A fast method for Stokes profile synthesis

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

doi:10.1051/0004-6361:200809981

Cited by 37 publications

(32 citation statements)

References 35 publications

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“…This was interpreted as the star's activity declining toward its magnetic minimum, due to which the spot distribution on the stellar surface was postulated to be more stochastic than during the high-activity state, veiling the possible drift due to the mechanism producing it underneath. We note that the results of Carroll et al (2007Carroll et al ( , 2009, computed from the same data with a different ZDI inversion method (Carroll et al 2008), also indicate a reduction in the magnetic field strength between the observing seasons 2004 and 2007. The authors reported, however, that a dominant and largely unipolar field seen in 2004 changed into two distinct, large-scale bipolar structures in 2007, not completely consistent with the proposed picture of the star declining toward its magnetic minimum with less pronounced, stochastic spot activity.…”

Section: Introductionmentioning

confidence: 73%

Multiperiodicity, modulations and flip-flops in variable star light curves

Lindborg

Mantere

Olspert

et al. 2013

A&A

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Aims. According to previously published Doppler images of the magnetically active primary giant component of the RS CVn binary II Peg, the surface of the star was dominated by one single active longitude that was clearly drifting in the rotational frame of the binary system during 1994-2002; later imaging for 2004-2010, however, showed decreased and chaotic spot activity, with no signs of the drift pattern. Here we set out to investigate from a more extensive photometric dataset whether this drift is a persistent phenomenon, in which case it could be caused either by an azimuthal dynamo wave or be an indication that the binary system's orbital synchronization is still incomplete. On a differentially rotating stellar surface, spot structures preferentially on a certain latitude band could also cause such a drift, the disruption of which could arise from the change of the preferred spot latitude. Methods. We analyzed the datasets using the carrier fit (CF) method, which is especially suitable for analyzing time series in which a fast clocking frequency (such as the rotation of the star) is modulated with a slower process (such as the stellar activity cycle).Results. We combined all collected photometric data into one single data set and analyzed it with the CF method. We confirm the previously published results that the spot activity has been dominated by one primary spotted region almost through the entire data set and also confirm a persistent, nearly linear drift. Disruptions of the linear trend and complicated phase behavior are also seen, but the period analysis reveals a rather stable periodicity with P spot = 6. d 71054 ± 0. d 00005. After removing the linear trend from the data, we identified several abrupt phase jumps, three of which are analyzed in more detail with the CF method. These phase jumps closely resemble what is called a flip-flop event, but the new spot configurations do not persist for longer than a few months in most cases. Conclusions. There is some evidence that the regular drift without phase jumps is related to the high state, while the complex phase behavior and disrupted drift pattern are related to the low state of magnetic activity. The most natural explanation of the drift is weak anti-solar (pole rotating faster than the equator) differential rotation with a coefficient k ≈ 0.002 combined with the preferred latitude of the spot structure.

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Section: Introductionmentioning

confidence: 73%

Multiperiodicity, modulations and flip-flops in variable star light curves

Lindborg

Mantere

Olspert

et al. 2013

A&A

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“…All maps were computed with our Doppler Imaging (DI) and (Zeeman Doppler Imaging) ZDI-code iMap (Carroll et al 2007(Carroll et al , 2008(Carroll et al , 2009(Carroll et al , 2012. Here we give just a brief description of the code, for further details see Carroll et al (2012).…”

Section: The Imap Codementioning

confidence: 99%

“…The code performs a multiline inversion of a large number of photospheric line profiles simultaneously. For the local line profile calculation, the code utilizes a full (polarized) radiative transfer solver (Carroll et al 2008). The atomic line parameters are taken from the VALD database (Kupka et al 1999).…”

