Wolf-Rayet stars are among the rarest objects in the Galaxy, with only 500 examples known in the whole of the Milky Way. These massive stars, observed en route to their destruction in powerful supernovae, are the subjects of an exciting new paper from a team of European and US astronomers, who have captured the first ever high-resolution X-ray spectrum of just such a star.
Wolf-Rayets are the most massive stars known. Despite being millions of times more luminous than the Sun, the stars are quite faint in visible light, because most of the energy emerges as energetic short-wave radiation, in the form of ultraviolet and X-rays. This unusual behaviour is due to their temperature: a typical Wolf-Rayet might have a surface temperature of tens or even hundreds of thousands of degrees Celsius, compared with the Sun’s relatively puny 5,500”C.
The stars first came to attention in the 1860s when astronomers noticed their unusual spectra, which contained features that indicated the presence of gas moving at enormous speeds. This is now believed to be the result of a powerful stellar wind, and it’s this wind that is most prominent and confusing in the new X-ray observations. The observations make use of ESA’s XMM-Newton space telescope, which was aimed at a star known as WR6 or EZ CMa.
Thanks to this powerful wind, which reaches a velocity of more than l,500km/s, WR6 is losing 40,000 billion tonnes of matter into space each year. Impressive though those statistics are, perhaps the most mind-boggling figure revealed by the observations is the existence of gas as hot as 45 million “C within the stellar wind.
The team suggests that such hot gas can only be produced by a dramatic shock, and invokes suggestions from ultraviolet observations that some portion of the wind is moving much faster than the average. If such a fast-moving component were to smash into the main, slower moving body of the gas, there might just be enough energy around to heat some of the debris up to the observed temperature.
The real mystery is how such a collision might occur, and the team writes that the wind must include previously unsuspected slow-moving clumps of gas. Such clumps would have to assemble near the surface of the star and would need to be able to resist, at least while forming, whatever force accelerates the rest of the wind to such high speeds.
If that sounds a little hand-wavy, I don’t think the authors would disagree. Demonstrating that the most exciting scientific results are precisely those which make little sense at the time, they conclude that some unknown mechanism must be operating on hot, strange WR6. That provides a challenge, but also a rare opportunity for us to learn more about how stellar winds work elsewhere in the Universe. By Chris Lintott