X-ray pulsations

A team of astrophysicists, including Yaël Nazé, a FRS-FNRS research associate in astrophysics at the University of Liege, has observed X-ray pulsations from a massive star for the first time. The pulsation is monoperiodic and is similar to those observed at visible wavelengths. This behavior was previously unknown up to this point and was not part of the theoretical predictions associated with this type of star. The discovery opens up a new window into the study of massive stars, challenging  the theoretical knowledge of these space giants. These pulsations are linked to the extremely powerful stellar winds of the star. The details remains a mystery. The quest to understand these massive stars continues.  

Massive stars are among the most surprising objects to study and still hide a great number of secrets. Why? Simply because they are not only situated at thousands of light years away from the Earth but also are relatively rare. For the formation of each massive star, many smaller stars are created at the same time: massive stars thus represent a small fraction of the stellar population. In addition, their lives are very short: such stars only last a few million years. By comparison, our Sun has long since blown out its four billionth birthday candle and is still in good health. The short life of massive stars can be explained by the fact that they burn the candle at both ends and with a very high intensity. Indeed, they have a luminosity that is one hundred thousand to one million times brighter than that of the Sun. This luminosity depends on the rate of nuclear reactions (fusion of hydrogen) in the core of the star (this hydrogen fusion phase occurs during 90% of the life of a star). While massive stars are indeed 10 to 100 times bigger than the Sun and therefore have 10 to 100 times more hydrogen, they consume the gas one hundred thousand to a million times faster which explains their short life-spans.

As long as they shine, these stars are visible from great distances, which helps observing these distant objects. They also have an energetic impact. The greater part of their luminosity is emitted in the ultraviolet (UV), at wavelengths that generate powerful stellar winds (see below; “The ultraviolet and matter in movement”). In addition to UV rays there are also X-rays, which contain even more energy and which represent one ten-millionth of the luminous emission. This may not seem like a lot, but at this kind of scale, it means that the biggest stars emit the equivalent of one-tenth of the solar luminosity in X-rays alone, which is considerable. This energetic emission has an important impact on the environment of stars, notably the heating of this environment through UV or the shaping of the environment by stellar winds. Finally, when they die, massive stars explode into supernovae and end up as neutron stars or as black holes. Many important phenomena are therefore caused by their presence. But their great distance, and the fact that the current scientific world gives priority to the study of exoplanets or  the confines of the universe, means that they remain unknown in many ways.

A study that resulted in an unexpected discovery

Among these massive stars is Xi1 CMa, a spectral B-type star, that is to say, the second category of stars if we class them according to temperature. Located in the Canis Major constellation, around 1400 light-years from the Earth, it is visible with the naked eye despite the great distance involved. It is on this star that a team of researchers including Yaël Nazé, a FRS-FNRS research associate in astrophysics at the University of Liege focused their attention. “We became interested in this star because it is very magnetic”, explains the researcher. “It is 5000 greater than the overall magnetic field of the Sun and 10 000 times more than that of the Earth, a value which is enormous”. It has been long suspected that massive stars have magnetic fields, but their signature could not be detected before 2002 because of the lack of sensitive instruments. Another important property of massive stars is the presence of stellar winds which are much more powerful than the solar wind. These stars eject hundreds of millions of times more matter than the solar wind, and these stellar winds can reach speeds of around five million kilometers an hour, ten times faster than the average speed of the solar wind. “By way of comparison, the ‘small’ solar wind is already powerful enough to strip certain planets of their atmosphere (Mars and Venus) so we cannot even begin to imagine the effect of these massive stars”. 

“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”.

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.”