Section: The Imap Codementioning

confidence: 99%

Spot evolution on the red giant star XX Triangulum

Künstler

Carroll

Strassmeier

2015

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Context. Solar spots appear to decay linearly proportional to their size. The decay rate of solar spots is directly related to magnetic diffusivity, which itself is a key quantity for the length of a magnetic-activity cycle. Is a linear spot decay also seen on other stars, and is this in agreement with the large range of solar and stellar activity cycle lengths? Aims. We investigate the evolution of starspots on the rapidly-rotating (P rot ≈ 24 d) K0 giant XX Tri, using consecutive time-series Doppler images. Our aim is to obtain a well-sampled movie of the stellar surface over many years, and thereby detect and quantify a starspot decay law for further comparison with the Sun. Methods. We obtained continuous high-resolution and phase-resolved spectroscopy with the 1.2-m robotic STELLA telescope on Tenerife over six years, and these observations are ongoing. For each observing season, we obtained between 5 to 7 independent Doppler images, one per stellar rotation, making up a total of 36 maps. All images were reconstructed with our line-profile inversion code iMap. A wavelet analysis was implemented for denoising the line profiles. To quantify starspot area decay and growth, we match the observed images with simplified spot models based on a Monte Carlo approach. Results. It is shown that the surface of XX Tri is covered with large high-latitude and even polar spots and with occasional small equatorial spots. Just over the course of six years, we see a systematically changing spot distribution with various timescales and morphology, such as spot fragmentation and spot merging as well as spot decay and formation. An average linear decay of D = −0.022 ± 0.002 SH/day is inferred. We found evidence of an active longitude in phase toward the (unseen) companion star. Furthermore, we detect a weak solar-like differential rotation with a surface shear of α = 0.016 ± 0.003. From the decay rate, we determine a turbulent diffusivity of η T = (6.3 ± 0.5) × 10 14 cm 2 /s and predict a magnetic activity cycle of ≈26 ± 6 yr. Finally, we present a short movie of the spatially resolved surface of XX Tri.

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“…A PCA decomposes a set of observed Stokes profiles into an empirical set of orthogonal eigenprofiles (e.g., Carroll et al 2008). As has been shown by Asensio Ramos & Lopez Ariste (2010) and Asensio Ramos & Allende Prieto (2010), a PCA expansion can result in a compact and sparse representation of Stokes profiles.…”

Section: Sparsity Of Stokes Profilesmentioning

confidence: 99%

Detecting and quantifying stellar magnetic fields

Carroll¹,

Strassmeier²

2014

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Context. In recent years, we have seen a rapidly growing number of stellar magnetic field detections for various types of stars. Many of these magnetic fields are estimated from spectropolarimetric observations (Stokes V) by using the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of this method rapidly deteriorates with increasing noise and thus calls for a more robust procedure that combines signal detection and field estimation. Aims. We introduce an estimation method that provides not only the effective or mean longitudinal magnetic field from an observed Stokes V profile but also uses the net absolute polarization of the profile to obtain an estimate of the apparent (i.e., velocity resolved) absolute longitudinal magnetic field. Methods. By combining the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able to decompose observed Stokes profiles with an overcomplete dictionary of wavelet-basis functions to reliably reconstruct the observed Stokes profiles in the presence of noise. The elementary wave functions of the sparse reconstruction process were utilized to estimate the effective longitudinal magnetic field and the apparent absolute longitudinal magnetic field. A multiresolution analysis complements the OMP algorithm to provide a robust detection and estimation method.Results. An extensive Monte-Carlo simulation confirms the reliability and accuracy of the magnetic OMP approach where a mean error of under 2% is found. Its full potential is obtained for heavily noise-corrupted Stokes profiles with signal-to-noise variance ratios down to unity. In this case a conventional COG method yields a mean error for the effective longitudinal magnetic field of up to 50%, whereas the OMP method gives a maximum error of 18%. It is, moreover, shown that even in the case of very small residual noise on a level between 10 −3 and 10 −5 , a regime reached by current multiline reconstruction techniques, the conventional COG method incorrectly interprets a large portion of the residual noise as a magnetic field, with values of up to 100 G. The magnetic OMP method, on the other hand, remains largely unaffected by the noise, regardless of the noise level the maximum error is no greater than 0.7 G.

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A fast method for Stokes profile synthesis

Cited by 37 publications

References 35 publications

Multiperiodicity, modulations and flip-flops in variable star light curves

Multiperiodicity, modulations and flip-flops in variable star light curves

Spot evolution on the red giant star XX Triangulum

Detecting and quantifying stellar magnetic fields

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