The VLT Unravels the Nature of the Fastest Binary Star

An amazing stellar binary system

One year is the time it takes the Earth to move once around the
Sun, our central star. This may seem quite fast when measured on the
scale of the Universe, but this is a snail’s motion compared to the
the speed of two recently discovered stars. They revolve around each
other 100,000 times faster; one full revolution takes only 321
seconds, or a little more than 5 minutes! It is the shortest
period ever observed in a binary stellar system.

This is the surprising conclusion reached by an international team
of astronomers led by GianLuca Israel of the Astronomical
Observatory of Rome [1], and based on detailed
observations of the faint light from these two stars with some of the
world’s most advanced telescopes. The record-holding binary stellar
system bears the prosaic name RX J0806.3+1527 and it is located
north of the celestial equator in the constellation Cancer (The
Crab).

The scientists also find that the two partners in this hectic dance
are most likely a dying white dwarf star, trapped in the strong
gravitational grip of another, somewhat heavier star of the same
exotic type. The two Earth-size stars are separated by only 80,000
kilometers, a little more than twice the altitude of the
TV-broadcasting satellites in orbit around the Earth, or just one
fifth of the distance to the Moon.

The orbital motion is very fast indeed – over 1,000 km/sec, and the
lighter star apparently always turns the same hemisphere towards its
companion, just as the Moon in its orbit around Earth. Thus, that star
also makes one full turn around its axis in only 5 minutes, i.e. its
“day” is exactly as long as its “year”.

The discovery of RX J0806.3+1527

The visible light emitted by this unusual system is very faint, but
it radiates comparatively strong X-rays. It was due to this emission
that it was first detected as a celestial X-ray source of unknown
origin by the German ROSAT space
observatory in 1994. Later it was found to be a periodically variable
source [2]. Once every 5 minutes, the X-ray
radiation disappears for a couple of minutes. It was recently studied
in greater detail by the NASA Chandra observatory.

The position of the X-ray source in the sky was localised with
sufficient accuracy to reveal a very faint visible-light emitting
object in the same direction, over one million times weaker than the
faintest star that can be seen by unaided eye (V-magnitude
21.1). Follow-up observations were carried out with several world
class telescopes, including the ESO Very Large Telescope
(VLT) at the Paranal
Observatory in Chile, and also the Telescopio Nazionale Galileo
(TNG), the Italian 4-m class observatory at the Roche de
Muchachos Observatory on La Palma in the Canary Islands.

The nature of RX J0806.3+1527

The observations in visible light also showed the same effect:
RX J0806.3+1527 was getting dimmer once every 5 minutes, while
no other periodic modulation was seen. By observing the spectrum of
this faint object with the FORS1 multi-mode instrument on the 8.2-m
VLT ANTU telescope, the as2omers were able to determine the
composition of RX J0806.3+1527. It was found to contain
large amounts of helium; this is unlike most other stars, which
are mainly made up of hydrogen.

“At the outset, we thought that this was just another of the usual
binary systems that emit X-rays”, says Gianluca Israel. “None
of us could imagine the real nature of this object. We finally solved
the puzzle by eliminating all other possibilities one by one, while we
kept collecting more data. As the famous detective said: when you have
eliminated the impossible, whatever remains, however improbable, must
be the truth!”.

Current theory predicts that the two stars, which are bound
together by gravity in this tight system, produce X rays when one of
them acts as a giant “vacuum cleaner”, drawing gas off its companion.
That star has already lost a significant fraction of its mass during
this process.

The incoming matter impacts at high speed on the surface of the
other star and the corresponding area – a “hot spot” – is heated to
some 250,000 °C, whereby X rays are emitted. This radiation
disappears for a short time during each orbital revolution when this
area is on the far side of the accreting star, as seen from the
Earth.

A very rare class of stars

Our Sun is a normal star of comparatively low mass and it will
eventually develop into a white dwarf star. Contrary to the violent
demise of heavier stars in a glorious supernova explosion, this is a
comparatively “quiet” process during which the star slowly cools while
losing energy. It shrinks until it finally becomes as small as the
Earth.

The Sun is a single star. However when a solar-like star is a
member of a binary system, the evolution of its component stars is
more complicated. During an initial phase, one star continues to move
along an orbit that is actually inside the outer, very tenuous
atmospheric layers of its companion. Then the system rids itself of
this matter and develops into a binary system with two orbiting white
dwarf stars, like RX J0806.3+1527.

Systems in which the orbital period is very short (less than 1
hour) are referred to as AM Canis Venaticorum (AM CVn) systems,
after first known binary star of this rare class. It is likely that
such systems, after having reached a minimum orbital period of a few
minutes, then begin to evolve towards longer orbital periods. This
indicates that RX J0806.3+1527 is now at the very beginning of
the “AM CVn phase”.

Gravitational waves

With its extremely short orbital period, RX J0806.3+1527 is
also a prime candidate for the detection of the elusive
gravitational waves, predicted by Einstein’s General Theory of
Relativity. They have never been measured directly, but their
existence has been revealed indirectly in binary neutron star
systems.

A planned gravitational wave space experiment, the European Space
Agency’s Laser
Interferometer Space Antenna (LISA) that will be launched in
about 10 years’ time, will be sufficiently sensitive to be able to
reveal this radiation from RX J0806.3+1527 with a high degree
of confidence. Such an observational feat would open an entirely new
window on the universe.

More information

The results described in this Press Release were announced in IAU
Circular 7835 and will shortly appear in print in the European
research journal Astronomy & Astrophysics Letters (“RX
J0806.3+1527: a double degenerate binary with the shortest known
orbital period (321 s)” by G.L. Israel and co-authors), cf. astro-ph/0203043.
The 5-min optical modulation was detected independently by another
group led by G. Ramsay, cf. astro-ph/0203053.

Note

[1]: The team consists of GianLuca
Israel and Luigi Stella at the Astronomical Observatory of
Rome (Italy), Stefano Covino and Sergio Campana at the
Astronomical Observatory of Brera (Milan, Italy), Wolfgang Hummel,
Gianni Marconi and Gero Rupprecht at the European Southern
Observatory, Immo Appenzeller and Otmar Stahl at the
University of Heidelberg (Germany), Wolfgang Gassler and
Karl-Heinz Mantel at the University of Munich (Germany),
Christopher Mauche at the Lawrence Livermore National
Laboratory (USA), Ulisse Munari at the Astronomical Observatory
of Padua (Italy), Ignacio Negueruela at the Astronomical
Observatory of Strasbourg (France), Harald Nicklas at the
University of Göttingen (Germany), and Richard Smart at
the Astronomical Observatory of Turin (Italy).

[2]: See the research article by Israel et al.
(1999, Astronomy &A, Vol. 349, p. L1).

Contact

GianLuca Israel
Osservatorio Astronomico di Roma
Italy
Tel.: +39 06 9428 6437
email: gianluca@ulysses.mporzio.astro.it