Low-mass stars (such as the Sun and less massive stars) possess a radiative core – meaning that energy is transported outwards by light – and a convective envelope – meaning that hotter material bubbles outwards while colder material sinks back inwards (similar to a pot of boiling water!). Furthermore, these stars are rotating, but unlike a solid sphere, they undergo differential rotation – points on the stellar surface located at different latitudes rotate at different rates. For instance, the Sun’s rotation period is about 26 days at the equator, but surface rotation close to the poles is slower, with a period of about 34 days.
The interplay of convection and differential rotation leads to the creation of a stellar dynamo – a generator capable of amplifying and sustaining magnetic fields. Therefore, magnetism is found in essentially all of these stars. It leads to important observable consequences, such as the dark spots seen on the Sun – or sunspots – occurring at the footpoints of magnetic loops. It can also explain the activity cycles of stars such as the Sun, as well as space weather as experienced here on Earth.
On the other hand, more massive stars have an inverted structure compared to their lower-mass counterparts: while their core is convective, their outer envelope is radiative. As such, the latter is not well-suited to the formation of dynamos, and consequently, most massive stars do not exhibit strong magnetism on their surface. However, despite that, about 10% of massive stars do host detectable surface magnetic fields.
Stellar magnetism is best detected and characterised through the use of spectropolarimetry – a combination of spectroscopy (breaking down light into its constituent wavelengths, or “colors”, like when light filters through a prism to form a rainbow) and polarimetry (breaking down light into its “orientations”, similar to polarised sunglasses). There are a few dedicated instruments in the world that can carry out such observations: ESPaDOnS and SPIRou at the Canada-France-Hawaii Telescope, Neo-NARVAL at the Télescope Bernard-Lyot, HARPS(pol) at the ESO 3.6-m telescope at La Silla, FORS2 at the ESO Very Large Telescope and MIRADAS at the Gran Telescopio Canarias.
Large spectropolarimetric surveys have led to the discovery of magnetic massive stars and have led to the description of this subpopulation: magnetic OBA stars possess strong (of order kG), large-scale (typically mostly dipolar) magnetic fields. However, many open questions remain, especially about the origin of the fields and their subsequent evolution. One important limitation is that spectropolarimetry requires long integration times. This means that further advances in the field cannot rely solely on that technique.
Thankfully, magnetic massive stars exhibit a host of other observational signatures that can be leveraged to discover more of them and help answer some of these questions. An important feature in particular is variability, especially on the timescale of rotation. This can be understood in terms of two different phenomena, depending on the mass of a star: magnetospheres and persistent surface spots.
The most massive stars possess intense radiation fields that drive dense and electrically charged outflows from their surface. When a magnetic field is present, it interacts with this stellar wind, shaping and confining it into a magnetosphere. However, since the magnetic and rotation axes of the star are not generally aligned, this means that we can observe various parts of the magnetosphere throughout a rotation cycle, leading to periodic variations in many observables.
For intermediate-mass stars, the magnetic field can affect diffusion processes, leading to the formation of chemical abundance spots. This also changes the brightness across the surface of the star, meaning that, as it rotates, it can appear brighter or dimmer over time. Once again, this happens periodically and can be observed across several diagnostics.
One particularly useful and accessible type of observation is photometry – light is collected, similarly to rain accumulated in buckets during a storm – and the variation of an object’s brightness over time can give us different sorts of information about the nature of the object. In our case, the light curves of magnetic massive stars often show periodic variations related to rotation. However, these variations can be quite subtle. Such a signature can help identify magnetic candidates. Therefore, to discover more magnetic massive stars, thus increasing our sample size and advancing the state of our field, we require very precise light curves of a large number of stars.
Enter the Transiting Exoplanet Survey Satellite (TESS), NASA’s newest space photometer. Not only does it obtain exquisitely precise data, but it also does so by observing most of the sky. As a result, we formed the MOBSTER (Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties) Collaboration – our aim is to use TESS data to identify high-probability magnetic candidate stars and to characterise known magnetic stars. We can then follow up on our candidates using spectropolarimetry (as well as other diagnostics). So far, MOBSTER has detected or confirmed the presence of a field in 21 new massive stars, and has the potential to do so in hundreds more. You’ll be able to follow the progress and new discoveries of MOBSTER on this website!
MOBSTER Collaboration 