Illustration of J075141 & J174140
Credit X-ray: NASA/CXC/Univ of Oklahoma/M.Kilic et al, Optical: SDSS,
Credit X-ray: NASA/CXC/Univ of Oklahoma/M.Kilic et al, Optical: SDSS,
Illustration: NASA/CXC/M.Weiss
In the middle of the twentieth century, an unusual star was spotted in the constellation of Canes Venatici
(Latin for "hunting dogs"). Years later, astronomers determined that
this object, dubbed AM Canum Venaticorum (or, AM CVn, for short), was,
in fact, two stars. These stars revolve around each other every 18
minutes, and are predicted to generate gravitational waves - ripples in
space-time predicted by Einstein.
The name AM CVn came to represent a new class of objects where one white dwarf star is pulling matter from a very compact companion star, such as a second white dwarf. (White dwarf stars are dense remains of Sun-like stars that have run out of fuel and collapsed
to the size of the Earth.) The pairs of stars in AM CVn systems orbit
each other extremely rapidly, whipping around one another in an hour,
and in one case as quickly as five minutes. By contrast, the fastest
orbiting planet in our Solar System, Mercury, orbits the Sun once every 88 days.
Despite being known for almost 50 years, the question has remained: where do AM CVn systems come from? New X-ray and optical observations
have begun to answer that with the discovery of the first known systems
of double stars that astronomers think will evolve into AM CVn systems.
The two binary systems - known by their shortened names of J0751 and J1741 - were observed in X-rays by NASA's Chandra X-ray Observatory and ESA's XMM-Newton telescope.
Observations at optical wavelengths were made using the McDonald
Observatory's 2.1-meter telescope in Texas, and the Mt. John Observatory
1.0-meter telescope in New Zealand.
The artist's illustration depicts what these systems are like now and
what may happen to them in the future. The top panel shows the current
state of the binary that contains one white dwarf (on the right) with
about one-fifth the mass of the Sun and another much heavier and more
compact white dwarf about five or more times as massive (unlike Sun-like
stars, heavier white dwarfs are smaller).
As the two white dwarfs orbit around each other, gravitational waves
will be given off causing the orbit to become tighter. Eventually the
smaller, heavier white dwarf will start pulling matter from the larger,
lighter one, as shown in the middle panel, forming an AM CVn system.
This process continues until so much matter accumulates on the more
massive white dwarf that a thermonuclear explosion may occur in about
100 million years.
Credit: X-ray: NASA/CXC/Univ of Oklahoma/M.Kilic et al,
Optical: SDSS
One possibility is that the thermonuclear explosion could destroy the larger white dwarf completely in what astronomers call a Type Ia supernova (the type of supernova used to mark large distances across the Universe by serving as so-called standard candles.) However, it's more likely that a thermonuclear explosion will occur only on the surface of the star, leaving it scarred but intact. The resulting outburst is likely to be about one tenth the brightness of a Type Ia supernova. Such outbursts have been named - somewhat tongue-in-cheek - as .Ia supernovae. Such .Ia outbursts have been observed in other galaxies, but J0751 and J1741 are the first binary stars known which can produce .Ia outbursts in the future.
The optical observations were critical in identifying the two white
dwarfs in these systems and ascertaining their masses. The X-ray
observations were needed to rule out the possibility that J0751 and
J1741 contained neutron stars.
A neutron star - which would disqualify it from being a possible parent
to an AM CVn system - would give off strong X-ray emission due to its
magnetic field and rapid rotation. Neither Chandra nor XMM-Newton
detected any X-rays from these systems.
AM CVn systems are of interest to scientists because they are
predicted to be sources of gravitational waves, as noted above. This is
important because even though such waves have yet to be detected, many
scientists and engineers are working on instruments that should be able
to detect them in the near future. This will open a significant new
observational window to the universe.
The paper reporting these results is available online
and is published in the Monthly Notices of the Royal Astronomical
Society Letters. The authors are Mukremin Kilic, from the University of
Oklahoma in Norman, OK; J.J. Hermes from the University of Texas at
Austin in TX; Alexandros Gianninas from the University of Oklahoma;
Warren Brown from Smithsonian Astrophysical Observatory in Cambridge,
MA; Craig Heinke from University of Alberta, in Edmonton, Canada; Marcel
Agüeros from Columbia University in New York, NY; Paul Chote and Denis
Sullivan from Victoria University of Wellington, New Zealand; and Keaton
Bell and Samuel Harrold from University of Texas at Austin.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory in Cambridge, Mass., controls
Chandra's science and flight operations.
Scale: Image is 7 arcmin across (about 11 light years).
Category: White Dwarfs & Planetary Nebulas
Coordinates (J2000): RA 07h 51m 41.20s | Dec -01° 41' 20.90"
Constellation: Monoceros
Observation Dates: 22 Dec 2012
Observation Time: 1 hours 7 min
Obs. IDs: 14608
Instrument: ACIS
References: Kilic, M. et al, 2013, MNRAS Letters (in press); arxiv:1310.6359
Color Code: X-ray (Pink); Optical (Red, Green, Blue)
Distance Estimate: About 5,500 light years
