Figure 1. Artist's rendition of the future evolution of the WD 0931+444 system. Credit: David A. Aguilar (Harvard-Smithsonian Center for Astrophysics).
Figure 2. Gemini time-resolved spectroscopy of H-gamma (top) and H-beta
lines (bottom) over 45 minutes. Both lines clearly show a 20 minute
periodicity.
Figure 3. The radial velocities of the Balmer lines in WD 0931+444. The
bottom panel shows all of these data points phased with the best-fit
period. The dotted line represents the best-fit model for a circular
orbit with a period of 19.8 minutes.
Figure 4. Gemini time-resolved spectroscopy of the Na I doublet (top)
and the H-alpha line (bottom) over 90 minutes. The Na I lines and the
H-alpha line from the M dwarf are stationary, whereas the H-alpha line
from the WD clearly shows a 20 minute periodicity.
Einstein's Theory of General Relativity predicts that accelerated masses
emit gravitational waves, or ripples in space-time. Even though
gravitational waves have yet to be detected directly, we expect that
there are more than hundred million gravitational wave sources in our
own galaxy. However, as of today, we know of only a few such sources.
A team of researchers, led by Dr. Mukremin Kilic of the University of
Oklahoma and Dr. Warren Brown of the Smithsonian Astrophysical
Observatory, have recently identified one of the best (and perhaps the
most powerful) gravitational wave sources currently known using the
Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope
and the Blue Channel spectrograph on the 6.5-meter MMT Telescope. Known
as WD 0931+444, the object was first identified in 1982, and classified
as a white dwarf with a low-mass M-dwarf stellar companion.
This new data from Gemini and MMT reveal that the white dwarf in this
system is not in a binary with the M dwarf. Instead, it is orbiting
another invisible white dwarf every 20 minutes. Thanks to the large
collecting area of the Gemini and MMT telescopes, the team was able to
obtain high quality optical spectroscopy of this system every 2 minutes
and resolve the 20 minute orbital period.
The orbital separation of the two stars is only 20 percent of the size
of the Sun. These two stars will lose angular momentum through
gravitational wave radiation and merge in less than nine Million years.
Depending on the inclination angle (which is currently unknown) the pair
may merge even faster, in only a few million years. This previously
unknown source is believed to be stretching everything around us (due to
gravity waves) by a factor of 10-22 (or more) every 10 minutes!
The discovery of the true nature of WD 0931+444 indicates that there are
likely many other strong gravitational wave sources hiding in plain
sight. Some of these hidden sources can be identified through further
optical follow-up observations as in this work. However, the direct
detection of gravitational waves from these sources has to wait for a
space-based gravitational wave mission like the evolved Laser
Interferometer Space Antenna, which will likely not be operational until
2034 as currently envisioned.”
The paper can be accessed at: http://arxiv.org/abs/1406.3346
Source: Gemini Observatory
