Stellar magnetic fields and surface structures

Magnetic fields play a key role at many stages of stellar formation and evolution. The fields represent a key ingredient of stellar and planetary system formation processes. They are also responsible for the angular momentum loss in young stars and is the main energy source behind a broad range of dynamic phenomena (flares, X-ray emission, star spots) occurring at the surface layers of the Sun and other stars. However, magnetic fields are challenging to detect and model directly.

We use time-series spectropolarimetric observations and advanced remote sensing algorithms, coupled with detailed theoretical calculations, to detect the feeble signatures of magnetic fields and to study their topologies. Magnetic and star spot maps reconstructed by our group reveal intricate details of the magneto-hydrodynamical processes on the stellar surfaces and in circumstellar environments. Our studies provide unique constraints to the theories of stellar magnetism, activity, and structure formation.

Time-series spectroscopy and spectropolarimetry

High-resolution spectroscopy and spectropolarimetry are the primary tools for studies of stellar magnetic fields and surface structures. Inhomogeneities on stellar surfaces distort spectral line profile shapes, producing characteristic signatures that can be detected and interpreted. Magnetic fields also give rise to polarisation signatures in spectral lines via the Zeeman effect. This enables detection of stellar magnetic fields and reconstruction of their topologies. Both spectroscopic and spectropolarimetric observations must be obtained repeatedly to resolve the time-dependent phenomena such as pulsations and rotational modulation.

We are contributing to the development of instrumentation for the UV, optical, and near-infrared spectroscopy and spectropolarimetry at space and large ground-based  telescopes. We participate in major international projects (e.g. MiMeS and Binamics collaborations) aimed at collecting observational data with these instruments. We also provide the community with advanced tools for the spectroscopic and spectropolarimetric data reduction and multi-line analysis. We develop and maintain computer codes for the calculation of theoretical stellar spectra and an interactive comparison of these calculations with observations.

Indirect imaging of stellar surfaces

The characteristic distortions produced by magnetic fields and star spots move across Doppler-broadened intensity and polarisation line profiles. This line profile variability provides enough information to reconstruct a two-dimensional map of stellar surface. This technique, known as Doppler imaging (DI) and Magnetic/Zeeman Doppler imaging (MDI/ZDI), represents the highest resolution indirect imaging method currently used in astronomy.

We have developed and applied to different types of stars a variety of DI codes for the reconstruction of temperature, chemical abundance, and pulsation velocity maps from intensity spectra. We have also developed ZDI inversion codes to recover vector magnetic field maps from spectropolarimetric observations. While most of stellar magnetism studies use only circular polarisation, we have pioneered the use of the full Stokes vector observations for stellar magnetic field mapping.

Simulation showing hof star spot effects intensity profile of a spectral line..Simulation showing how spectral lines are effected by stellar magnetic fields.

Left: variation of the intensity profile of a spectral line due to cool spots on the stellar surface. Right: variation of the four Stokes parameter profiles of a magnetically sensitive spectral line due to an oblique dipolar magnetic field.

Magnetic fields and star spots across the H–R diagram

We use spectropolarimetry and ZDI to detect and reconstruct topologies of surface magnetic fields in stars across the entire Hertzsprung–Russell diagram. These studies provide key constraints to stellar magnetism theories, including stellar dynamos, relaxation of fossil magnetic fields, etc. Our investigations are also relevant in the context of many other astrophysical phenomena where magnetic fields are often invoked but seldom observed directly.

Massive stars

We study magnetic field geometries of early-B and O stars in the context of “Magnetism in Massive Stars” (MiMeS) collaboration. Our magnetic maps provide the basis for investigation of magnetospheres surrounding of massive stars.

Intermediate-mass stars

These late-B and A-type stars possess remarkably strong global magnetic fields and non-uniform distributions of chemical elements. For these stars we use four Stokes parameter observations to produce detailed models of magnetic field topologies. We also reconstruct horizontal and vertical maps of chemical spots. These surface structure studies enable sensitive tests of stellar magneto-hydrodynamic theories.

Late-type active stars

We study magnetic field geometries of different classes of rapidly rotating, magnetically active late-type stars. Our ZDI studies aim at following evolution of the dynamo-generated magnetic fields through stellar activity cycles. We also carry out detailed modelling of the spectroscopic and spectropolarimetric data with the goal to measure magnetic fields and assess thermodynamic properties of star spots.

