Scattering linear polarization of late-type active stars

Yakobchuk, T. M.; Berdyugina, S. V.

doi:10.1051/0004-6361/201730513

Cited by 2 publications

(2 citation statements)

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“…Even for a cooler star with a spot covering 5% of the disk, this will not exceed a few ppm. Only in very densely spotted stars will it be higher (Yakobchuk & Berdyugina 2018).…”

Section: Discussionmentioning

confidence: 95%

“…Strong localised fields might be produced in starspots (Huovelin & Saar 1991;Saar & Huovelin 1993). Or starspots might produce polarization by breaking symmetry, not in the spectral lines, but on the disk of the star instead (Yakobchuk & Berdyugina 2018). In red super/giant stars, stellar hotspots have also been found to produce linear polarization (Schwarz 1986;Aurière et al 2016).…”

Section: Introductionmentioning

confidence: 99%

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The rotationally modulated polarization of ξ Boo A

Cotton

Evensberget

Marsden

et al. 2018

Monthly Notices of the Royal Astronomical Society

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We have observed the active star ξ Boo A (HD 131156A) with high precision broadband linear polarimetry contemporaneously with circular spectropolarimetry. We find both signals are modulated by the 6.43 day rotation period of ξ Boo A. The signals from the two techniques are 0.25 out of phase, consistent with the broadband linear polarization resulting from differential saturation of spectral lines in the global transverse magnetic field. The mean magnitude of the linear polarization signal is ∼4 ppm/G but its structure is complex and the amplitude of the variations suppressed relative to the longitudinal magnetic field. The result has important implications for current attempts to detect polarized light from hot Jupiters orbiting active stars in the combined light of the star and planet. In such work stellar activity will manifest as noise, both on the time scale of stellar rotation, and on longer time scales -where changes in activity level will manifest as a baseline shift between observing runs.

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“…Even for a cooler star with a spot covering 5% of the disk, this will not exceed a few ppm. Only in very densely spotted stars will it be higher (Yakobchuk & Berdyugina 2018).…”

Section: Discussionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

The rotationally modulated polarization of ξ Boo A

Cotton

Evensberget

Marsden

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Spectroscopic long-term monitoring of RZ Cas

Lehmann

Dervişoğlu

Mkrtichian

et al. 2020

A&A

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Context. RZ Cas is a short-period Algol-type system showing episodes of mass transfer and δ Sct-like oscillations of its massgaining primary component. This system exhibits temporal changes in orbital period, v sin i, and the oscillation pattern of the primary component. Aims. We analyse high-resolution spectra of RZ Cas that we obtained during a spectroscopic long-term monitoring lasting from 2001 to 2017. In this first part we investigate the atmospheric parameters of the stellar components and the time variation of orbital period, v sin i, and radial velocities (RVs), searching for seasonal changes that could be related to episodes of mass exchange and to a possible activity cycle of the system triggered by the magnetic cycle of the cool companion. Methods. We used spectrum synthesis to analyse the spectra of both components of RZ Cas. The study of variations of the orbital period is based on published times of primary minima. We used the LSDbinary program to derive separated RVs and least-squares deconvolved (LSD) profiles of the components. From the LSD profiles of the primary we determined its v sin i. Using Markov Chain Monte Carlo simulations with the PHOEBE program, we modelled the RV variations of both components. Results. Spectrum analysis resulted in precise atmospheric parameters of both components, in particular in surface abundances below solar values. We find that the variation of orbital period is semi-regular and derive different characteristic timescales for different epochs of observation. We show that the RV variations with orbital phase can be modelled when including two cool spots on the surface of the secondary component. The modelling leads to very precise masses and separation of the components. The seasonal variation of several parameters, such as v sin i, rotation-orbit synchronisation factor, strength of the spots on the cool companion, and orbital period, can be characterised by a common timescale of the order of nine years. Conclusions. We interpret the timescale of nine years as the magnetic activity cycle of the cool companion. In particular the behaviour of the dark spots on the cool companion leads us to the interpretation that this timescale is based on an 18-year magnetic dynamo cycle. We conclude that the mass-transfer rate is controlled by the variable depth of the Wilson depression in the magnetic spot around the Lagrangian point L1. In the result, based on available data, we observe a damped activity cycle of the star, starting with a high mass-transfer episode around 2001 with a calculated mass-transfer rate of 1.5 × 10 −6 M yr −1 , followed by quiet periods in 2006 and 2009, slightly higher activity around 2013 and 2014, and again followed by quiet periods in 2015 and 2016. However, owing to missing data for years 2010 and 2011, we cannot exclude that a second high mass-transfer episode occurred within this time span.

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Scattering linear polarization of late-type active stars

Cited by 2 publications

References 44 publications

The rotationally modulated polarization of ξ Boo A

The rotationally modulated polarization of ξ Boo A

Spectroscopic long-term monitoring of RZ Cas

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