“Following theoretical predictions, we thought that the powerful magnetic field around Xi1 CMa influenced the wind and that the wind flows from both hemispheres of the star would follow the lines of the field to collide at the equator. At such speeds and since such great quantities of matter are involved, this collision must be enormous and generate huge amounts of energy and therefore X-ray light. This is an easily recognizable feature which we wanted to observe to verify our theories on the influence of the magnetic fields on the stellar winds”. The team obtained viewing time on the ESA’s XMM-Newton telescope, which has been orbiting the Earth since 1999, and, on a private note, was tested in the Liege Space Center (ULg was also involved in the construction of one of its instruments). For 29 hours, XMM-Newton pointed its mirrors in the direction of Xi1 CMa. “Once we gathered our data, I had to analyze the X-ray light curve – that is the variation in light intensity of the star during observation. I immediately saw that there was an abnormal periodic variation that was totally different from what we had predicted: a luminous pulsation that was periodically stable. The discovery was both astonishing and new for a massive star”.  

A new phenomenon to identify

The light emitted by stars varies, including in the X-ray range: that is not extraordinary. That this variation is periodic was not extraordinary either. This periodicity could be observed in binary systems (in which two stars orbit each other and whose solar winds collide -see article; “The stellar wind reveals its secrets”). “But in the present case, the star is alone and its pulsation is almost perfectly sinusoidal”, explains Yaël Nazé. Yet another surprise was in store for the researcher. When she determined the pulsation period for the X-rays, which is approximately five hours and corresponds to the light pulsation in the visible domain. “Therefore the star has only one pulsation period, whatever the wavelength. Once again, this was a phenomenon that we had never observed before. Other pulsating massive stars had already been observed before, but we had not detected any variation in their X-ray emission”! Icing on the cake: the variations in the X-rays are the result of a very high-energy emission, the most difficult type to produce. She continues, “Typically, the strength of any high-energy emission is quite reduced compared to emissions in other colours of the light spectrum. However, in the present case, the variations are weaker in the visible domain than in X-rays. The phenomenon responsible for this pulsation generates a large high-energy emission, which is unusual”.

While observing Xi1 CMa, the researchers therefore did not find what they were expecting, but they did find something more valuable: a new phenomenon! It is probably linked to the launching of stellar winds near the surface. “However, for the moment, we do not know the details of the phen omenon. We are just identifying a brand-new phenomenon whose exact causes are still unknown to us. We will continue to observe the star at other wavelengths and develop new models to attempt to understand why this particular pulsation occurs. I think that it will be necessary to combine two models, one which takes account of the oscillation of the surface, and the other which model the stellar winds, whose behavior is linked to these light variations”.

Light is the cause of the winds

In the visible domain, as in X-rays, the light variations are linked to the surface of the star which oscillates like a drumskin. These pulsations are the result of the propagation of light inside the star. Inside there are zones which act as heat engines, storing energy while propagating light and forcing matter  to move with a back-and-forth motion, which creates the observed light variations. “so we have a stellar surface whose light emission vary and we have winds which are very powerful and very unstable. The reason these winds are so strong, so rapid and so dense is because they are propelled by the UV light which is abundant in the spectrum of Xi1 CMa. This means that there is a connection between the winds and the light emitted on the surface. Therefore, if the surface changes, we can imagine that it will influence the conditions that drive the winds. As these winds are very unstable, any variation can quickly have an important impact, creating shocks that lead to the emission of X-rays. But in order to verify this we need new models”.