Behind the apparently simple brightness of certain stars there is
often a fascinating complexity. That is the case for the particularly
interesting Castor stellar system upon which XMM-Newton has shed new

Castor is the second brightest star in the Gemini constellation (The Twins) after Pollux. Its magnitude as seen by the naked eye is 1.6. But this brightness is the combined result of not one but six stars, in three pairs, all waltzing around each other.

This sextuplet consists first of a pair of A-type stars, orbiting each other with a period of 467 years. They are just observable with an amateur optical telescope with a separation of only 3.9 arcseconds. Slightly to the south of this pair is the M-type dwarf star YY Gem which has long attracted astronomers for its frequent flare outbursts. (Spectral type-A stars have surface temperatures respectively in the range 7,500-11,000 Kelvin and are white in colour. M-type stars are cooler, with surface temperatures between 2,400-3,480 Kelvin and are red in colour.)

But to complicate matters, each of these three stars is a binary system by itself with very intriguing and different properties. The brighter of the A-stars is accompanied by an invisible dwarf in a highly eccentric orbit. The other A-star hosts a similar companion but in a circular orbit. YY Gem, finally, is made up of two identical M-dwarfs that orbit each other in only 19 hours. As we look at this system from the side, every 9.5 hours one of the dwarfs is almost completely eclipsed by its companion.

The Castor family reveals its real splendour in the X-ray domain as XMM-Newton has shown on 25 April when for performance/verification purposes it observed the sextuplet for a full day, using all three EPIC cameras and both Reflection Grating Spectrometers (RGS).

While it has been known that both YY Gem and a source in the double A-star formation are bright X-ray objects, ESA’s new observatory has now given conclusive evidence of who does what in this system.

The EPIC MOS1 image clearly reveals two X-ray sources (Castor B and Castor A) at the position where the two A-type stars are supposed to be. This resolves a long-standing debate on whether both stars, or more probably their respective low-mass companions, are magnetically active.

Previous data from the ROSAT X-ray mission had suggested that the optically brighter system was the likely X-ray source. Radio astronomy data obtained in 1994 by Manuel Guedel, one of the RGS consortium team members, had shown that both stars were radio sources. So on theoretical grounds, it was believed that both components would show up as X-ray sources. XMM-Newton has confirmed this model very nicely in one EPIC image.

Spectra obtained by the RGS instruments provide more information on the Castor system, nicely separating the spectra of YY Gem and of the Castor A+B system, getting two spectra for the price of one. “The spectra for the YY Gem and the Castor sources are strikingly different” – says Manuel Guedel – “The ratio between the line fluxes indicates either different dominant temperatures or different elemental compositions”.

Given XMM-Newton’s unprecedented effective collecting area and the excellent resolving power of the RGS, studying the spectral lines will allow analysis of the motions of the order of 100 km/s in this system.

Throughout the 25-hour observation of the sextuplet, the EPIC images reveal that all three X-ray sources are blinking, indicating that all are flaring stars. Flares are giant releases of energy that can evolve in a few minutes or over several hours. A consequence of magnetic instabilities in the hot outer stellar atmospheres, they are well studied on the Sun notably by ESA’s SOHO, and on other stars.

But the frequency of the flaring on Castor is quite surprising. At no period during XMM-Newton’s observation is the emission at a constant level, perhaps indicating that much of the observed X-ray radiation stems from such eruptions.

“We are really pleased that so much data on Castor has come out of one single observation. It is a beautiful example of what can be obtained with time-resolved high-resolution X-ray spectroscopy,” says Manuel Guedel. The investigation into the Castor system is to be pursued after the conclusion of XMM-Newton’s calibration phase. It promises more exciting insights into this fascinating sextuplet.

Bert Brinkman of SRON/The Netherlands is the Principal Investigator of the Reflection Grating Spectrometer. Co-Investigator institutes include Columbia University/USA, MSSL/UK, and PSI/Switzerland. Martin Turner of Leicester University in the UK is the Principal Investigator of EPIC. Co-Investigator institutes are: IFC Milan, MPE Garching, CEA Saclay, AI Tuebingen, Birmingham University, IAS Orsay, CESR Toulouse, ITESRE Bologna, OA Palermo.