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.

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.