The William Herschel telescope find the best candidate for a supernova explosion
An international team of astronomers using the Utrecht Echelle Spectrograph on the William Herschel
Telescope has identified the bright star Rho Cassiopeiae as the best candidate to
undergo a supernova explosion in the near future. The results of this investigation are to be published in a research paper in The Astrophysical Journal on February 1.
Rho Cassiopeiae is one of the brightest yellow “hypergiant”
stars in the Milky Way.
In spite of being 10,000 light-years away from the Earth, this star is
visible to the naked eye as it is over half a million times more luminous
than the Sun.
Yellow hypergiants are rare objects; there are only 7 of them known in our
Galaxy. They are very luminous and have surface temperatures between 3,500
and 7,000 degrees. It is believed that these stars are at a very
evolved stage of their life and will ultimately explode as supernovae.
Yellow hypergiants are peculiar stars because they display an uncommon
combination of brightness and temperature, which places them in a so-called Yellow
Evolutionary Void. When approaching the
Void these stars may show signs of peculiar
instability. Theoretically, they cannot cross the Void unless they have lost
sufficient mass. During this process these stars end up in a supernova
explosion: their ultimate and violent fate. The process of approaching the
Void however, has not yet been studied observationally in sufficient detail
as these events are very rare.
The highly efficient Utrecht Echelle
Spectrograph has allowed astronomers to monitor Rho Cassiopeiae in detail from
1993 to 2002. The observations were aimed at investigating the processes occurring
when yellow hypergiants approach and bounce against the
Yellow Evolutionary Void, and the
results revealed almost regular variations of temperature within a few
hundred degrees. However, what happened with Rho Cassiopeiae during the
summer of 2000 went beyond anybody’s expectations.
The star suddenly cooled down from 7,000 to 4,000 degrees within a few months. Astronomers discovered molecular
absorption bands of titanium-oxide (TiO) formed in the slowly
expanding atmosphere, suggesting that they
had witnessed the formation of a cool and extended shell which was detached
from the star by a shock wave carrying a mass equal to 10% of our Sun or
10,000 times the mass of the Earth. This is the highest amount of ejected
material astronomers have ever witnessed in a single stellar eruption.
The
outburst was similar to the shock wave a jet aircraft produces, which can be heard
as a sonic boom. Gas rushed outwards at four times the speed of sound.
Dr. Garik Israelian, one of the members of the discovery team, said: “Rho Cassiopeiae
could end up in a supernova
explosion at any time as it has almost consumed the nuclear fuel at its core. It is
perhaps the best candidate for a supernova in our Galaxy and the monitoring
of this and other unstable evolved stars may help us to shed some light on
the very complicated evolutionary episodes that precede supernova explosions.”
Rho Cassiopeiae experienced periods of excessive mass loss in 1893 and
around 1945, that appeared to be associated with a decrease in effective
temperature and the formation of a dense envelope. The results suggest that Rho Cassiopeiae goes through these events every 50 years
approximately.
Since the event in the year 2000, Rho Cassiopeiae’s atmosphere has been pulsating in a
strange manner. Its
outer layer now seems to be collapsing again, an event that
looks similar to one that preceded the last outburst. The researchers think
another eruption, possibly a stronger one, is imminent.
Dr. Israelian comments: “Given the large distance it is possible that Rho
Cassiopeiae has already exploded and become a black hole or a neutron star.
In 10,000 years the star will undergo 200 outbursts (if the frequency is 50
years). In each event it will lose 0.1 times the mass of the Sun and
therefore 20 solar
masses will be lost in 10,000 years! Very likely Rho Cassiopeiae does not
exist any more.”
The science team involved in this research consists of Drs. Alex Lobel,
Andrea Dupree, Robert Stefanik and Guillermo Torres (CfA, USA), Garik
Israelian (Instituto de Astrofísica de Canarias, Spain), Nancy
Morrison (University of Toledo, USA), Cornelis de Jager and Hans
Nieuwenhuijzen (SRON, The Netherlands), Ilya Ilyin (University of Oulu,
Finland) and Faig Musaev (SAO, Russia).
The Utrecht Echelle Spectrograph (UES) was designed and built by the
Astronomical Institute at the University of Utrecht, The Netherlands, under a contract placed by the Royal
Greenwich Observatory and the Netherlands Foundation for Radio Astronomy.
The acquisition and guidance unit was constructed at the Kapteyn Institute
at Roden, The Netherlands. First light on UES took place in 1991 and until
recently it was mounted at one of the
Nasmyth foci of the William Herschel Telescope.
The Isaac Newton Group of Telescopes (ING) is an establishment of the
Particle Physics and Astronomy Research Council (PPARC) of the United
Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)
of the Netherlands and the Instituto de Astrofísica de Canarias (IAC)
in Spain. The ING operates the 4.2 metre William Herschel Telescope, the 2.5
metre Isaac Newton Telescope, and the 1.0 metre Jacobus Kapteyn Telescope.
The telescopes are located in the Spanish Roque de Los Muchachos Observatory
on La Palma which is operated by the Instituto de Astrofísica de Canarias (IAC).
