CANADA'S SPACE TELESCOPE CRACKS OPEN A MASSIVE STAR

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 today at the Canadian Astronomical Society meeting in Montréal, 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 (Université de Montréal), 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 Université de Montréal. "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.

For more information:

Laure Lefevre
Phone: 514-343-6111 ext 1089,
Email: lefevre@astro.umontreal.ca

Professor Anthony Moffat
Phone: 514-343-6682
Email: moffat@astro.umontreal.ca

Figure captions :

WR123 variations : This figure shows the full oscillation of WR123 over the whole 38-day interval observed non-stop by MOST. The upper panel shows the original data as it was extracted. The middle panel shows the signal extracted from the comparison star C2 on the same scale. The lower panel (vertical scale expanded by a factor of 2) shows the data for WR123 where all variability with time scales longer than about 1 day have been filtered out.

http://www.astro.umontreal.ca/~casca/PR/PR_sub_1orb_0.4j.jpg

Hubble image of WR124 : WR124 is a close cousin of WR123. This Hubble Space Telescope image in the light of excited hydrogen gas reveals the gas being ejected at high speed from the central star (overexposed and blotted out by a white dot) as it catches up and collides with the slower wind from a previous phase. Although both stars are almost identical, WR123 must be slightly older, having already dissipated its colliding gas. Credit: Yves Grosdidier (Université de Montréal and Observatoire de Strasbourg), Anthony Moffat (Université de Montréal), Gilles Joncas (Université Laval), Agnes Acker (Observatoire de Strasbourg), and NASA.

http://www.astro.umontreal.ca/~casca/PR/WR124.jpg