Center-to-limb variation of intensity and polarization in continuum spectra of FGK stars for spherical atmospheres

Костогрыз, Н. М.; Milić, Ivan; Berdyugina, S. V.; Hauschildt, P. H.

doi:10.1051/0004-6361/201527598

Cited by 9 publications

(18 citation statements)

References 36 publications

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“…Besides limb darkening, continuum radiation also carries some small degree of polarization; this depends on the wavelength and could be detected in exoplanet transit polarimetry (Kostogryz et al 2015(Kostogryz et al , 2016Wiktorowicz & Laughlin 2014).…”

Section: Exoplanet Detection Limitsmentioning

confidence: 99%

Spatially resolved spectroscopy across stellar surfaces

Dravins

Ludwig

Dahlén

et al. 2017

A&A

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Context. High-precision stellar analyses require hydrodynamic modeling to interpret chemical abundances or oscillation modes. Exoplanet atmosphere studies require stellar background spectra to be known along the transit path while detection of Earth analogs require stellar microvariability to be understood. Hydrodynamic 3D models can be computed for widely different stars but have been tested in detail only for the Sun with its resolved surface features. Model predictions include spectral line shapes, asymmetries, and wavelength shifts, and their center-to-limb changes across stellar disks. Aims. We observe high-resolution spectral line profiles across spatially highly resolved stellar surfaces, which are free from the effects of spatial smearing and rotational broadening present in full-disk spectra, enabling comparisons to synthetic profiles from 3D models. Methods. During exoplanet transits, successive stellar surface portions become hidden and differential spectroscopy between various transit phases provides spectra of small surface segments temporarily hidden behind the planet. Planets cover no more than ∼1% of any main-sequence star, enabling high spatial resolution but demanding very precise observations. Realistically measurable quantities are identified through simulated observations of synthetic spectral lines. Results. In normal stars, line profile ratios between various transit phases may vary by ∼0.5%, requiring S /N 5000 for meaningful spectral reconstruction.While not yet realistic for individual spectral lines, this is achievable for cool stars by averaging over numerous lines with similar parameters. Conclusions. For bright host stars of large transiting planets, spatially resolved spectroscopy is currently practical. More observable targets are likely to be found in the near future by ongoing photometric searches.

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Section: Exoplanet Detection Limitsmentioning

confidence: 99%

Spatially resolved spectroscopy across stellar surfaces

Dravins

Ludwig

Dahlén

et al. 2017

A&A

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“…To calculate the flux and the Stokes parameters during a transit, the center-to-limb variations of the intensity and the linear polarization should be known. We use the CLVI and the CLVP calculated for PPM and SphM (Kostogryz et al 2016) geometries of stellar atmospheres to study the variations in resulting flux and polarization during planetary transits.…”

Section: Calculation Of the Transit Polarizationmentioning

confidence: 99%

“…a plane-parallel model (PPM) or a spherical model (SphM). We recently computed the CLVI and CLVP in the continuum assuming a PPM ) and a SphM (Kostogryz et al 2016) stellar atmospheres at different effective temperatures and surface gravity. calculated the polarization during the transits of 88 known extrasolar planets assuming a PPM stellar atmosphere, and selected the most promising targets for observations.…”

Section: Introductionmentioning

confidence: 99%

“…calculated the polarization during the transits of 88 known extrasolar planets assuming a PPM stellar atmosphere, and selected the most promising targets for observations. In the present paper, we calculate the polarimetric curves during transits for 405 known exoplanets with SphM and PPM stellar atmospheres using the CLVI and CLVP calculated for these two geometries from Kostogryz et al (2016) and , respectively. The SphM geometry is closer to reality; however, the PPM is computationally simpler.…”

Section: Introductionmentioning

confidence: 99%

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Polarimetry of transiting planets: Differences between plane-parallel and spherical host star atmosphere models

