How magnetic fields change the sound of stars

The solar 11-year cycle affects many of the observable values of the Sun. In the course of the solar cycle, the oscillation frequencies of the Sun and the amplitudes of these oscillations change marginally but measurably. Solar oscillations are standing sound waves inside the Sun. Similarly to sound waves here on Earth, the pressure within the medium is key to the way in these waves travel. This is why these oscillations are also referred to as p modes (p for pressure). Changes in the p-mode characteristics throughout the solar cycle are due to the varying strength of the magnetic field over time at different depths inside the Sun.

The upper half of the diagram shows the measured frequency shifts of the oscillations of star KIC 8006161 as a function of time. The bottom half shows the measured oscillation amplitudes of this star as a function of time.

Between 2009 and 2013, the NASA space telescope Kepler continuously observed over 150’000 stars. The main purpose of this mission was the discovery of planets outside our solar system. However, the same data can also be used to engage in asteroseismology, i.e. to explore stellar properties by looking into their inherent oscillations.

A recently published study (see reference) investigates the characteristics of the oscillations of 24 solar-like stars observed by Kepler. Of the 24 stars examined, 23 showed temporal variations in their oscillation frequency. In 6 of these 23 stars, any increase in oscillation frequency was associated with a decrease in amplitude, while any decrease in frequency was associated with an increase in amplitude. This connection between frequency and amplitude is well-known from the Sun and is considered as an indication that the observed changes in the oscillation properties are due to temporal variations of the magnetic fields in these stars.

The clearest example of this is illustrated in the diagram for star KIC 8006161, also referred to as Doris. Doris is a star that is very similar to the Sun. Its mass is 1.04 times the solar mass, its radius 0.947 times the solar radius and its age 5.04 billion years. During the observation period, the oscillation frequency of this star increased by almost 1µHz. Over the same period, the oscillation amplitudes decreased significantly. For comparison: Over its 11-year cycle of magnetic activity, the Sun’s oscillation frequencies change by about 0.5µHz.

Observing the activity cycles in a larger number of stars helps to better understand the processes governing these cycles. Even in the case of the Sun, we still do not know in full detail the operation principles of the dynamo maintaining and driving the solar cycle. In this respect, asteroseismic analyses of other stars - like the one presented here - can provide valuable additional information.

References:

Stellar magnetic activity and variability of oscillation parameters: An investigation of 24 solar-like stars observed by Kepler, R. Kiefer, A. Schad, G. Davies, M. Roth, Astronomy & Astrophysics, In press, DOI: 10.1051/0004-6361/201628469

Link: NASA ADS