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Doppler Imaging

Doppler Imaging of Stellar Surface Structure

Written by K. G. Strassmeier

Friday, 01 June 2001 18:34


Scientific Goals of this Project

One of the most challenging observational goals of today's stellar activity research is to obtain two-dimensional images of inhomogeneous stellar surfaces to relate disk integrated observations to spatially resolved surface features.

Spots cooler than the undisturbed photosphere of a late-type star are a manifestation of surface magnetic fields and a time-dependent study of their latitudinal and longitudinal behavior provides a direct link to the internal stellar dynamo. Our long-term objective is thus to provide conclusive observational constraints for a generalized theory of stellar magnetism.
The near-term goal, however, is to enlarge the available sample of stars with a Doppler map and to investigate the surface spot morphology as a function of stellar rotation.




What is Doppler imaging?

Doppler imaging is an inversion technique to recover a 2-D image of a rapidly rotating star from a series of high-resolution spectral line profiles. The inverse problem for stars with cool spots amounts to solving the integral equation relating the surface temperature distribution to the observed line profiles and light and color curve variations, while controlling the effects of noise in the data through a regularizing functional. Note that we solve for the photometric variations simultaneously with the line profiles, but that we can handle only one line per solution. If the spectroscopic phase coverage contains gaps of more than 25 degr on the stellar surface more weight is shifted to the photometry. In a recent application to the K-dwarf LQ Hya (Strassmeier, Rice, Wehlau et al. 1993, A&A 268, 671) we made up an average map from nine spectral lines and two broad-band colors. This ensured that spurious features from a single line, caused by noise or any other systematic error, will be suppressed in the final map.

Download PDF file (19MB): Resolving stellar surface spots (K.G. Strassmeier, Stellar Coronae, MPIfR, 2006 December)


Optical, Ultraviolet, and X-ray Observations Planned and in Progress

The first star investigated in this project was the well-known FK Comae star HD 199178 = V1794 Cyg, the second star was UZ Librae, a RS CVn binary with a K giant as the target star. Other targets currently being observed include T Tauri stars, young single main-sequence stars, RS CVn binaries, and single rapidly-rotating field giants.

The optical observations are being conducted at KPNO, NSO, CFHT, and ESO. X-ray observations were made or are in progress with the HRI on board of ROSAT. Ultraviolet observations are scheduled in cycle-5 with the Hubble Space Telescope. This will enable us to obtain Doppler maps at different heights in the stellar atmosphere and thereby obtain 3-D information on the temperature/magnetic flux distribution.


Last Updated on Tuesday, 08 December 2009 22:09