Astronomers Identify A 'Planet-Swallowing' Giant Star

http://www.usyd.edu.au/news/newseven.../16_star.shtml

Astronomers identify a 'planet-swallowing' giant star
University of Sydney News Release
16 September 2003

Astronomers from Sydney University have come forth with a solution
to a mysterious new object recently discovered in our Milky Way.

In a letter soon to be published in the journal Monthly Notices of the
Royal Astronomical Society, Dr Alon Retter and Dr Ariel Marom from
the Department of Physics suggest that this phenomenon is an
expanding giant star swallowing nearby planets, an event which may
one day befall our own planet.

Their research provides data to support the theory that the
multi-stage eruption of the 'red giant' known as V838 Monocerotis
observed last year was fuelled as it engulfed three near orbiting
planets. This could be the first evidence for an event that had been
predicted but not known to have been observed so far. The work
identifies a new group of objects with stars that swallow planets.

Astronomers had previously been unable to explain a spectacular
explosion that transformed a dim innocuous star into the brightest
cool supergiant in the Milky Way. The event was originally
discovered by Australian amateur astronomer, Nicholas Brown in
January 2002, when V838 Monocerotis suddenly became 600,000
times more luminous than our Sun. In an ordinary nova explosion, the
outer layers of a compact star are ejected into space, exposing the
super hot core where nuclear fusion was taking place. By contrast,
V838 Monocerotis increased enormously in diameter and its outer
layers cooled and were very disrupted but still conceal the giant's
core. Beautiful images taken by the Hubble Space Telescope showed
evidence of a previous eruption that ejected material from this object
in the past. This too is very unusual.

The Sydney team suggests that the outburst of V838 Monocerotis
took place as it swallowed three massive Jupiter-like planets in
succession. Evidence for this is provided through study of the shape of
the light curve and comparison between the observed properties of the
star and several theoretical works. In their scenario, in addition to the
gravitational energy generated by the process, there may also have
been a rapid release of nuclear energy as 'fresh' hydrogen was driven
into the hydrogen burning shell of the post-main sequence star.

Interestingly past studies have also suggested that the inner planets
in our solar system, Mercury, Venus and maybe even Earth, should
be eventually swallowed by the Sun. Previous research has proposed
that this is in fact a common characteristic and that many giant stars
have consumed planets during their evolution. The current work
suggests that the engulfment of a massive planet can cause an
eruption of the host star.

Explaining the methods used during their study, Dr Retter said: 'The
careful inspection of the light curve of V838 Monocerotis showed that
the three peaks have a similar structure, namely each maximum is
followed by a decline and a very weak secondary peak. The shape of
the light curve prompts us to argue that V838 Mon had three events
of similar nature, but probably of different strengths. The obvious
candidate for such behaviour is the swallowing of massive planets in
close orbits around a parent star.'

According to this work, there should be more examples of expanding
giants that swallow less and lighter planets thus showing weaker and
less spectacular eruptions.

For further information please contact:

Jacob O'Shaughnessy
Media Officer
ph: +61 2 9351 4312

Alon Retter

ph: +61 2 9351-4058