X-ray pulsations
7/10/14

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.  

Canis Major constellationMassive 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”. 

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