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Öğe Amplification of brightness variability by active-region nesting in Solar-like stars(Iop Publishing Ltd, 2020) Işık, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A.Kepler observations revealed that hundreds of stars with near-solar fundamental parameters and rotation periods have much stronger and more regular brightness variations than the Sun. Here we identify one possible reason for the peculiar behavior of these stars. Inspired by solar nests of activity, we assume that the degree of inhomogeneity of active-region (AR) emergence on such stars is higher than on the Sun. To test our hypothesis, we model stellar light curves by injecting ARs consisting of spots and faculae on stellar surfaces at various rates and nesting patterns, using solar AR properties and differential rotation. We show that a moderate increase of the emergence frequency from the solar value combined with the increase of the degree of nesting can explain the full range of observed amplitudes of variability of Sun-like stars with nearly the solar rotation period. Furthermore, nesting in the form of active longitudes, in which ARs tend to emerge in the vicinity of two longitudes separated by 180 degrees, leads to highly regular, almost sine-like variability patterns, rather similar to those observed in a number of solar-like stars.Öğe Amplification of Brightness Variability by Active-region Nesting in Solar-like Stars (vol 901, L12, 2020)(Iop Publishing Ltd, 2020) Isik, Emre; Shapiro, Alexander I.; Solanki, Sami K.; Krivova, Natalie A.[No abstract available]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 Where have all the solar-like stars gone? Rotation period detectability at various inclinations and metallicities(IOP Publishing, 2021) Reinhold, Timo; Shapiro, Alexander I.; Witzke, Veronika; Nèmec, Nina E.; Işık, Emre; Solanki, Sami K.The plethora of photometric data collected by the Kepler space telescope has pro moted the detection of tens of thousands of stellar rotation periods. However, these periods are not found to an equal extent among different spectral types. Interest ingly, early G-type stars with near-solar rotation periods are strongly underrepre sented among those stars with known rotation periods. In this study we investigate whether the small number of such stars can be explained by difficulties in the period determination from photometric time series. For that purpose, we generate model light curves of early G-type stars with solar rotation periods for different inclination angles, metallicities and (magnitude-dependent) noise levels. We find that the de tectability is determined by the predominant type of activity (i.e. spot or faculae domination) on the surface, which defines the degree of irregularity of the light curve, and further depends on the level of photometric noise. These two effects significantly complicate the period detection and explain the lack of solar-like stars with known near-solar rotation periods. We conclude that the rotation periods of the majority of solar-like stars with near-solar rotation periods remain undetected to date. Finally, we promote the use of new techniques to recover more periods of near-solar rotators.
