Doppler Imaging Code Research Articles

First Magnetic Doppler Images of a roAp star

Inhalt (u.a.): M. BREGER: Delta Scuti stars: Observational aspects; M.S. Cunha: Theory of rapidly oscillating Ap stars; M. SACHKOV et al.: Vertical structure of pulsations in roAp stars; G. HOUDEK: Theoretical asteroseismology of solar-like oscillations; B.L. POPIELSKI: Core modes as a seismic probe of mixing beyond the convective core; M. GODART: Temperature gradients in the core overshooting region; W.A. DZIEMBOWSKI – J. DASZYŃSKA-DASZKIEWICZ – A.A. PAMYATNYKH: Interpretation of the Be star HD 163868 oscillation spectrum based on the MOST observations; D. KILKENNY: Pulsating Hot Subdwarfs – An Observational Review; M.H. MONTGOMERY: Mapping Convection using Pulsating White Dwarf Stars; M. ROTH: The Network Activities in HELAS; I. KUDZEJ et al.: A New Slovak Observatory 500 km from Vienna; W.W. WEISS: Microsatellites; G. KOPACKI – S. FRANDSEN: RR Lyrae stars in M4

Doppler Images of ζ Andromedae

Doppler images are presented for the RSCVn-type binary ζ And. Our upgraded Doppler imaging code TempMap∈ takes into account the distorted geometry of the primary giant component. On the maps several low latitude spots are restored with a temperature contrast of about 1000 K. Some weak polar features are also found. Crosscorrelation of the consecutive Doppler-maps suggests solar-like differential surface rotation.

Starspot Activity of V711 Tau from November to December 2005

AbstractTwo new high-resolution spectroscopic datasets of the active binary V711 Tau are analyzed here. Using the Least-Squares Deconvolution method, we have derived the time series profiles of V711 Tau with high signal-to-noise ratio, which have been used to reconstruct the starspot patterns for its two components by means of the Doppler imaging code DoTS. The results show that the high latitude spots are main features for the primary, the low latitude spots are clear for the secondary in both of two observing runs, and the starspot pattern changes obviously during one month. The lifetime of intermediate-low latitude spots is shorter than the high latitude spots for the primary, the low latitude spots of the secondary seem to have a longer lifetime.

Doppler imaging of Speedy Mic using the VLT

We study the short-term evolution of starspots on the ultrafast-rotating star HD 197890 (“Speedy Mic” = BO Mic, K 0-2 V, d) based on two Doppler images taken about 13 stellar rotations apart. Each image is based on spectra densely sampling a single stellar rotation. The images were reconstructed by our Doppler imaging code CLDI (Clean-like Doppler imaging) from line profiles extracted by spectrum deconvolution. Our Doppler images constructed from two independent wavelength ranges agree well on scales down to 10° on the stellar surface. In conjunction with nearly parallel V-band photometry our observations reveal a significant evolution of the spot pattern during as little as two stellar rotations. We suggest that such a fast spot evolution demands care when constructing Doppler images of highly active stars based on spectral time series extending over several stellar rotations. The fast intrinsic spot evolution on BO Mic impedes the determination of a surface differential rotation; in agreement with earlier results by other authors we determine an upper limit of .

Magnetic Doppler imaging of 53 Camelopardalis in all four Stokes parameters

We present the first investigation of the structure of the stellar surface magnetic field using line profiles in all four Stokes parameters. We extract the information about the magnetic field geometry and abundance distributions of the chemically peculiar star 53 Cam by modelling time-series of high-resolution spectropolarimetric observations with the help of a new magnetic Doppler imaging code. This combination of the unique four Stokes parameter data and state-of-the-art magnetic imaging technique makes it possible to infer the stellar magnetic field topology directly from the rotational variability of the Stokes spectra. In the magnetic imaging of 53 Cam we discard the traditional multipolar assumptions about the structure of magnetic fields in Ap stars and explore the stellar magnetic topology without introducing any global a priori constraints on the field structure. The complex magnetic model of 53 Cam derived with our magnetic Doppler imaging method achieves a good fit to the observed intensity, circular and linear polarization profiles of strong magnetically sensitive spectral lines. Such an agreement between observations and model predictions was not possible with any earlier multipolar magnetic models, based on modelling Stokes I spectra and fitting surface averaged magnetic observables (e.g., longitudinal field, magnetic field modulus, etc.). Furthermore, we demonstrate that even the direct inversion of the four Stokes parameters of 53 Cam assuming a low-order multipolar magnetic geometry is incapable of achieving an adequate fit to our spectropolarimetric observations. Thus, as a main result of our investigation, we discover that the magnetic field topology of 53 Cam is considerably more complex than any low-order multipolar expansion, raising a general question about the validity of the multipolar assumption in the studies of magnetic field structures of Ap stars. In addition to the analysis of the magnetic field of 53 Cam, we reconstruct surface abundance distributions of Si, Ca, Ti, Fe and Nd. These abundance maps confirm results of the previous studies of 53 Cam, in particular dramatic antiphase variation of Ca and Ti abundances.

