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Öğe Connecting measurements of solar and stellar brightness variations(Edp Sciences S A, 2020) Nemec, Nina; Işık, Emre; Shapiro, A. I.; Solanki, Sami K.; Krivova, N. A.; Unruh, Y.Context. A comparison of solar and stellar brightness variations is hampered by the difference in spectral passbands that are used in observations, and also by the possible difference in the inclination of the solar and stellar rotation axes from the line of sight.Aims. We calculate the rotational variability of the Sun as it would be measured in passbands used for stellar observations. In particular, we consider the filter systems used by the CoRoT, Kepler, TESS, and Gaia space missions. We also quantify the effect of the inclination of the rotation axis on the solar rotational variability.Methods. We employed the spectral and total irradiance reconstruction (SATIRE) model to calculate solar brightness variations in different filter systems as observed from the ecliptic plane. We then combined the simulations of the surface distribution of the magnetic features at different inclinations using a surface flux transport model with the SATIRE calculations to compute the dependence of the variability on the inclination.Results. For an ecliptic-bound observer, the amplitude of the solar rotational variability, as observed in the total solar irradiance (TSI), is 0.68 mmag (averaged over solar cycles 21-24). We obtained corresponding amplitudes in the Kepler (0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag), Gaia G(RP) (0.62 mmag), and Gaia G(BP) (0.86 mmag) passbands. Decreasing the inclination of the rotation axis decreases the rotational variability. For a sample of randomly inclined stars, the variability is on average 15% lower in all filter systems we considered. This almost compensates for the difference in amplitudes of the variability in TSI and Kepler passbands, making the amplitudes derived from the TSI records an ideal representation of the solar rotational variability for comparison to Kepler stars with unknown inclinations.Conclusions. The TSI appears to be a relatively good measure of solar variability for comparisons with stellar measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia G(RP) filters. Whereas the correction factors can be used to convert the variability amplitude from solar measurements into the values expected for stellar missions, the inclination affects the shapes of the light curves so that a much more sophisticated correction than simple scaling is needed to obtain light curves out of the ecliptic for the Sun.Öğe Faculae cancel out on the surfaces of active suns(IOP Publishing, 2022) Nemec, N. E.; Shapiro, A. I.; Işık, Emre; Sowmya, K.; Solanki, S. K.; Krivova, N. A.; Cameron, R. H.; Gizon, L.Surfaces of the Sun and other cool stars are filled with magnetic fields, which are either seen as dark compact spots or more diffuse bright structures like faculae. Both hamper detection and characterization of exoplanets, affecting stellar brightness and spectra, as well as transmission spectra. However, the expected facular and spot signals in stellar data are quite different, for instance, they have distinct temporal and spectral profiles. Consequently, corrections of stellar data for magnetic activity can greatly benefit from the insight on whether the stellar signal is dominated by spots or faculae. Here, we utilize a surface flux transport model to show that more effective cancellation of diffuse magnetic flux associated with faculae leads to spot area coverages increasing faster with stellar magnetic activity than that by faculae. Our calculations explain the observed dependence between solar spot and facular area coverages and allow its extension to stars that are more active than the Sun. This extension enables anticipating the properties of stellar signal and its more reliable mitigation, leading to a more accurate characterization of exoplanets and their atmospheres.Öğe Faculae Cancel out on the Surfaces of Active Suns (vol 934, L23, 2022)(Iop Publishing Ltd, 2022) Nemec, N. -E.; Shapiro, A. I.; Isik, E.; Sowmya, K.; Solanki, S. K.; Krivova, N. . A.; Cameron, R. H.[No abstract available]Öğe Forward modelling of brightness variations in Sun-like stars II. Light curves and variability(Edp Sciences S A, 2023) Nemec, N. -E.; Shapiro, A. I.; Isik, E.; Solanki, S. K.; Reinhold, T.Context. The amplitude and morphology of light curves of Sun-like stars change substantially with increasing rotation rate: brightness variations are amplified and become more regular. This has not been explained so far.Aims. We develop a modelling approach for calculating brightness variations of stars with various rotation rates and use it to explain the observed trends in stellar photometric variability.Methods. We combined numerical simulations of magnetic flux emergence and transport with a model for stellar brightness variability to calculate synthetic light curves of stars as observed by the Kepler telescope. We computed the distribution of the magnetic flux on the stellar surface for various rotation rates and degrees of active-region nesting (i.e. the tendency of active regions to emerge in the vicinity of recently emerged regions). Using the resulting maps of the magnetic flux, we computed the rotational variability of our simulated stellar light curves as a function of rotation rate and nesting of magnetic features and compared our calculations to Kepler observations.Results. We show that both the rotation rate and the degree of nesting have a strong impact on the amplitude and morphology of stellar light curves. In order to explain the variability of most of the Kepler targets with known rotation rates, we need to increase the degree of nesting to values that are much higher than the values on the Sun.Conclusions. The suggested increase in nesting with the rotation rate can provide clues about the flux emergence process for high levels of stellar activity.
