Zeeman-Doppler imaging: old problems and new methods

Carroll, T. A.; Kopf, M.; Strassmeier, K. G.; Ilyin, I.

doi:10.1017/s1743921309031469

Cited by 13 publications

(13 citation statements)

References 19 publications

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“…One popular and very successful extraction technique which follows this line of arguments is the so called least-squares deconvolution (LSD, Donati et al 1997;Kochukhov et al 2010). Another method is the principal component analysis (PCA, Carroll et al 2007;Martínez González et al 2008;Carroll et al 2009;Paletou 2012) or the simple but very effective coherent addition of line profiles in the velocity or logarithmic wavelength domain (Semel et al 2009;Ramírez et al 2010).…”

Section: Methodsmentioning

confidence: 99%

“…In this study, we use the iMap code (Carroll et al , 2009 to simultaneously reproduce the temperature and magnetic vector field distribution from a sequence of observed Stokes I and V profiles. The forward modeling in iMap is based on polarized radiative transfer to allow for the best possible accuracy in line profile modeling (Carroll et al 2009).…”

Section: A Brief Description Of Imapmentioning

confidence: 99%

“…The forward modeling in iMap is based on polarized radiative transfer to allow for the best possible accuracy in line profile modeling (Carroll et al 2009). All line parameters necessary for the synthetic calculation are taken from the Vienna atomic line list (VALD; Kupka et al 1999.…”

Section: A Brief Description Of Imapmentioning

confidence: 99%

“…The three major issues that constitute an ill-posed, inverse problem are: (i) a solution may not exist; (ii) the problem may not be unique; (iii) the solution does not depend continuously on the data. We will not go into problem (i) which would require a discussion about the adequacy of the underlying ZDI/DI model assumptions (see Carroll et al 2009, for a discussion), but instead we assume (as usual) that our model admits a solution. Then, we are left with the uniqueness problem (ii) and the numerical stability problem (iii) which both depend also on the noise level of the data.…”

Section: A Brief Description Of Imapmentioning

confidence: 99%

“…(22) down to the threshold δ we would ensure that the reduced χ 2 is close to one. But let us emphasize here that DI as well as ZDI (given we have correctly A95, page 6 of 18 A95, page 7 of 18 modeled the problem) are non-linear problems (see Carroll et al 2009) and moreover the problem is generally ill-posed, which makes it by no means a simple task to adequately determine the real degree of freedom of the problem. The reduced χ 2 as a measure of the goodness-of-fit may therefore only of limited use.…”

Section: A Brief Description Of Imapmentioning

confidence: 99%

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The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Carroll

Strassmeier

Rice

et al. 2012

A&A

114

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Aims. In a follow-up investigation we present Zeeman-Doppler maps of the weak-lined T Tauri star (WTTS) V410 Tau. As a rapid rotating star and a typical WTTS the stellar surface of V410 Tau is accessible to surface imaging techniques and allows us to detect and reconstruct the major magnetic surface features on this pre-main sequence star. Methods. The polarized signals we are measuring are on the order of 10 −4 to 10 −3 and are hidden well below the noise level of a single observation. A new line profile reconstruction technique based on a singular value decomposition (SVD) allows us to extract the weak polarized line profiles (Stokes V) as well as the intensity profiles (Stokes I). One of the key features of the line profile reconstruction is that the SVD line profiles are amenable to radiative transfer modeling within our Zeeman-Doppler Imaging code iMap. The code also utilizes a new iterative regularization scheme which is independent of any additional surface constraints. To provide more stability a vital part of our inversion strategy is to invert both Stokes I and Stokes V profiles to simultaneously reconstruct the temperature and magnetic field surface distribution of V410 Tau. A new image-shear analysis is also implemented to allow the search for image and line profile distortions induced by a differential rotation of the star. Results. The magnetic field structure we obtain for V410 Tau shows a good spatial correlation with the surface temperature and is dominated by a strong field within the cool polar spot. The Zeeman-Doppler maps exhibit a large-scale organization of both polarities around the polar cap in the form of a twisted bipolar structure. The magnetic field reaches a value of almost 2 kG within the polar region but smaller fields are also present down to lower latitudes. The pronounced non-axisymmetric field structure and the nondetection of a differential rotation for V410 Tau supports the idea of an underlying α 2 -type dynamo, which is predicted for WTTS.

