First discovery of a magnetic delta Scuti star.

Illustration of a magnetic delta Scuti star © Sylvain Cnudde LESIA / Observatoire de Paris

Coralie Neiner from the Laboratory for Space Studies and Astrophysics Instrumentation, LESIA (CNRS / Observatoire de Paris / UPMC / Université Paris Diderot) and Patricia Lampens (Royal Observatory of Belgium) discovered the first magnetic delta Scuti star, through spectropolarimetric observations at CFHT. The delta Scuti stars are pulsating stars some of which show signatures attributed to a second type of pulsations. The discovery shows that it is actually the signature of a magnetic field. This has important implications for understanding the interior of stars.

Measuring the magnetic field (top), test pollution measurement (middle), and average profile of HD188774 spectral lines at two different dates. The nonzero visible signature in the upper panel shows that the star is magnetic. © Neiner et al. 2015, MNRAS Letters, sous presse.

Two types of pulsating stars exist among stars with a mass between 1.5 and 2.5 solar masses: the delta Scuti stars and the gamma Dor stars. Theory tells us that stars with a temperature between 6900 and 7400 degrees Kelvin may have both types of pulsations. These are then called "hybrid stars." However, Kepler NASA satellite has detected a large number of hybrids stars at colder or warmer temperatures. The existence of these hybrid stars on a larger temperature range is very controversial as it challenges our understanding of pulsating delta Scuti and gamma Dor stars.

Coralie Neiner from LESIA(CNRS / Observatoire de Paris / UPMC / Université Paris Diderot) and Patricia Lampens (Royal Observatory of Belgium) have therefore sought which physical phenomena could mimic the signatures of gamma Dor pulsations in delta Scuti stars, making them only appear as hybrid when they were really not. One explanation could be the presence of a magnetic field that would produce spots on the surface of the star: When the star rotates, the passage of the spot in front of the observer would mimic the signature of gamma Dor type of pulsations. However, no magnetic field had never been observed in a delta Scuti star ...

Through spectropolarimetric telescope observations at CFHT, they looked for the presence of a magnetic field in a putative hybrid Kepler star: HD188774. They discovered that this star is actually a magnetic delta Scuti star and that the signature of this magnetic field is confused with the signature of gamma Dor type pulsations. HD188774 is thus not a true hybrid, but the first known magnetic delta Scuti star. It is likely that many other stars thought to be hybrids among Kepler stars are actually magnetic Scuti delta stars, which would resolve the controversy between the theoretical predictions and Kepler's observations. This discovery brings new light to the interpretation of the Kepler observations, especially on the structure inside these stars.

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