By John Debes, ESA/AURA astronomer at STScI
It is quite the April Fool’s prank to hide in plain sight, and that’s just what the rather pedestrian-appearing WD 1242-105 did for several years, masquerading as a single white dwarf. The secret it held, and the story of how my team of researchers unraveled it, makes for a useful lesson in how science sometimes progresses—not always by careful predictions, but sometimes by serendipity. Furthermore, our discovery also demonstrates how astronomy often requires close collaboration.
WD 1242-105 (See Figure 1), resides near the constellation Virgo, and was first discovered as part of the large Palomar-Green Survey of UV-excess sources (Green et al., 1986). In that survey, it was promptly misclassified as a subdwarf star. Sixteen years later, Salim & Gould (2002) recognized that this might be a white dwarf candidate based on its apparent motion on the sky—it was large compared to its faint apparent magnitude. This is often how new white dwarfs are discovered, since astrophysical objects closer to the Earth have larger apparent motions, and white dwarfs are intrinsically faint.
Figure 1: A false color image of WD 1242-105 (center), which is a rather inconspicuous star. The hint of its high motion on the sky is given by the slight blue/red color–this is due to the fact that the binary had moved between when the two photographs of this star were taken.
Surprisingly, low-resolution spectra were taken and did not detect anything unusual about the white dwarf (Kawka et al. 2004; Kawka & Vennes 2006). Based on this spectrum, it appeared to be a relatively close, single white dwarf that was about 75 lightyears away. By a quirk of fate, there was a large high resolution spectroscopic survey of white dwarfs looking for binary objects and using the Very Large Telescope, but this particular white dwarf was not included.
This is where I came in. As a postdoctoral researcher within the Department of Terrestrial Magnetism (DTM) at the time, I had access to the premiere high resolution spectrograph at Las Campanas Observatory’s Magellan Telescope, called MIKE. I was using it to survey as many southern hemisphere white dwarfs as I possibly could for traces of metals in their atmospheres. Since no one had yet published a high resolution spectrum of WD 1242-105, and it appeared to reside within the solar neighborhood, it was a perfect target. I consequently observed the star three times, each exposure separated by ten minutes.
When I finally got around to analyzing the spectrum of WD 1242-105 a few months later, I was in for a shock. Instead of the usual single spectral line of Hydrogen (See Figure 2) one sees in white dwarf spectra, there were two separate lines—indicative of a binary system consisting of two hydrogen white dwarfs. By a sheer stroke of luck, I had caught the binary when both stars were at their maximal relative velocities to our line of sight—the lines were separated by almost 6.5 Angstroms, or 300 km/s. They were moving so fast, that I could detect changes in their radial velocities on the timescale of each exposure, or ten minutes!
Figure 2: (left) A comparison between synthetic model white dwarf spectra (red lines) with the MIKE observations of WD 1242-105. The two Hydrogen line components are most visible around the Hα spectral line. (right) A comparison between our model photometry and the observed photometry of this star from optical to mid-IR wavelengths.
Over the next year, I prepared to take more spectra, but by this time I needed to call in some favors. This binary could have been a progenitor of the famous Type Ia supernovae, and so far not many convincing progenitors have been found. Many of my colleagues at DTM also used MIKE, and I asked for help in taking additional spectra. Others were experienced at taking high precision time series photometry of stars, and so I asked them to monitor WD 1242-105. Finally, another friend of mine was working on a project to measure the parallaxes of nearby stars for evidence of planetary systems—I enlisted that team’s help to measure the distance to this interesting binary.
In the end we had a complete picture for this system’s binary parameters, which enabled us to estimate the individual white dwarf masses (See Figure 3). We also were able to estimate the masses of the two components using the hydrogen lines themselves as tracers of the white dwarfs’ temperatures and gravities. Since this was a binary, the distance obtained from our parallax measurement was actually about 60% larger than originally assumed, or 40 pc (130 light years). The binary has an orbital period just shy of 3 hours, and a total mass of 0.9 M☉. While its mass makes it too light to be a progenitor of a Type Ia supernova, the two components of WD 1242-105 will merge within the next 800 million years in what will no doubt be a remarkable show. Binaries like WD 1242-105 are believed to be the progenitors of objects such as R Corona Borealis stars, which are believed to arise from the merger of two white dwarfs.
Figure 3: Radial Velocity curve of the WD 1242-105 double degenerate binary system. Red points are derived from the less massive component, while the blue points represent the more massive white dwarf. The second panel shows the residuals after subtracting off the orbital fit to the data.
Since it is so close to Earth, it is one of the largest known sources of expected gravitational wave radiation at frequencies within the mHz range. Unfortunately, this is too low a frequency for detection with some proposed space-based future gravitational wave observatories such as eLisa, but nevertheless, it will “shine” brightly in the gravitational wave sky, despite its rather bland optical appearance.
Sometimes, the most interesting things are hiding in plain sight, and they just require the right approach or instrument to discover them.
The paper related to WD 1242-105 can be found at this link.
Green, R. F., Schmidt, M., & Liebert, J. 1986, ApJS, 61, 305
Kawka, A. & Vennes, S. 2006, ApJ, 643, 402
Kawka, A., Vennes, S., & Thorstensen, J. R. 2004, AJ, 127, 1702
Salim, S. & Gould, A. 2002 ApJ, 575, L83
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