The remarkable star V838 Monocerotis could be the coolest supergiant ever
observed according to new observations by a team of researchers from Keele
University and the Gemini Observatory. The star’s dim appearance after an
episode of rapid expansion is due to its exceptionally low temperature and
not a concealing veil of dust as previously thought.

The team have been monitoring this unusual object with the United Kingdom
Infrared telescope (UKIRT), since an Australian amateur astronomer, Nicholas
J. Brown, found it in the throes of an outburst of light on 6 January 2002.
This marked the start of an extraordinary change to the star over a
remarkably short time. Initially a normal-looking star, V838 Mon expanded
into a cool supergiant in just a few months. The transformation was marked
by three episodes of brightening, followed by a dramatic fade. At the time,
a logical explanation for the fading seemed to be obscuring dust that could
have formed from material expelled when the star puffed up. But a spectrum
obtained in March 2002 was characteristic of a typical cool supergiant star
with a surface temperature around 4000 Kelvin.

Due to the motion of the Earth around the Sun, V838 Mon was too close to the
Sun in the sky to permit further observations until October 2002. The
infrared spectrum obtained on 28 October 2002, which is due to be published
in the Monthly Notices of the Royal Astronomical Society, showed the star
had cooled significantly in the 7 months since March. “The spectrum was
reminiscent of the coolest of all stellar-like objects – brown dwarfs. It
corresponded to a surface temperature little more than 1000 Kelvin, well
within the temperature range of brown dwarfs,” says Mark Rushton.
“Decreasing surface temperature, rather than dust, was mainly responsible
for the dramatic fade from view.”

Whereas brown dwarfs are typically less than a tenth of the size of the Sun,
V838 Mon is at least 800 times larger than the Sun, making it the coolest
supergiant ever observed. However, there are notable differences between the
spectrum of V838 Mon and those of brown dwarfs, because of their difference
in size.

An unusual insight into the environment around V838 Mon was gained when
pre-existing and previously unseen material around the star began to reflect
light after V838 Mon brightened, giving rise to a spectacular “light-echo”.
This phenomenon was reported by Arne Henden of the Universities Space
Research Association and US Naval Observatory, Ulisse Munari of Osservatori
Astronomico di Padova, and Michael Schwartz of Tenagra Observatory, and
gives the impression of an expanding nebula, which can be used to determine
the distance to the object. Howard Bond (Space Telescope Science Institute)
and colleagues, using images taken by the Hubble Space Telescope (HST),
estimate this to be around 20,000 light-years.

Why V838 Mon has displayed such unique and unusal behaviour is a mystery.
One fascinating suggestion, proposed by Noam Soker and Romuald Tylenda of
the Copernicus Astronomical Center, is that we have witnessed the merging of
two individual stars.

Curious about what will happen next the team will continue to monitor this
enigmatic object.

CONTACT

Mr Mark T. Rushton, Astrophysics Group, School of Chemistry and Physics,
Keele University, Staffordshire, ST5 5BG
Phone (+44) (0)1782-583530
email: mtr@astro.keele.ac.uk

TEAM MEMBERS

Professor Nye Evans, Astrophysics Group, School of Chemistry and Physics,
Keele University, Staffordshire, ST5 5BG

Dr Thomas R. Geballe, Gemini Observatory, 670 N. A’ohoku Place, University
Park, Hilo, HI 96720, USA

Dr Barry Smalley, Astrophysics Group, School of Chemistry and Physics, Keele
University, Staffordshire, ST5 5BG

Dr Jacco Th. van Loon, Astrophysics Group, School of Chemistry and Physics,
Keele University, Staffordshire, ST5 5BG

NOTE

V838 Mon lies in the constellation Monoceros, the Unicorn. At maximum
brightness it reached magnitude 6.5 and is now 16th magnitude.