dust away, or cause them to slow down and spiral into the star and be swallowed up.

So where is the dust around these K giants coming from? Nobody really knows yet, but there are several hypotheses. One is that dust is coming from the star itself. Another is that the dust is in fact interstellar dust in our galaxy. A third is that giants are breaking up more comets. Whatever the cause, we have a lot of K giants in our list of DDOIs that potentially have dusty disks–so once we can follow these up with telescopes we will be able to help solve this mystery.

Dawoon Jung (@dirkpitt2050) is a graduate student at the International Space University currently at NASA Goddard Space Flight Center doing a summer internship with the Disk Detective team. He was born in Korea, and is interested in exoplanets and space flight.

​Herschel image of κ Coronae Borealis 31.1 parsecs away. This K giant is about twice as massive as our Sun. The red regions correspond to dust orbiting the star. Interestingly, this star also hosts at least one exoplanet with a mass of about 2 Jupiters. Credit: Bonsor et al. 2013.

​Herschel image of κ Coronae Borealis 31.1 parsecs away. This K giant is about twice as massive as our Sun. The red regions correspond to dust orbiting the star. Interestingly, this star also hosts at least one exoplanet with a mass of about 2 Jupiters. Credit: Bonsor et al. 2013.

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Chasing Dust Around Dead Stars

The typical place to find dusty debris disks is orbiting around ordinary stars like the Sun, or younger stars that  are in the process of forming terrestrial planets.  But some dusty disks that you might spot in Disk Detective surround tiny, exotic dead stars called white dwarfs.

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An artist’s impression of the evolution of a planetary system from middle-age to the star’s eventual death as a white dwarf. In the process, surviving asteroids can form a dusty disk around the white dwarf. Credit: Mike Garlick

Most ordinary stars like the Sun will end their lives by bloating up into red giants.  The cores of the red giant stars are balls of mostly carbon and oxygen, the spent fuel of the nuclear burning that powers the star.  Red giant stars blow away much of their mass in winds, and eventually become stripped down to naked cores. It is called a white dwarf when those winds are done blowing, and there is nothing left of the star but its carbon and oxygen core covered with a very thin layer of hydrogen gas. White dwarfs are just a bit larger than the Earth in diameter, though they generally weigh around three-quarters as much as the Sun!

For a long time people thought that the process of stellar evolution into white dwarfs meant certain death for any planetary systems that might orbit the original stars.  Indeed, inner terrestrial planets may indeed be destroyed by this process. But  large asteroids, giant planets, and potentially even icy bodies might survive if they are big enough and far enough away to weather the drastic changes that a star goes through. Astronomers are now studying the nearest white dwarfs for signatures of these possible surviving planets.

One telltale signature is the presence of dust orbiting a white dwarf.  If we look at a white dwarf in infrared light, with a telescope like the WISE telescope, a dusty white dwarf will be brighter there than at shorter wavelengths, just like the targets of Disk Detective.  Thanks to many observations of white dwarfs with the Spitzer Space Telescope, WISE, and NASA’s ground-based Infrared Telescope Facility, we have discovered a few dozen dusty white dwarfs in the last 10 years.  We believe the dust around these stellar ghosts comes from asteroids kicked too close to the central white dwarf by a larger planet. When this happens, the asteroid shreds apart much like Comet Shoemaker-Levy 9 did when it collided with Jupiter in 1994, and the pieces of it form a disk around the white dwarf.  The size of the disk is not much larger than Saturn’s rings.

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A collection of nearby white dwarfs and their accompanying dust disks. The dust disks are drawn to scale with Saturn’s rings. Credit: John Debes

Dust from the disk continues to rain down on the white dwarf surface in the form of atomic gas.  Astronomers can measure the composition of this atomic gas—and thereby that of the dust—by taking optical and ultraviolet spectra of the white dwarfs.  Just like characters from TV shows like CSI break materials found in a crime scene down to their component elements to identify them, astronomers compare the pattern of elements to Solar System bodies to understand the origin of the dust. Sure enough, they look just like asteroids in our own Solar System.

By chasing this elusive dust around dead stars, astronomers are using these clues to piece together the chemical history of terrestrial planet formation around other stars using the shredded remnants of exo-asteroids.  The information they gain from studying white dwarfs might be able to tell us whether terrestrial planets have similar properties to our own Earth.

John Debes

@johndebes

Disk Detective and Planet Hunters

A few folks have asked us: what’s the relationship between Disk Detective and Planet Hunters? Planet Hunters, of course, is the Zooniverse citizen science website that invites users to examine data from NASA’s Kepler mission to search for extrasolar planets.

The success of Planet Hunters helped inspire us to launch Disk Detective!  But beyond that, there are several scientific connections between the two projects. Both are about extrasolaKepler Field of View Star Chartr planets. As you probably know, in Planet Hunters, users look at measurements of a star’s brightness, checking for sudden dips that could indicate a planet crossing in front of the star (called “transits”).  In Disk Detective, we search for the homes of planets: stars surrounded by disks where planets form and often dwell.

Let’s talk more specifically–about what stars the two projects have in common.  First of all, the data from the WISE mission that we’re examining at Disk Detective covers the whole sky.  So it overlaps with everything, including the part of the sky that Kepler/Planet Hunters has already studied and whatever parts of the sky Kepler will image in the future. Indeed, the part of the sky Kepler has already examined has already been searched for disks at least once; Samantha Lawler and Brett Gladman claimed to find eight debris disks around stars with Kepler planets in 2012, using data from the WISE mission. However, further studies of the Kepler field were unable to replicate this result. The map above illustrates the current Kepler field, mostly located within the constellation of Cygnus.

But there will be more such Kepler/WISE disks for us to find via Disk Detective and Planet Hunters.  For one, both the Kepler and WISE databases have improved substantially since that work was done.  Kepler has found more transiting planets, and WISE scanned the sky again, leading to the new ALLWISE data release this fall.

Moreover, plans are afoot to extend the Kepler mission.  The extended mission, called “K2” will search for planets in a different region of sky, near the plane of the Earth’s orbit. Here at Disk Detectives, we will already be searching that region for disks.  And I’m pretty sure the new K2 data will be searchable at Planet Hunters as well.

So stay tuned–and keep digging for new disks!  You might find one around a star that Kepler has already found planets around, or that it will find planets around soon.  And even if there is not a direct match, we still learn by combining the statistical information from both surveys about how and where planets form.

“It has long been an axiom of mine that the little things are infinitely the most important”

Marc Kuchner