The white dwarf star GD 362 has been cooling for up to five billion years. You might think of it as an image of our Sun’s future, although it was originally about seven times more massive. As the Sun’s will do five billion years from now, this star’s core simply ran out of fuel, reaching a point where it could no longer create the heat needed to counterbalance gravity. As the star died, it would have given off stellar material, initially swelling dramatically, then dying back to the dwarf we see today.
But what has astronomers studying Gemini Observatory data talking is that GD 362 seems to be surrounded by an extensive band of dust and debris. The find is striking — gravity and radiation should long ago have removed such materials from the star’s proximity. The only reasonable explanation is that an asteroid, or perhaps something as large as a planet, has survived the demise of the star and is now contributing material for the debris disk. “The parallel to our own solar system’s eventual demise,” says Eric Becklin, UCLA astronomer and Principal Investigator for the Gemini observations, “is chilling.”
Image: Artist’s visualization of what a dust disk might look like around the white dwarf GD 362. A more distant planet (shown at upper left) might be responsible for “shepherding” the dust ring and promoting ongoing collisions. This striking image (click to enlarge) is the work of noted space artist Jon Lomberg (www.jonlomberg.com).
From a Gemini Observatory news release:
“There are just precious few scenarios that can explain so much dust around an ancient star like this,” said UCLA’s Mike Jura, who led the effort to model the dust environment around the star. “We estimate that GD 362 has been cooling now for as long as five billion years since the star’s death-throes began and in that time any dust should have been entirely eliminated.” Jura likens the disk to the familiar rings of Saturn and thinks that the dust around GD 362 could be the consequence of the relatively recent gravitational destruction of a large “parent body” that got too close to the dead star.
And GD 362 raises other mysteries. Perhaps most surprising is the abundance of metals — calcium, magnesium and iron at levels similar to our own Sun — where no heavier elements would have been expected. UCLA’s Benjamin Zuckerman, a co-author on the Gemini-based paper that will appear in an upcoming issue of the Astrophysical Journal, calls the finding “a complete surprise.” You can hear audio clips of Zuckerman discussing the star by following links on this page.
An independent team working with the NASA Infrared Telescope Facility (IRTF) has produced data that support the idea of a dust disk around GD 362. And note this comment by University of Texas grad student Mukremin Kilic, who led the team making the IRTF observations. Here he makes reference to the only other white dwarf known to have a dust disk:
“Both of these stars’ atmospheres are continuously polluted by metals — that is, heavy chemical elements — almost surely accreted from the disk,” Kilic said. “If the accretion from a debris disk can explain the amounts of heavy elements we find in white dwarfs, it would mean that metal-rich white dwarfs — and this is fully 25% of all white dwarfs — may have debris disks, and therefore planetary systems, around them. Planetary systems may be more numerous than we thought.” More on the IRTF work can be found here.
Astrophysics, abstract
astro-ph/0701549
From: Mukremin Kilic [view email]
Date: Thu, 18 Jan 2007 21:05:13 GMT (52kb)
A Dusty Disk Around WD1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks
Authors: Mukremin Kilic (Ohio State), Seth Redfield (Texas)
Comments: ApJ, in press
We report the discovery of excess K-band radiation from a metal-rich DAV white dwarf star, WD1150-153. Our near infrared spectroscopic observations show that the excess radiation cannot be explained by a (sub)stellar companion, and is likely to be caused by a debris disk similar to the other DAZ white dwarfs with circumstellar debris disks.
We find that the fraction of DAZ white dwarfs with detectable debris disks is at least 14%. We also revisit the problem of explaining the metals in white dwarf photospheres by accretion from the interstellar medium (ISM). We use the observed interstellar column densities toward stars in close angular proximity and similar distance as DAZ white dwarfs to constrain the contribution of accretion from the ISM.
We find no correlation between the accretion density required to supply metals observed in DAZs with the densities observed in their interstellar environment, indicating that ISM accretion alone cannot explain the presence of metals in nearby DAZ white dwarfs.
Although ISM accretion will certainly contribute, our analysis indicates that it is not the dominant source of metals for most DAZ white dwarfs. Instead, the growing number of circumstellar debris disks around DAZs suggests that circumstellar material may play a more dominant role in polluting the white dwarf atmospheres.
http://arxiv.org/abs/astro-ph/0701549
Astrophysics, abstract
astro-ph/0701560
From: E.J.M. van den Besselaar [view email]
Date: Fri, 19 Jan 2007 12:26:40 GMT (281kb)
DE CVn: A bright, eclipsing red dwarf – white dwarf binary
Authors: E.J.M. van den Besselaar, R. Greimel, L. Morales-Rueda, G. Nelemans, J.R. Thorstensen, T.R. Marsh, V.S. Dhillon, R.M. Robb, D.D. Balam, E.W. Guenther, J. Kemp, T. Augusteijn, P.J. Groot
Comments: 12 pages, 9 figures, 6 tables. Accepted for publication in A&A
Close white dwarf – red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf – red dwarf binary with a relatively short (~8.7 hours) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters which we discuss in the frame work of common-envelope evolution. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average low-resolution spectrum of DE CVn we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. The derived ephemeris is HJD_min = 2452784.5533(1) + 0.3641394(2) x E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8000 K. The inclination of the system is 86 (+3, -2) deg and the mass and radius of the red dwarf are 0.41 +/- 0.06 M_sun and 0.37 (+0.06, -0.007) R_sun, respectively, and the mass and radius of the white dwarf are 0.51 (+0.06, -0.02) M_sun and 0.0136 (+0.0008, -0.0002) R_sun, respectively. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively low-mass star (M