Despite an abundance of indirect evidence of the importance of magnetic fields, direct detection and study of magnetic fields on stars other than the Sun is an extremely challenging task. Such analysis was impossible until recently, leading astronomers to speculate about stellar magnetism based on theoretical models extrapolated from the Sun using unreliable indirect proxy indicators (flaring, X-ray emission).
During the last decade new instruments at large telescopes, especially high-resolution spectropolarimeters, have allowed detecting magnetic field in essentially all types of stars. These discoveries led to a completely new and, often, unexpected perspective on the geometry and evolution of stellar magnetic topologies. Theoretical studies are only now starting to explore implications of this new picture of stellar magnetism.
The main topic of my research is observational investigation of magnetic fields and reconstruction of magnetic field topologies in massive, intermediate-mass and low-mass stars. In particular, I carry out the following research projects:
|Spectropolarimetric studies and modelling of magnetic field topologies in massive O and B stars|
|Four Stokes parameter observations and reconstruction of magnetic field topologies in magnetic Ap/Bp stars|
|Polarimetric detection and mapping the structure of magnetic fields in young active Sun-like stars|
|Study of magnetic fields in low-mass stars (M dwarfs and T Tauri stars) using high-resolution infra-red spectroscopy|
Magnetic field structure of the Ap star
α2 CVn reconstructed using spectropolarimetric
observations in all four Stokes parameters.
Top panel: field strength distribution
Bottom panel: orientation of magnetic field vectors
Magnetic Ap stars represent are among the most interesting targets for applications of asteroseismology. These stars exhibit non-radial p-mode oscillations with periods 5-15 minutes in the presence of highly non-uniform atmospheric chemistry and strong magnetic fields. I have pioneered spectroscopic time-resolved observations of pulsating Ap stars using large telescopes. These studies revealed a puzzling pulsational behaviour, characterized by a large diversity of radial velocity amplitudes and phase, and an unusual profile variations of the lines of rare-earth elements.
We have succeeded in interpreting and modeling pulsations in magnetic Ap stars in terms of magneto-acoustic waves, propagating outwards in stellar atmospheres with increasing amplitude and changing phase. This pulsational behaviour allows reconstructing from observations a three-dimensional picture of pulsation waves, chemical stratification, and magnetic fields, as can be done for no other star but the Sun.
Typical rapid line profile variations in an oscillating Ap star.
Left panel: the average spectrum and time evolution of the residuals showing pulsation in rare-earth lines.
Right panel: pulsational radial velocity curves showing the outward propagation of the magneto-acoustic wave.
I am part of the world's leading group in imaging of stellar surfaces. I have developed Doppler and Zeeman-Doppler inversion codes for mapping of
|chemical and isotopic spot distributions in Ap/Bp stars|
|temperature spot distributions in active late-type stars|
|magnetic field structure in both early-type and late-type stars|
|velocity field in non-radially pulsating stars|
Using this software, I have carried out, for the first time, reconstruction of the stellar magnetic field geometries from the spectropolarimetric observations in all four Stokes parameters.
A brief overview of the scientific results obtained in our DI studies and an illustration of the basic principles of the DI technique can be found in my DI Gallery.
|Chemical spot distribution at the surface of Ap star HR 3831.|
Standard 1-D theoretical codes are available to model atmospheres of cool and hot stars, assuming solar or scaled-solar chemical composition. Such tools are generally not applicable to B, A, and F stars with anomalous surface abundances. In these stars line opacity differs considerably from normal stars and also varies substantially from one star to another, depending on individual chemical composition and occasional presence of strong magnetic fields.
Together with my collaborators, I utilize an advanced opacity sampling stellar model atmosphere code LLMODELS to study effects of non-solar abundances, modification of the line opacity due to Zeeman splitting, magneto-hydrostatic equilibrium with the Lorentz force included, and an inhomogeneous vertical and horizontal distribution of chemical elements.
My theoretical work with the LLMODELS code is coupled with observational investigations using world's largest telescopes. These studies are aimed at testing new model atmospheres, finding and interpreting the signatures of anomalous atmospheric structure in chemically peculiar B-F stars.
|Spectral energy distribution of the cool Ap star alpha Cir modeled taking into account non-solar abundances and vertical stratification of chemical elements.|
The standard assumption of the abundance analyses is that stellar atmosphere is chemically homogeneous. Although reasonable for cool stars with convective envelopes, this may not apply to hotter stars without surface convection zones. The latter stars can possess considerable abundance gradients within the line forming regions due to operation of chemical transport processes.
Taking advantage of the wealth of information available in modern high-quality stellar spectroscopic observations, I have pioneered development and application of the inversion techniques to determine radial distribution of chemical elements in stellar atmospheres. My studies have added a new dimension to stellar abundance analysis, revealing, for the first time, vertical separation of elements in the atmospheres of chemically peculiar B-F stars.
|Radial distribution of chemical elements in the atmosphere of late-A star HR 1217. Chemical abundances are given on the logarithmic scale relative to the Sun.|
I have developed codes for simulating composite spectra of multiple stars and for disentangling contributions of individual stars using time-series spectroscopic observations. These techniques are applied in the context of chemical abundance studies of early-type binary stars. A puzzling result of these studies is existence of significant difference in the chemical composition of close binary components with very similar fundamental parameters.
|Disentangling of composite spectra of the eclipsing binary star AR Aur. The primary component of this system is a spotted chemically peculiar star of the rare HgMn-type.|
I also employ high-resolution spectropolarimeters at the 2-4-m telescopes, in particular CFHT and NOT, to carry out investigations of stellar magnetic fields and to study inhomogeneities and dynamical phenomena at stellar surfaces.
A consortium including our group at Uppsala university has recently developed full Stokes vector polarimeter, HARPSpol, for the HARPS instrument at the ESO 3.6-m telescope. I am leading science verification observations and exploitation of the truly unique polarization data delivered by this most advanced stellar polarimeter.
I use existing and develop my own computer programs for 3-D visualization of the models of magnetic fields, non-radial pulsations, and surface inhomogeneities at the surfaces of stars.
Left: density distribution in 3-D hydrodynamic simulation of
convection in red supergiant star.
Right: 3-D rendering of the magnetic field structure in the He-strong Bp star HD 37776.