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1\31 Spain - The William Herschel Telescope finds the best candidate for a
supernova explosion
Part 1 of 2

Isaac Newton Group of Telescopes
Apartado de Correos, 321
E-38700 Santa Cruz de La Palma
Canary Islands; SPAIN

FOR FURTHER INFORMATION PLEASE CONTACT: 

Dr. Garik Israelian 
Instituto de Astrofísica de Canarias 
Phone: +34 922 605 258 
Fax: +34 922 605 210 
E-mail: gil{at}ll.iac.es 

Mr. Javier Méndez 
Public Relations Officer 
Isaac Newton Group of Telescopes 
Phone: +34 922 425 464, +34 616 464 111 
Fax: +34 922 425 442 
E-mail: jma{at}ing.iac.es

For immediate release: Friday 31 January 2003

THE WILLIAM HERSCHEL TELESCOPE FINDS 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).

FOR FURTHER INFORMATION ON THE WEB: 

More information on the William Herschel Telescope
     http://www.ing.iac.es/PR/wht_info/

More information on the Isaac Newton Group of Telescopes
(ING):
     http://www.ing.iac.es/PR/

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