A massive star known for violent and erratic behaviour shows a surprisingly predictable and regular pattern in brightness, reveals the most detailed study ever done on the star. But the clockwork signal has stumped astronomers, who hope to unravel how other such stars sow the universe with heavy elements.
The object, called WR123, is one of 200 so-called 鈥淲olf-Rayet鈥� stars known in our galaxy. These represent a short-lived stage 鈥� lasting just a few hundred thousand years 鈥� in the lives of stars born with the mass of at least 25 Suns.
Wolf-Rayet stars haemorrhage trillions of tonnes of material into space every second and are also thought to produce fleeting blasts of high-energy photons, called gamma-ray bursts, when they die as supernovae.
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The objects are 鈥渧ery important astrophysically鈥�, says Michael Corcoran, an astronomer with the Universities Space Research Association in Greenbelt, Maryland, US. 鈥淭hey produce the heavy elements we need for life 鈥� like carbon and oxygen 鈥� and they distribute them by blowing up.鈥�
Explosive departures
But Wolf-Rayet stars are not well understood, and WR123 is a member of an even more mysterious subclass 鈥� called WN8 鈥� whose 17 known members lie far from any stellar nursery. That suggests WN8 stars once had stellar companions that exploded as supernovae, which pushed the surviving stars into regions of empty space. The brightness of WN8 stars appears to vary chaotically.
Indeed, previous ground-based observations showed WR123 dimmed and brightened on a number of different timescales 鈥� from once every 10 hours to once every few days. But the periodic signals tended to disappear after a short time, and observing runs were generally limited to evenings over a single week, providing limited statistics.
Now, a team of astronomers using Canada鈥檚 MOST (Microvariability and Oscillations of Stars) satellite has observed the star continuously for 39 days. And it has ruled out all the apparent brightness variation periods with the exception of the 10-hour cycle.
Star within star
That short period 鈥渋s hard to explain in terms of normal stellar processes鈥�, says Corcoran, who was not involved with the research. If the star, which may be as massive as 13 Suns, is spinning once every 10 hours, models predict the star should tear itself apart. And if the star is orbiting an unseen companion, theory predicts the companion would have to actually be within WR123 to complete the 鈥渙rbit鈥� in such a short time.
Either of those situations might point to a recent or ongoing merger with another star, which could have sped up WR123鈥檚 rotation, says Corcoran. MOST team member Anthony Moffat at the University of Montreal, Canada, says future observations at other wavelengths might reveal the traces of such a merger.
But colleague Laure Lefevre says the most probable explanation for the 10-hour period is an intrinsic property of the star itself. 鈥淟ight is trapped inside the star and pushes out on the different layers of the star, which causes the pulsations,鈥� she told New 杏吧原创.
But Moffat says theoretical models fail to show how such pulsations could be maintained stably over long periods of time. 鈥淨uite frankly, we don鈥檛 know what it is,鈥� he admits.
Lefevre presented the results on Tuesday at a meeting of the Canadian Astronomical Society in Montreal.
