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John H. Debes

ESA/AURA Astronomer, COS/STIS Instrument Team

The WISE Infrared Excesses around Degenerates (WIRED) Survey

The WISE mission is a NASA all-sky Infrared Survey Telescope that recently finished observing at 4 different wavelengths in the mid-IR. The shortest two wavelengths, 3.4 and 4.6 microns, are perfect for detecting the excess emission around a white dwarf from a companion brown dwarf or dusty disk. The WIRED team, (led by Stefanie Wachter), of which I am a member, is searching for new excesses using a combination of optical, near-infrared, and WISE photometry for new IR excess WDs. To learn more about why dusty white dwarfs are interesting, check out my page on the Structure and Origin of Dusty White Dwarfs

GALEX J193156.8+011745: A Dusty, Metal-Rich White Dwarf

The first WIRED result comes from the WISE detection of a newly discovered WD called GALEX J193156.8+011745 (or GALEX J1931+0117 for short). It was a fairly nearby, bright WD that had never been discovered before, but was easily found with data from the recent NASA GALEX satellite, which observed the sky in the UV. When another group took spectra of the WD, it showed strong absorption lines from various metals, like Fe, Si, and Ca, and showed a possible NIR excess due to a disk or companion (for more info, see its discovery paper. The WIRED team quickly sifted through available WISE data to discover that they too found an excess at longer wavelengths. Our team, however, took a much closer look in the Near-IR and found that the photometry of GALEX J1931+0177 was a bit off thanks to some nearby faint objects that weren't noticed before. These little objects are in the background and look like stars, and so don't contaminate the WISE photometry--thus providing better information on determining the origin of the excess observed. We think that the extra infrared emission is coming from dust a few tens of white dwarf radii from the GALEX J1931+0117, that probably arrived from a recently disrupted asteroid. The image below shows the spectrum of GALEX J1931+0117:

The squares are photometry of GALEX J1931+0117, while the asterisks show the expected photometry from the WD if it had no dust. The solid and dashed lines show different dust models, including a very narrow ring of dust, or a wider ring of dust--both of which are allowed by the data.

How many Dusty White Dwarfs are hiding in a group of 20,000?

The WIRED team has just submitted a paper to ApJ Supplements using WISE data to take the measure of almost 20,000 white dwarfs that have been identified in the preliminary Sloan Digital Sky Survey DR7 Catalogue of White Dwarfs. Of those almost 20,000 objects, we found over 1500 detections with WISE. Taking a combination of SDSS, 2MASS, UKIDSS, and WISE photometry, we can search these new IR detections for new dusty white dwarfs and white dwarf+brown dwarf systems. Watch this space for new developments from our program, but a preview is below, where we show some plots of our white dwarfs detected with WISE:

This first plot shows (top) a histogram of all the WDs detected with the W1 WISE channel (lambda=3.36 micron), as well as histograms of (middle) the WDs detected in W2 (lambda=4.6 micron) and (bottom) the total numbers of WDs as a function of W1-W2 color. We detect a handful of objects fainter than the nominal sensitivities of WISE.

This second figure shows various color-color plots of our targets, grouped by type. Orange circles represent WD+M dwarf systems, black circles represent naked WDs, blue triangles represent candidate WD+brown dwarf systems, red circles represent candidate dusty disk systems, and green upside down triangles represent systems whose origin is uncertain between a companion or a dust disk. Open squares represent known dusty WDs that have been observed with WISE.