Andy Pollock


Main Research Fields

My name is Andy Pollock and I am the Calibration Scientist for the RGS on XMM. I have been thinking about X-rays from hot stars for about 25 years. The most massive stars of more than, say, 10 or 20 solar masses have temperature of 30,000K or more and are blue or UV. They are of spectral types O and Wolf-Rayet. UV radiation pressure drives a strong wind from the surface of these stars to velocities of a few thousand km/s, about 1% of the speed of light. Even though the temperature of the wind is about 10,000K, you see X-rays typical of temperatures of millions of degrees. These X-rays come from both single hot stars and binary systems of two hot stars. In binary systems, it's easy to imagine how X-rays get produced as two winds crash into each other between the stars. The best example of this is a 7th magnitude star in Cygnus called WR140, which does wonderful things nearly everywhere - radio, IR, X-rays. It's the brightest hot-star X-ray source. Until recently, everybody including me has thought of these colliding winds in terms of ordinary gas dynamics and shocks. It's obvious that this is wrong because we are dealing with plasmas rather than neutral gas and plasmas are strange. Not only are they plasmas but they are collisionless plasmas. This means that the colliding winds in a hot-star binary system have things in common with, say, the interaction of the solar wind with the Earth's magnetosphere. WR140 is a very eccentric 8-year binary system. I got some high-resolution X-ray spectra near the last periastron in 2001, when the stars were closest, and some more a couple of months ago near apastron, when they were much further apart. The predictions made about both these were wrong. I'm now trying to understand them with plasma physics. Single hot stars also give you X-rays, though they are softer. It's more difficult to image where the X-rays come from as there's nothing obviously in the way. X-rays from million-degree plasmas in thermal equilibrium are caused by electrons colliding with ions. This is the usual assumption behind the plasma emissions codes that people use in XSPEC like mekal or APEC. There are reasons to suppose that there are no hot electrons in the winds of single hot stars. I have suggested that the X-rays are produced instead by ions colliding with ions. The paper I wrote on this idea was submitted to A&A 342 days ago. The referee doesn't like it. In summary, X-rays from hot stars encompass microphysics and macrophysics. There are lots of data to be analyzed in the XMM archive. If you want to join in, I would be very happy to discuss projects that could be small, medium-sized or big. I have a list of about 20 possible projects. You can find out more this time next week in a talk which should last about 25 times longer than this one.