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Section: Methodsmentioning

confidence: 99%

Section: A Brief Description Of Imapmentioning

confidence: 99%

Section: A Brief Description Of Imapmentioning

confidence: 99%

Section: A Brief Description Of Imapmentioning

confidence: 99%

Section: A Brief Description Of Imapmentioning

confidence: 99%

See 3 more Smart Citations

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Carroll

Strassmeier

Rice

et al. 2012

A&A

114

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Rotation, activity, and lithium abundance in cool binary stars

et al. 2012

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We have used two robotic telescopes to obtain time-series high-resolution optical echelle spectroscopy and VI and/or by photometry for a sample of 60 active stars, mostly binaries. Orbital solutions are presented for 26 double-lined systems and for 19 single-lined systems, seven of them for the first time but all of them with unprecedented phase coverage and accuracy. Eighteen systems turned out to be single stars. The total of 6609 R = 55 000échelle spectra are also used to systematically determine effective temperatures, gravities, metallicities, rotational velocities, lithium abundances and absolute Hα-core fluxes as a function of time. The photometry is used to infer unspotted brightness, V − I and/or b − y colors, spot-induced brightness amplitudes and precise rotation periods. An extra 22 radial-velocity standard stars were monitored throughout the science observations and yield a new barycentric zero point for our STELLA/SES robotic system. Our data are complemented by literature data and are used to determine rotation-temperature-activity relations for active binary components. We also relate lithium abundance to rotation and surface temperature. We find that 74 % of all known rapidly-rotating active binary stars are synchronized and in circular orbits but 26 % (61 systems) are rotating asynchronously of which half have Prot > P orb and e > 0. Because rotational synchronization is predicted to occur before orbital circularization active binaries should undergo an extra spin-down besides tidal dissipation. We suspect this to be due to a magnetically channeled wind with its subsequent braking torque. We find a steep increase of rotation period with decreasing effective temperature for active stars, Prot ∝ T −7 eff , for both single and binaries, main sequence and evolved. For inactive, single giants with Prot > 100 d, the relation is much weaker, Prot ∝ T −1.12 eff . Our data also indicate a period-activity relation for Hα of the form RHα ∝ P −0.24 rot for binaries and RHα ∝ P −0.14 rot for singles. Its power-law difference is possibly significant. Lithium abundances in our (field-star) sample generally increase with effective temperature and are paralleled with an increase of the dispersion. The dispersion for binaries can be 1-2 orders of magnitude larger than for singles, peaking at an absolute spread of 3 orders of magnitude near T eff ≈ 5000 K. On average, binaries of comparable effective temperature appear to exhibit 0.25 dex less surface lithium than singles, as expected if the depletion mechanism is rotation dependent. We also find a trend of increased Li abundance with rotational period of form log n(Li) ∝ −0.6 log Prot but again with a dispersion of as large as 3-4 orders of magnitude.

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Magnetic field measurements of ϵ Eridani from Zeeman broadening

et al. 2015

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We present new magnetic field measurements of the K2 main-sequence star Eri based on principal components analysis (PCA) line-profile reconstructions. The aim of this paper is to quantify the surface-averaged magnetic field and search for possible variations. A total of 338 opticaléchelle spectra from our robotic telescope facility STELLA with a spectral resolution of 55 000 were available for analysis. This time-series was used to search for the small line-profile variations due to a surface magnetic field with the help of a PCA. Evidence for a spatial and temporal inhomogeneous magnetic field distribution is presented. The mean, surface averaged, magnetic field strength was found to be B = 186 ± 47 G in good agreement with previous Zeeman-broadening measurements. Clear short-term variations of the surface averaged magnetic field of up to few tens Gauss were detected together with evidence for a three-year cycle in the surface-averaged magnetic field of Eri.

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Zeeman-Doppler imaging: old problems and new methods

Cited by 13 publications

References 19 publications

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Rotation, activity, and lithium abundance in cool binary stars

Magnetic field measurements of ϵ Eridani from Zeeman broadening

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