Canada's space telescope cracks open a massive star

UNIVERSITE DE MONTREAL NEWS RELEASE

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Posted: May 18, 2005

The MOST space telescope has given astronomers new clues about an exotic star, at least ten times more massive than our Sun, spewing gas into space at a rate of more than 100 trillion tonnes per second.

And according to results presented at the Canadian Astronomical Society meeting in Montreal, Canada, the star - with the misleadingly bland name of WR123(*) - is even weirder than astronomers ever suspected.

The new findings, by Laure Lefevre and Anthony Moffat (Universite de Montreal), Sergey Marchenko (Western Kentucky University) and the international MOST science team, are based on over five weeks of non-stop monitoring of the light variations of WR123. WR123 is a member of the relatively rare class of Wolf-Rayet stars (named after two French astronomers who discovered their telltale strong plasma winds using a simple spectroscope in the Paris suburbs in 1867, ironically the same year Canada became a nation).

Wolf-Rayet stars like WR 123 have long been known to exhibit complex - seemingly chaotic - brightness variations associated with the turbulent high-speed winds they eject into space. But the nearly continuous coverage possible with the MOST (Microvariability & Oscillations of STars) satellite has revealed a clock in the chaos - a stable variation repeating every 10 hours (see figure illustrating WR123 variations).

"Finding a clock in a star like WR123 is like finding the Rosetta stone for astronomers studying massive stars," explained Ms. Lefevre, a PhD student at the Universite de Montreal. "However, although WR123 may vary like clockwork, it must be a very strange mechanism indeed."

The only theories to explain the 10-hour clock in WR123 would be: (1) the rotation of the star itself, (2) the orbit of another small star around WR123, or (3) vibrations in the structure of WR123 that are transmitted to its dense enveloping wind. All of these ideas are equally strange. If WR123 is spinning at that rate, the surface would be moving so fast (about 2000 kilometres per *second*, or over 7 million kph) that the star should throw itself apart, unless that is the actual source of the wind! If the star is in a tight binary system, it's so tight that its companion would be orbiting *inside* the star itself. If pulsations are the right answer, theoreticians will have to completely revise their current understanding of this class of massive stars.

The same period was hinted at in spectroscopic data obtained from an Earthbound observatory a year earlier, but the MOST results leave little doubt as to the bizarre timing of this stellar clock.

One hundred times fainter than what the unaided eye can see, WR123 is located about 19,000 light-years from Earth, in the direction of the constellation Aquila ("the Eagle"). WR123 and other similar Wolf-Rayets (see the Hubble image of WR124) are believed to have had very violent births, ejected by a supernova explosion in a binary system, or by a gravitational slingshot from a dense star cluster. "Either way, WR123 was probably kicked out from the nest rather abruptly," jokes Dr. Moffat, who helped develop these formation theories in the late 1970's.

Stars that start off their lives with ten or more times the Sun's mass are capable of "burning" hydrogen into helium, helium into carbon, and so on up to the final nuclear ash, iron, before the iron-rich core collapses on itself in less than a second and produces the greatest of all stellar explosions, a supernova. Since H-burning lasts by far the longest, some 90% of stars that shine are actually consuming hydrogen in their cores at a prodigious rate. Then, somewhat under 10% of stars are in the next stage, that of He-burning, while only a miniscule fraction occurs in the subsequent, ever-faster evolving stages. WR123 represents the fleeting final stages of helium-burning, before the rapid death-spiral to supernova.

The gases ejected from stars like WR123 will enrich the interstellar medium, and contribute to future generations of stars. Understanding such stars is vital if we are to properly understand the evolution of the Milky Way and other galaxies. "We may be seeing an example of one of the key stages in the stellar lifecycle that led to the Sun, Earth, and us, being here," noted Ms. Lefevre.

The MOST mission was financed by the Canadian Space Agency and supported by the Natural Sciences and Engineering Research Council of Canada.

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*- WR stars are so rare that less than 200 of them have been cataloged.

The current theory is that WR stars are not a specific class of star as much as they are a later stage in the evolution of the hot, massive O and B class stars. In fact, WR stars are only found in clusters called OB associations.

WR stars are so powerful (luminious) that a mere sprinkling of them can influence the spectra of an entire galaxy. In fact the phenemenon that astronomers called Wolf-Rayet galaxies are spiral galaxies which show emission spectra. This is interperted as active star formation since O and B class stars from which WR stars evolve have relatively short lifespans- between 10-60 million years.

WR123 indicates that this star is listed in van der Hutch's 7th General Catalog of Wolf Rayet Stars.