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First discovery of a magnetic delta Scuti star

16 octobre 2015

Coralie Neiner,CNRS scientist at the Paris Observatory, and Patricia Lampens at the Belgian Royal Observatory, have discovered the very first magnetic delta Scuti star, through spectropolarimetric observations at the CFHT telescope in Hawaii. Delta Scuti stars are pulsating stars, some of which exhibit signatures attributed to a second type of pulsation This discovery shows that the latter is in fact the signature of a magnetic field. This has an important impact on our understanding of the internal structure of stars.

Vue d’artiste d’une étoile delta Scuti magnétique.
© Sylvain Cnudde LESIA / Observatoire de Paris

There are two types of pulsating stars in the mass range 1,5 to 2,5 solar masses : delta Scuti stars and gamma Dor stars.
Theoretically, stars in the temperature range 6900 to 7400 degrees Kelvin can have simultaneously both types of pulsation. They are then referred to as "hybrid stars ".
However, Nasa’s Kepler satellite has also detected a large number of hybrid stars at temperatures which are either colder or hotter. The existence of such stars over a larger temperature range is a very controversial isue, since they challenge our understanding of delta Scuti and gamma Dor pulsating stars.
Coralie Neiner from the Paris Observatory, at the "Laboratoire d’études spatiales et d’instrumentation en astrophysique" (the laboratory for space research and instrumentation in astrophysics - Paris Observatory /CNRS/UPMC/ Paris Diderot University), and Patricia Lampens at the Royal Observatory of Belgium have thus looked for phenomena which could mimic the signatures of the gamma Dor pulsations in delta Scuti stars, making them look like hybrid stars while they are not.
One explanation could be the presence of a magnetic field which gives rise to spots on the stellar surface : as the star turns, the motion of the spots across the line of sight of the observer could mimic the signature gamma Dor type pulsations.
However, a magnetic field had so far never been observed in a delta Scuti star...

Mesure du champ magnétique (en haut), test de la pollution de la mesure (au milieu), et profil moyen des raies spectrales de HD188774, à deux dates différentes. La signature non nulle visible dans le panneau du haut indique que l’étoile est magnétique.
© Neiner et al. 2015, MNRAS Letters

Using spectropolarimetic observations made at the CFHT (Canada-France-Hawaii Telescope) in Hawaii, they searched for a magnetic field in one of the supposedly hybrid stars observed by the Kepler satellite : HD188774.
They discovered that this delta Scuti star is in fact magnetic, and that the signature of the magnetic field had been taken for the signature of pulsations of the gamma Dor type.

HD188774 is thus not a hybrid star, but the first known magnetic delta Scuti star.
It is quite likely that many other supposedly hybrid stars targetted by Kepler are in fact magnetic delta Scuti stars, which would end the controversy between theoretical predictions and the Kepler observations.
This discovery throws new light on the interpretation of the observations of these stars by the Kepler satellite, and in particular on their internal structure.
This work appeared in the October 14th 2015 issue of the Monthly Notices of the Royal Astronomical Society (MNRAS) published by Oxford University Press.

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