Magnetic fields can have important consequences for the life and death of stars.
We show for the first time that it is possible to determine the presence of strong magnetic fields in the inner regions of an evolved star. This is done in the context of asteroseismology, a discipline that –similar to seismology– exploits the existence of waves propagating through an astronomical object to determine its inner properties. This is also analogous to a medical ultrasound, which uses sound waves to image otherwise invisible parts of the human body.
The outer regions of red giants, stars more evolved than our Sun and with larger radii, are characterized by turbulent motions that excite sound waves. This is akin to the noise produced by water boiling in a pan. These waves can propagate in the star and excite another class of waves (gravity waves), whose restoring force is buoyancy. Gravity waves are somewhat analogous to surf waves in the ocean (buoyancy is the force that makes a buoy oscillate). Basically, the sound waves propagate in the outer layers of the star, but can resonate with the gravity waves that probe the inner region of the star. This cross-talk requires sound waves to share some of their energy with the gravity waves. If strong magnetic fields are present in the stellar core, magnetic forces can become comparable to the buoyancy force. One can imagine the magnetic fields as stiff rubber bands embedded in the stellar gas that affect the propagation of the gravity waves. In this situation, the gravity waves are altered and become trapped in the stellar core: we call this the “magnetic greenhouse effect”. When the gravity waves become trapped, some of the wave energy is lost in the core, which makes the amplitude of the observable surface oscillation smaller compared to a star where no trapping occurs (a star with a non-magnetic or a weakly-magnetized core).
A suppression of the oscillations caused by waves reaching the stellar core is indeed observed in a group of red giant stars using the Kepler satellite, a space telescope that can measure stellar brightness variations of order one part in a million. We can then use these observations to put a limit on (or even measure) the internal magnetic fields for these stars. The result is mind-blowing: The measured magnetic fields are a million times or more stronger than a typical fridge magnet. This is exciting, as internal magnetic fields play an important role both for the evolution of stars and for the properties of their remnants. For example, some of the most powerful explosions in the universe (long gamma-ray bursts) could be associated with the death of stars ten or more times more massive than our Sun that ended their lives with strong magnetic fields in their core.
What is “Asteroseismology”?
Asteroseismology is the science that studies the internal structure of stars by the interpretation of their pulsations. In fact certain stars pulsate and can effectively be considered huge musical instruments: depending on their size and internal structure they can oscillate at different frequencies, somewhat similar to spherical drums of different size, shape and material. The waves that propagate through the star can bring to surface precious information about the internal regions they crossed. This is similar to what seismologists do with our planet: they study at the surface waves excited by earthquakes to retrieve information about the structure and properties of the Earth’s inner regions they crossed.
What is a “Dipole mode”? There are many ways in which a sphere (and therefore a star) can oscillate. Mathematically any possible oscillation can be described as a sum of functions called spherical harmonics. The simplest one represent a radial oscillation (harmonic degree \(\ell =0\)), in which the whole sphere expands and contracts around its center. Then we have a dipole (\(\ell =1\)), a quadrupole (\(\ell =2\)), an octupole (\(\ell =3\)) etc. Check here movies showing a number of spherical harmonics.
What is the Magnetic Greenhouse Effect?
We call magnetic greenhouse effect the trapping of gravity waves (waves for which the restoring force is buoyancy, akin to ocean waves) in the interior of a star due to the presence of a strong internal magnetic field. The trapping occurs because the (simple) geometry of an incoming wave is altered by the presence of a magnetic field, which generally has a complex geometry. As a result of this interaction, the incoming wave is “scattered” into a collection of waves with a lower degree of symmetry. These waves see a much larger evanescent region (a region they can hardly traverse, since there their amplitude decays exponentially) compared to the incoming wave and are prevented from escaping the core, where they remain trapped.
While the physics is somewhat different, the analogy is with the greenhouse effect in the Earth’s atmosphere. In that case part of the light coming from our Sun and hitting our planet’s surface is re-emitted as heat (longer wavelength, infrared radiation). This radiation is then trapped by greenhouse gases (like carbon dioxide), and prevented to leave the atmosphere.