Костогрыз

Yakobchuk

Berdyugina

et al. 2017

A&A

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Context. To properly interpret photometric and polarimetric observations of exoplanetary transits, accurate calculations of centerto-limb variations of intensity and linear polarization of the host star are needed. These variations, in turn, depend on the choice of geometry of stellar atmosphere. Aims. We want to understand the dependence of the flux and the polarization curves during a transit on the choice of the applied approximation for the stellar atmosphere: spherical and plane-parallel. We examine whether simpler plane-parallel models of stellar atmospheres are good enough to interpret the flux and the polarization light curves during planetary transits, or whether more complicated spherical models should be used. Methods. Linear polarization during a transit appears because a planet eclipses a stellar disk and thus breaks left-right symmetry. We calculate the flux and the polarization variations during a transit with given center-to-limb variations of intensity and polarization. Results. We calculate the flux and the polarization variations during transit for a sample of 405 extrasolar systems. Most of them show higher transit polarization for the spherical stellar atmosphere. Our calculations reveal a group of exoplanetary systems that demonstrates lower maximum polarization during the transits with spherical model atmospheres of host stars with effective temperatures of T eff = 4400−5400 K and surface gravity of log g = 4.45−4.65 than that obtained with plane-parallel atmospheres. Moreover, we have found two trends of the transit polarization. The first trend is a decrease in the polarization calculated with spherical model atmosphere of host stars with effective temperatures T eff = 3500−5100 K, and the second shows an increase in the polarization for host stars with T eff = 5100−7000 K. These trends can be explained by the relative variation of temperature and pressure dependences in the plane-parallel and spherical model atmospheres. Conclusions. For most cases of known transiting systems the plane-parallel approximation of stellar model atmospheres may be safely used for calculation of the flux and the polarization curves because the difference between two models is tiny. However, there are some examples where the spherical model atmospheres are necessary to get proper results, such as the systems with grazing transits, with Earth-size planets, or for the hot host stars with effective temperatures higher than 6000 K.

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“…calculated the tables for continuum spectra of FGK stars (T eff = 4500 K -6900 K, logg = 2.0 -5.0, and wavelength range 4000 -7000 Å) for the Phoenix grid of plane-parallel models, and in Kostogryz et al (2016) similar calculations were made for a wider range of models (T eff = 4000 K -7000 K and logg = 1.0 -5.5) assuming a spherical stellar atmosphere. In addition, we used unpublished data on intensity and polarization variations for both plane-parallel and spherical cooler models with temperatures down to 3000 K, obtained with the same code by N. Kostogryz (2016, private communication).…”

Section: Integrating the Diskmentioning

confidence: 99%

Scattering linear polarization of late-type active stars

Yakobchuk

Berdyugina

2018

A&A

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Context. Many active stars are covered in spots, much more so than the Sun, as indicated by spectroscopic and photometric observations. It has been predicted that star spots induce non-zero intrinsic linear polarization by breaking the visible stellar disk symmetry. Although small, this effect might be useful for star spot studies, and it is particularly significant for a future polarimetric atmosphere characterization of exoplanets orbiting active host stars. Aims. Using models for a center-to-limb variation of the intensity and polarization in presence of continuum scattering and adopting a simplified two-temperature photosphere model, we aim to estimate the intrinsic linear polarization for late-type stars of different gravity, effective temperature, and spottedness. Methods. We developed a code that simulates various spot configurations or uses arbitrary surface maps, performs numerical disk integration, and builds Stokes parameter phase curves for a star over a rotation period for a selected wavelength. It allows estimating minimum and maximum polarization values for a given set of stellar parameters and spot coverages. Results. Based on assumptions about photosphere-to-spot temperature contrasts and spot size distributions, we calculate the linear polarization for late-type stars with T eff = 3500 K -6000 K, logg = 1.0 -5.0, using the plane-parallel and spherical atmosphere models. Employing random spot surface distribution, we analyze the relation between spot coverage and polarization and determine the influence of different input parameters on results. Furthermore, we consider spot configurations with polar spots and active latitudes and longitudes.

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Center-to-limb variation of intensity and polarization in continuum spectra of FGK stars for spherical atmospheres

Cited by 9 publications

References 36 publications

Spatially resolved spectroscopy across stellar surfaces

Spatially resolved spectroscopy across stellar surfaces

Polarimetry of transiting planets: Differences between plane-parallel and spherical host star atmosphere models

Scattering linear polarization of late-type active stars

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