Our First Paper and the First Debris Disk with a White Dwarf Companion!

Congratulations, us!  Our first paper was accepted for publication in the Astrophysical Journal. The journal will copy-edit it and typeset it nicely.  But in the meantime, you can read a draft online here at the astro-ph archive. If you spot any errors, please let us know. The paper describes how the project works (you’re probably familiar with that) and announces 37 new disk candidates, including what looks like…

the first debris disk around a star with a companion white dwarf!  You met this star, HD 74389,  as subject AWI00000wz and several of you (including Artman40 and  Dolorous_Edd) immediately recognized it as a good candidate, roughly two years ago.  Well, amazingly, this critter is the first of its kind.  Stars with white dwarf companions are common; some of the brightest stars in the sky have white dwarf companions, like Sirius and Procyon.  And this star is an “early A” type star, meaning its about twice as hot as the Sun. Debris disks around “A” stars are fairly common; maybe 15-20% of A stars host debris disks.  But for some reason, nobody had ever found a star with BOTH a debris disk and a white dwarf companion.

One possible reason for why these objects are rare is that the birth of a white dwarf is somewhat violent.  In astronomy, we generally assume that the all the stars in a binary or triple system formed at the same time. More massive stars live shorter lives; they turn into white dwarfs (or neutron stars or black holes) sooner.  So that means the white dwarf in this system probably came from a star slightly more massive than the A star that has the debris disk, maybe a B type star.  This B star lost most of its mass–more than the mass of the sun–into a wind that shines for a while as a planetary nebula. This wind can blow away the small dust grains that are the part of debris disks that we actually see with our telescopes.

Sirius, the brightest star in the sky, is an A star with a white dwarf companion.  Add to this picture another companion star AND a debris disk and you’ve got HD 74389. (HST/Kuchner & Brown 1999)

However, there are three stars known that have extrasolar planets orbiting them that have white dwarf companions.  So maybe this wind isn’t all that harsh on planetary systems.   Alternatively, some theorists have suggested that the wind from an evolved star can form a new disk around the star’s companion. So maybe this disk we saw is a kind of second-generation debris disk.

To make things even weirder, HD 74389 also has an M dwarf companion; it’s what’s sometimes called a hierarchical triple, meaning that the M dwarf and the A dwarf look like a somewhat close binary, and the white dwarf orbits much farther out.  The distance to this triple system is about 111 parsecs (360 light years), based on observations from the Hipparcos telescope.  Based on that distance and the separation between the images of the A star, the M dwarf and the white dwarf, we can estimate that the white dwarf orbits roughly 2200 astronomical units (AU) away from the A star with the disk.  The M star orbits much closer in, at about 400 AU.   For comparison, Pluto’s orbit around the Sun is about 39 AU.

Besides this exotic object HD 74389, our paper reports thirty two other new debris disk candidates that we found and it describes a new detection of 22 micron excess from a previously known debris disk host: the star HD 22128. About half of the new debris disks are close enough to the Sun that they are potential targets for imaging with coronagraphs to search for extrasolar planets that could be lurking within them.

The paper reports four more interesting objects that we discovered, classical Be stars HD 6612, HD 7406, HD 164137, and HD 218546.  A classical Be star is a funny kind of beast: a rapidly rotating star surrounded by a disk made of gas. Nobody knows quite how these objects form, but it seems most likely that the gas disks are ejected from the stars themselves.  These objects are not the kind of disks that are thought to host planetary systems, but they are fun to think about nonetheless. Here’s a handy review paper about Be stars you might like.

The paper lists the user names of everyone who helped classify one of the new disks–and you may have noticed the eight of the authors of this paper are citizen scientists: Joseph Biggs, Milton Bosch, Tadeas Cernohous, Hugo A. Durantini Luca, Michiharu Hyogo, Lily Lau Wan Wah, Art Piipuu, Fernanda Pineiro.  They are members of the Disk Detective Advanced Users Group who helped do some of the more in depth background research on the stars discussed in the paper. If you’ve done more than 300 classifications and you’d like to join this group, just send an email to diskdetectives@gmail.com.

Nice work, everybody!  And we’re just getting started.  Stay tuned for more papers later this year.