Doppler Imaging of the Ap starϵUrsae Majoris: Ca, Cr, Fe, Mg, Mn, Ti, Sr

The surface abundance structure of seven elements of the brightest chemically peculiar A-type star, Ursae Majoris ( UMa, HD 112185, HR 4905) has been determined. Cr, Fe and, Mn are enhanced at the magnetic polar region of UMa while they avoid the magnetic equator. Sr seems to behave like Cr, Fe, and Mn, but is concentrated only at one of the two magnetic poles. Ti, in contrast, is accumulated at the magnetic equator and is depleted where Cr and Fe are accumulated, in accordance with theoretical predictions. Mg and Ca do not correlate with the distribution of Cr, Fe, Mn, and Sr or with Ti. The surface distribution of Mn, Sr, Ti, and Mg was determined for the first time for this star. A Doppler Imaging code was used that allows to analyse elements present in complex spectral line blends and thus to increase significantly the potential to map more elements. We compare our Cr and Fe distributions to already published maps. The high consistency of our results, based on dierent observations and Doppler Imaging codes, proves the reliability of the dierent methods.

Doppler Imaging of stellar magnetic fields

We used the new magnetic Doppler Imaging code to reconstruct the magnetic eld geometry and surface abundance distributions for the classical magnetic CP star 2 CVn. High-resolution spectropolarimetric observations in the Stokes I and V parameters were collected with the SOFIN echelle spectrograph at the Nordic Optical Telescope. This superb observational material in combination with the advanced modelling technique allowed to achieve the rst simultaneous self-consistent mapping of the vector magnetic eld and the abundance distributions of six chemical elements. In recovering the stellar magnetic distribution no prior assumptions about the eld geometry or strength were made. Instead, we restricted possible solutions of the inverse problem by means of the multipolar regularization method which searches for the eld map close to a general non-axisymmetric multipolar conguration but allows departures from this geometry if that is required by the observational data. We found that the magnetic eld of 2 CVn is dominated by a dipolar component and has a minor quadrupole contribution. The surface distributions of the chemical species form symmetric patterns which closely follow the magnetic geometry. This discovery constitutes one of the rst direct observational constraints on the horizontal diusion processes acting in the upper envelope of a strongly magnetized stellar atmosphere. In addition to the extensive magnetic Doppler Imaging analysis, we derived new accurate estimates of the atmospheric parameters and basic physical properties of 2 CVn using the energy distribution, hydrogen line proles and the recent

Doppler Imaging of stellar magnetic fields

We present numerical experiments designed to evaluate the performance of the new Magnetic Doppler Imaging (MDI) code INVERS10. Numerous test runs demonstrate what can be achieved with the MDI method and allow assessment of the systematic errors that can cause distortions of the maps. Our numerical experiments showed that given high-resolution observations in four Stokes parameters the code is capable of reconstructing abundance and magnetic field vector distributions simultaneously and without any prior assumptions about the magnetic field geometry. At the same time we found that in order to achieve reliable reconstruction using only circular polarization data it is necessary to impose additional constraints on the possible structure of the magnetic field. Numerical tests also reveal surprisingly different properties of the MDI maps from conventional scalar maps which we attribute to the complex relation between the orientation of the field and the polarization signal. We conclude that the information about the field is primarily extracted from the changes in magnetic orientation due to rotation rather than from the Doppler shifts critical for scalar Doppler Imaging maps making it possible to apply the MDI to even very slow rotators.

Doppler Imaging of stellar magnetic fields

We describe our latest Magnetic Doppler Imaging (MDI) code capable of simultaneously reconstructing the magnetic field vector and chemical composition distribution over the stellar surface. Input data consists of polarization measurements in the line profiles and the reconstruction is performed by solving the regularized inverse problem. The code incorporates the latest developments in the radiative transfer, the optimization and parallel computing techniques. We have also developed a new regularization method that allows application of our MDI code to incomplete data sets (e.g. only intensity and circular polarization) by restricting possible configurations of the field. Numerical experiments to be presented in a forthcoming paper demonstrate what can be achieved with MDI and allow studying of systematic errors that can cause serious distortions of the maps.

Comparisons of images derived from independent Zeeman Doppler imaging codes

We compare surface brightness and magnetic field surface maps for the K0 dwarf AB Doradus, obtained using two independently developed Zeeman Doppler imaging codes. We find that even the fine structure in the two sets of images shows excellent agreement. Magnetic field maps appear to be less sensitive to line-modelling effects than brightness maps. We argue that these results point to the consistency and reliability of Doppler imaging techniques even when using poorly sampled data sets, provided that the spectra used are of sufficiently high signal-to-noise ratio.

Reliability tests for Doppler imaging

In recent years, Doppler imaging codes have been developed by a number of different groups and successfully applied to map the spot distribution on stellar surfaces. As more and more images are obtained we tend to see a pattern of high-latitude spots, particularly on RS CVn systems. These high-latitude spots were and still are greeted with scepticism as they are inferred from non-varying profile deformations. Here, I describe some of the problems of Doppler imaging and their likely impact on the resulting images.