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    Predictions of astrometric jitter for sun-like stars. II. dependence on ınclination, metallicity, and active-region nesting
    (IOP Publishing, 2021) Krishnamurthy, Sowmya; Nemec, Nina Elisabeth; Shapiro, Alexander; Işık, Emre; Witzke, Veronika; Mints, Alexey; Krivova, Natalie; Solanki, Sami
    Ultra-precise astrometry from the Gaia mission is expected to lead to astrometric detections of more than 20,000 exoplanets in our Galaxy. One of the factors that could hamper such detections is the astrometric jitter caused by the magnetic activity of the planet host stars. In our previous study, we modeled astrometric jitter for the Sun observed equator-on. In this work, we generalize our model and calculate the photocenter jitter as it would be measured by the Gaia and Small-JASMINE missions for stars with solar rotation rate and effective temperature, but with various values of the inclination angle of the stellar rotation axis. In addition, we consider the effect of metallicity and of nesting of active regions (i.e., the tendency of active regions to emerge in the vicinity of each other). We find that, while the jitter of stars observed equator-on does not have any long-term trends and can be easily filtered out, the photocenters of stars observed out of their equatorial planes experience systematic shifts over the course of the activity cycle. Such trends allow the jitter to be detected with continuous measurements, in which case it can interfere with planet detectability. An increase in the metallicity is found to increase the jitter caused by stellar activity. Active-region nesting can further enhance the peak-to-peak amplitude of the photocenter jitter to a level that could be detected by Gaia.
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    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.