Deviations from a uniform period spacing of gravity modes in a massive star

The predictive power of current stellar evolution models is limited by the fact that little is known about the mixing process occurring at the stellar core. In theory, massive stars such as HD 50230 — seven times the mass of the Sun — should be ideal models for seismic probing of the stellar core via the distinct signature they leave in the gravity mode spectrum as it propagates from the core to the stellar surface. These gravity modes have now been detected observationally, in a continuous photometric light curve for HD 50230 collected with the CoRoT satellite. The data make it possible to estimate the extent of the convective core and constrain the location of the chemical transition zone at about 10% of the stellar radius. Measuring the oscillations of a star can allow the various mixing processes in its interior to be disentangled, through the signature they leave on period spacings in the gravity mode spectrum. Here numerous gravity modes in a young star of about seven solar masses are reported: the mean period spacing enables the extent of the convective core to be determined, and the clear periodic deviation from the mean constrains the location of the chemical transition zone — at about 10 per cent of the radius. The life of a star is dominantly determined by the physical processes in the stellar interior. Unfortunately, we still have a poor understanding of how the stellar gas mixes near the stellar core, preventing precise predictions of stellar evolution1. The unknown nature of the mixing processes as well as the extent of the central mixed region is particularly problematic for massive stars2. Oscillations in stars with masses a few times that of the Sun offer a unique opportunity to disentangle the nature of various mixing processes, through the distinct signature they leave on period spacings in the gravity mode spectrum3. Here we report the detection of numerous gravity modes in a young star with a mass of about seven solar masses. The mean period spacing allows us to estimate the extent of the convective core, and the clear periodic deviation from the mean constrains the location of the chemical transition zone to be at about 10 per cent of the radius and rules out a clear-cut profile.