X-ray and Observational Astronomy

News December 2003: X-rays from the Pleiades

A large scale colour optical image of the Pleiades showing the region observed by XMM-Newton. The haziness of the bright stars is due to particles of dust in the cluster.	A smaller scale optical image (false colour) with the XMM-Newton survey region in detail, stars of spectral type A to K are circled.	The XMM-Newton survey image showing the X-ray colours.

The Pleiades are a familiar and beautiful sight in the night sky. However, the famous 'Seven Sisters' are just the brightest stars in one of the nearest open clusters which actually contains around 800 stars. Such open clusters are interesting because they allow us to study stars which have all been born from the same gas cloud. So while the individual stars may have different masses, and so spectral types, they will all have the same distance and age and probably the same composition. For normal stars (those burning hydrogen to helium in their core) the spectral type (that is, its colour) tells us its surface temperature. Knowing the intrinsic brightness of such a star as well then tells us its mass. The mass of a star is most important in determining its evolution, high mass stars evolve quicker than lower mass stars because they have higher density and pressure in their core, allowing the nuclear reactions to proceed more quickly. The Pleiades are young stars, about 100 million years old. The Sun however, is around 4600 million years old.

When we look at stars with an X-ray telescope, we do not see the star's surface, but the very hot, lower density ionised gas above the surface. How this gas gets to be so hot is not well understood, but magnetic fields confine this gas in the solar corona. These magnetic fields are generated by convection, something which only occurs in atmospheres of stars with a mass similar to, or lower than, that of the Sun. There are superb X-ray pictures of the Sun's corona, but for more distant stars we can only measure the total X-ray emission and look at its variability and spectrum.

Drs Kevin Briggs and John Pye from the Department of Physics and Astronomy at the University of Leicester have used ESA's XMM-Newton X-ray satellite observatory to look in detail at the coronae of some of the Pleiades stars. As expected, all of the Sun-like (spectral types F-K) stars were detected as X-ray sources. In fact they are all much brighter in X-rays than the Sun (100-2,000x) because they are so young, stronger magnetic fields are found in younger stars because they rotate faster. It was more surprising to find that some of the more massive, hotter stars (spectral types B-A) were also seen. Such stars are not thought to have either convective regions or a stellar wind (in still higher mass stars such winds can give rise to X-ray emission due to shocks), and so are not expected to be X-ray sources.

A number of the stars showed the fast brightening and slower decline typical of a solar flare. These are due to the rapid heating and subsequent cooling of coronal gas which is magnetically confined. The Pleiades flares are 10-100 times more powerful than the strongest flares observed on the Sun, and span a considerable fraction of the size of the star involved. Flares are known to be quite common events in the X-ray activity on such young, fast-rotating, Sun-like stars. However, one fascinating example of a bright flare has a sharp drop to the non-flare level for 5 minutes before returning to its previous behaviour. While this could in principle be due to a combination of 2 flares (one of which would have to be very odd), or due to an extremely dense clump of gas getting in the way, it might also be the signature of a previously unknown planet eclipsing the hot flaring gas. Such a planet would be in the 'hot Jupiter' class, and would be the first found by X-ray observation.

The X-ray spectra of the Sun-like Pleiades stars show that coronal temperatures are higher in more rapidly rotating stars and in stars with lower surface temperatures, and that the relative iron abundance in the corona is low (20%-50%) compared to that of the Sun. These properties are not unusual in other Sun-like stars. Detailed hydrodynamic modelling of the flares gives flare loop lengths 0.07-0.50 times the stellar radii (i.e. 40 - 260,000 km), in the past less detailed models have tended to exaggerate the loop lengths. Finally 5 of the stars observed have spectral types which indicate that there would be no convection or stellar wind, yet 3 of these were detected as X-ray sources. It seems likely that these 3 stars have Sun-like companion stars, hidden in the glare of their brighter neighbour at optical wavelengths, which are nevertheless brighter in X-rays.

The paper detailing the results from the XMM observations by Briggs and Pye is available from the arXiv service; XMM-Newton and the Pleiades -- I: Bright coronal sources and the X-ray emission from intermediate-type stars. It has recently been published in the Monthly Notices of the Royal Astronomical Society (v345, p714, 2003). More information on the study of normal stars at the University of Leicester can be found here, and in the 2001 biennial report.

Dr Kevin Briggs now works at the Laboratory for Astrophysics of the Paul Scherrer Institut and at the Institute of Astronomy, ETH, Zurich in Switzerland.

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Last updated: 2003 December 10 by Julian Osborne