Connecting measurements of solar and stellar brightness variations

Nèmec, Nina-Elisabeth; Işık, Emre; Шапиро, А. И.; Solanki, S. K.; Krivova, N. A.; Unruh, Y. C.

doi:10.1051/0004-6361/202038054

Cited by 17 publications

(10 citation statements)

References 32 publications

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“…Compared to the Keplerband results, TESS-band variability is lower for all spectral types and coverage levels. This is a direct consequence of the spot and facular contrasts being smaller in redder wavelength intervals (Nèmec et al 2020b).…”

Section: Tess-band Variabilitymentioning

confidence: 98%

Forward modelling of Kepler-band variability due to faculae and spots

Johnson,

Norris,

Unruh

et al. 2021

Preprint

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Variability observed in photometric lightcurves of late-type stars (on timescales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) lightcurves of active G2, K0, M0 and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of lightcurve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density and active band latitudes is explored. For our lightcurve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates 2-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, lightcurves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte-Carlo simulations, calculating lightcurve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of 1%, the typical variability of a solar-type star is around 2 parts-per-thousand. The presence of faculae clearly affects the mean brightness and lightcurve shape, but has relatively little influence on the variability.

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Section: Tess-band Variabilitymentioning

confidence: 98%

Forward modelling of Kepler-band variability due to faculae and spots

Johnson,

Norris,

Unruh

et al. 2021

Preprint

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“…Next, the facular coverages were obtained by first, subtracting the magnetic flux of the spots from the magnetograms and then using the remaining flux to calculate the faculae areas following the approach of Krivova et al (2003) of setting a saturation threshold B sat . While we used the SFTM simulations from Cameron et al (2010) in N20b and Nèmec et al (2020a), the results in this study are based on the model in I18 in the present work. We discuss the differences in these two models in more detail in the next section.…”

Section: Surface Flux Transport and Filling Factorsmentioning

confidence: 99%

“…For example, Nèmec et al (2020b) have combined the Spectral And Total Irradiance Reconstruction (SATIRE, Fligge et al 2000, Krivova et al 2003) model together with a surface flux transport model (SFTM, Cameron et al 2010) to model the power spectra of solar brightness variations at various inclinations down to several days. This approach allowed to remove a number of important observational biases when comparing the solar variability to that of other stars (Nèmec et al 2020a. By employing the Nèmec et al (2020b) model, Reinhold et al (2021 have found that rotation periods of most of the G-dwarfs of near-solar age go undetected.…”

Section: Introductionmentioning

confidence: 99%

Power spectra of solar brightness variations at various inclinations

Nèmec

Шапиро

Krivova

et al. 2020

A&A

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Context. Magnetic features on the surfaces of cool stars cause variations of their brightness. Such variations have been extensively studied for the Sun. Recent planet-hunting space telescopes allowed measuring brightness variations in hundred thousands of other stars. The new data posed the question of how typical is the Sun as a variable star. Putting solar variability into the stellar context suffers, however, from the bias of solar observations being made from its near-equatorial plane, whereas stars are observed at all possible inclinations. Aims. We model solar brightness variations at timescales from days to years as they would be observed at different inclinations. In particular, we consider the effect of the inclination on the power spectrum of solar brightness variations. The variations are calculated in several passbands routinely used for stellar measurements. Methods. We employ the Surface Flux Transport Model (SFTM) to simulate the time-dependent spatial distribution of magnetic features on both near-and far-sides of the Sun. This distribution is then used to calculate solar brightness variations following the SATIRE (Spectral And Total Irradiance REconstruction) approach. Results. We have quantified the effect of the inclination on solar brightness variability at timescales down to a day. Thus, our results allow making solar brightness records directly comparable to those obtained by the planet-hunting space telescopes. Furthermore, we decompose solar brightness variations into the components originating from the solar rotation and from the evolution of magnetic features.

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“…This is because the rotation induced changes in the fractional coverage by faculae decreases as the inclination decreases and the variation produced by rotation disappears at i = 0 °(see e.g. Figure 7 of Nèmec et al 2020a). At i = 0 °the faculae simply move along in circles staying at roughly the same limb distances instead of crossing all µ values as at i = 90 °.…”

Section: Variations On the Rotational Timescalementioning

confidence: 97%

“…The cycle averages follow the same trend as the cycle maximum, as does the amplitude of the S-index variations. The horizontal dashed lines show the values averaged over all inclinations computed using (as done by Nèmec et al 2020a)…”

Section: Variations On the Activity Cycle Timescalementioning

confidence: 99%

Modeling stellar Ca II H & K emission variations. I. Effect of inclination on the S-index

Sowmya,

Shapiro,

Witzke

et al. 2021

Preprint

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The emission in the near ultraviolet Ca ii H & K lines is modulated by stellar magnetic activity. Although this emission, quantified via the S-index, has been serving as a prime proxy of stellar magnetic activity for several decades, many aspects of the complex relation between stellar magnetism and Ca ii H & K emission are still unclear. The amount of measured Ca ii H & K emission is suspected to be affected not only by the stellar intrinsic properties but also by the inclination angle of the stellar rotation axis. Until now such an inclination effect on S-index has remained largely unexplored. To fill this gap, we develop a physics-based model to calculate S-index, focusing on the Sun. Using the distributions of solar magnetic features derived from observations together with Ca ii H & K spectra synthesized in non-local thermodynamic equilibrium, we validate our model by successfully reconstructing the observed variations of solar S-index over four activity cycles. Further, using the distribution of magnetic features over the visible solar disk obtained from surface flux transport simulations, we obtain S-index time series dating back to 1700 and investigate the effect of inclination on S-index variability, both on the magnetic activity cycle and the rotational timescales. We find that when going from an equatorial to a pole-on view, the amplitude of S-index variations decreases weakly on the activity cycle timescale and strongly on the rotational timescale (by about 22% and 81%, respectively, for a cycle of intermediate strength). The absolute value of S-index depends only weakly on the inclination. We provide analytical expressions that model such dependencies.

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Connecting measurements of solar and stellar brightness variations

Cited by 17 publications

References 32 publications

Forward modelling of Kepler-band variability due to faculae and spots

Forward modelling of Kepler-band variability due to faculae and spots

Power spectra of solar brightness variations at various inclinations

Modeling stellar Ca II H & K emission variations. I. Effect of inclination on the S-index

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