The ultraviolet and matter in movement

How can light propel matter? It should be noted that the shorter the wavelength (UV, X-rays, Gamma rays…), the more energy light has. The intense ultraviolet light emitted by the massive stars is absorbed by the metals. “A word of caution”, says the researcher jokingly, “in astronomy we have quite a simple vision of the composition of matter. There are only three ‘elements’ in space: hydrogen, helium and the metals”. When the metallic ions absorb the UVs, the electrons move to more distant orbits. “The ion is then in what we call an excited state, and such a state does not last a very long time”.  When it returns to its normal state, the ion re-emits the light. If the initial light camefrom only one direction (the radial one, i.e., from the center of the star), the re-emission may occur in any direction. This difference between absorption and re-emission generates a driving force for the ion which begins to move outwards. The cycle repeats itself unceasingly, generating a powerful wind.

“On the whole, the ions are pushed towards the front. Just a little but because there are so many of them and they share what they gain in energy, the matter is propelled more and more strongly”. It is this very efficient process which explains how the winds of massive stars reach huge speeds and mass-lost rates compared to the solar wind. The important thing is therefore the light intensity and the proportion of UVs in this. The Sun emits very few UVs, while massive stars emit more than 90% of their light in that range: the solar wind is therefore not at all comparable to those of massive stars (it is simply hot gas in expansion, not material propelled by ultraviolet light).

Towards a better knowledge of stellar winds

To return to Xi1 CMa, the scientific team is facing a brand new phenomenon. “We think that X-rays are generated by the wind. But why does this pulsation dominate? We do not know the answer to this”. Xi1 CMa was already no ordinary massive star. Only a small percentage of massive stars contain strong magnetic fields. In fact, none should have such fields because they do not have convection motions under the surface capable of generating magnetic fields much like a dynamo. “Magnetic fields in massive stars are probably fossil in origin, coming from something that occurred at the beginning or their lives, or from the interstellar matter that form them.”.  In addition, Xi1 CMa is a pulsating star, which means that its surface oscillations are detectable from Earth; the researcher only knows of two other such stars, which combine this triple particularity (massive, magnetic and pulsating). “It was therefore already an exceptional object. But this X-ray pulsation  makes Xi1 CMa even more interesting. We do not know of any other case of a star producing such a pulsation in X-rays”.
 
Whatever the nature of Xi1 CMa may be, it has aroused the curiosity of the scientific community. These observations of the star were the subject of a published article in Nature Comms, and will be presented at several conferences. “I am also talking to other colleagues about making new observations and modelizations. But that will take some time to begin. We are just at the start of the project; we must not be too impatient.”