Candidate white dwarf stars within the area to be mapped by the Dark Energy Survey.
Images courtesy Douglas Tucker
The Experimental Astrophysics group’s Douglas Tucker spent some time early this month in Chile, where he observed white dwarf stars at the Cerro Tololo Inter-American Observatory for some preliminary work for the Dark Energy Survey (DES). The survey itself will also take place at the observatory using the Victor M. Blanco 4-meter telescope; however, Tucker and another researcher from Austin Peay State University in Tennessee used the 1-meter telescope for their 11-night run.
The purpose of the observation was to confirm whether stars suspected to be DA white dwarfs actually are. These particular stars are observed not only because there are a lot of them within the DES footprint, but also because they have a pure hydrogen atmosphere, which results in a smooth spectrum that makes taking measurements and analyzing data easier. Thanks to cooperation from the weather, about half of the candidate stars were successfully observed, many more than originally anticipated.
Scientists will use confirmed DA white dwarfs as calibration stars for DES. The images they take during observation provide data such as how many photons are detected—or how bright the star appears to be. “But observed brightness needs to relate to a physical quantity like watts per meter-squared or some other measure of brightness,” says Tucker. “You do that by using stars of known brightness, so when DES observes these stars later, we’ll already know their brightness and can then calibrate other stars and galaxies in the DES using these stars.”
To confirm that the target star (circled) is a white dwarf,
researchers use analysis software
to compare the star's magnitude of brightness to that of known stars (known as a "standard" stars). To do this, they must pass through various photometric filters (r=red, g=green, u=ultraviolet).
Click image for an animation to illustrate responses for various filter passbands.
Tucker and his colleagues took advantage of results from a previous study that were published a few months ago that cut down on the number of potential white dwarfs. Until then, no good catalogs of such stars in the southern skies existed. The study used data from a multi-decade-long photographic sky survey and then compared stars’ movement relative to other stars.
“White dwarfs are stars, like our sun,” says Tucker. “Unlike our sun, though, they’ve used most of their fuel and are near the end of their lifecycle, so they’re usually not very bright. If you see them, they’re probably pretty close to the earth.” And stars close to the earth appear to move faster in the sky.
To cull the sample from that study, Tucker observed in three different photometric filters developed by the Sloan Digital Sky Survey. “As it turns out, if you plot how bright they are in these different filters, DA white dwarfs appear to be very blue.”
The morning after each night's observing, raw data from the observatory was synchronized to the bluearc disk at Fermilab—a total of 15 Gigabytes for the 11-night run. The data will be processed on the EAG SDSS/DES computing cluster using a suite of python scripts based on code originally developed for another project by Liz Buckley-Geer and Huan Lin and extensively modified by Tucker.
Once the researchers have a sample of good white dwarfs, they will use a spectrograph during an additional observing run to take and measure spectra of the stars. This data will help further understand the characteristics of each star so that they may be used as calibration stars in DES.
~ Marcia Teckenbrock