Young stars

This is another type of cool stars displaying conspicuous surface activity related to the presence of strong magnetic fields. We measure magnetic fields though the Zeeman broadening and polarisation and study the field topologies of T Tauri stars of different masses. The general aim of this research is to establish how the field structure and strength change in the course of early stellar evolution. Of special interest are intermediate-mass T Tauri stars which start as cool objects with dynamo fields and then evolve into magnetic A and B stars with fossil fields.

Active M dwarfs

At the bottom of the main sequence one finds low-mass stars of M spectral classes. Some of these stars possess strong magnetic fields. The nature of these fields and their exact geometries are currently unknown. We study magnetic fields of M dwarfs with near-infrared high-resolution spectra and with optical spectropolarimetry. The focus of this work is to develop self-consistent magnetic topology models capable of explaining all available magnetic observations.

Illustrating the magnetic field topology of a star.
3-D rendering of the surface and circumstellar magnetic field topology of the massive star HD 37776 studied with magnetic Doppler imaging. The star is shown at two different rotation phases. The surface maps and magnetic field loops are colour coded according to the strength of the radial field component.
PLot showing chemical star spot maps.
Chemical star spot maps of the late-A magnetic star HD 83368. These chemical distributions were obtained by modelling line profile variations of individual elements with a DI inversion code.

Vertical stratification of chemical elements

Strong magnetic fields stabilise atmospheres of early-type magnetic stars, facilitating operation of the radiative diffusion process. Under the competing influence of gravity and radiation pressure chemical elements accumulate in distinct layers of under- or overabundance in stellar atmospheres. The resulting chemical gradients can reach several orders of magnitude within a thin atmospheric layer. This leads to characteristic anomalies of the spectral line strengths and profile shapes. We have developed and systematically applied inversion methods for reconstruction of the vertical chemical abundance gradients from high-resolution spectroscopic observations. These empirical chemical stratification models, now available for several dozen stars, enable sensitive tests of the theories of hydrodynamic processes (atomic diffusion, rotational mixing, weak mass loss) operating in stellar atmospheres.

Plot illustrating vertical chemical stratification profiles.
Vertical chemical stratification profiles for the cool magnetic A star HD 24712. Abundances of different elements are shown as a function of height in the stellar atmosphere. The colour code corresponds to the deviation from solar abundance values expressed in logarithmic units.

Pulsational tomography of stellar atmospheres

A handful of A-type magnetic stars (so-called “rapidly oscillating Ap” or roAp stars) pulsate in high-overtone non-radial p-modes, with periods on the order of 10 min. These pulsations, associated with the presence of strong magnetic fields in stellar envelopes, enable asteroseismic analysis that provides useful information about fundamental stellar parameters. Besides the classical asteroseismology high-resolution spectroscopy of roAp pulsations can provide a remarkably detailed view of the propagation of magneto-hydrodynamic waves in stellar atmospheres. Besides the Sun, this is the only class of stars for which such detailed vertical analysis of non-radial pulsations is possible.

We have carried out numerous time-resolved spectroscopic studies of roAp stars using spectrographs at large telescopes, such as ESO 3.6-m and 8-m VLT. These investigations led to discoveries of several new roAp stars. We have also reconstructed height dependence of the pulsational amplitude and phase by studying variations of individual chemical elements. In addition to these vertical pulsation tomography studies, we interpreted pulsational line profile variations and developed techniques to map the horizontal structures of non-radial pulsations. These empirical studies were confronted with predictions of theoretical magneto-hydrodynamic models to gain a better insight into the nature of magneto-acoustic pulsations.

Plot showing the effect of rapid line profile variations.
Typical rapid line profile variations in the time-resolved spectra of a rapidly oscillating Ap star. Left: the average spectrum and time evolution of the residuals. Pulsations are very weak in the lines of iron-peak elements but reach high amplitudes in rare-earth lines which form in the uppermost atmospheric layers. Right: radial velocity curves phased with pulsation period for the lines identified in the left panel. A gradual increase of the radial velocity amplitude and a change of phase indicate an outward propagating pulsation wave.


Axel Hahlin, Oleg Kochukhov, Alexis Lavail, Nikolai Piskunov

Last modified: 2021-01-29