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.