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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.Öğe Observing and modelling the young solar analogue EK Draconis: starspot distribution, elemental abundances, and evolutionary status(Oxford Univ Press, 2021) Senavci, H., V; Kilicoglu, T.; Isik, E.; Hussain, G. A. J.; Montes, D.; Bahar, E.; Solanki, S. K.Observations and modelling of stars with near-solar masses in their early phases of evolution are critical for a better understanding of how dynamos of solar-type stars evolve. We examine the chemical composition and the spot distribution of the pre-mainsequence solar analogue EK Dra. Using spectra from the HERMES Spectrograph (La Palma), we obtain the abundances of 23 elements with respect to the solar ones, which lead to a [Fe/H] = 0.03, with significant overabundance of Li and Ba. The s-process elements Sr, Y, and Ce are marginally overabundant, while Co, Ni, Cu, Zn are marginally deficient compared to solar abundances. The overabundance of Ba is most likely due to the assumption of depth-independent microturbulent velocity. Li abundance is consistent with the age and the other abundances may indicate distinct initial conditions of the pre-stellar nebula. We estimate a mass of 1.04 M-circle dot and an age of 27(-8)(+11) Myr using various spectroscopic and photometric indicators. We study the surface distribution of dark spots, using 17 spectra collected during 15 nights using the CAFE Spectrograph (Calar Alto). We also conduct flux emergence and transport (FEAT) simulations for EK Dra's parameters and produce 15-d-averaged synoptic maps of the likely starspot distributions. Using Doppler imaging, we reconstruct the surface brightness distributions for the observed spectra and FEAT simulations, which show overall agreement for polar and mid-latitude spots, while in the simulations there is a lack of low-latitude spots compared to the observed image. We find indications that cross-equatorial extensions of mid-latitude spots can be artefacts of the less visible southern-hemisphere activity.Öğe Predictions of astrometric jitter for sunlike stars. III. Fast Rotators(IOP Publishing, 2022) Sowmya, K.; Nemec, N. E.; Shapiro, A., I.; Işık, Emre; Krivova, N. A.; Solanki, S. K.A breakthrough in exoplanet detections is foreseen with the unprecedented astrometric measurement capabilities offered by instrumentation aboard the Gaia space observatory. Besides, astrometric discoveries of exoplanets are expected from the planned space mission, Small-JASMINE. In this setting, the present series of papers focuses on estimating the effect of the magnetic activity of G2V-type host stars on the astrometric signal. This effect interferes with the astrometric detections of Earth-mass planets. While the first two papers considered stars rotating at the solar rotation rate, this paper focuses on stars having solar effective temperature and metallicity but rotating faster than the Sun, and consequently more active. By simulating the distribution of active regions on such stars using the Flux Emergence And Transport model, we show that the contribution of magnetic activity to the astrometric measurements becomes increasingly significant with increasing rotation rates. We further show that the jitter for the most variable periodic Kepler stars is high enough to be detected by Gaia. Furthermore, due to a decrease in the facula-to-spot area ratio for more active stars, the magnetic jitter is found to be spot dominated for rapid rotators. Our simulations of the astrometric jitter have the potential to aid the interpretation of data from Gaia and upcoming space astrometry missions.
