Abstract
Context. Starspots are important manifestations of stellar magnetic activity. By studying their behaviour in young solar analogues, we can unravel the properties of their magnetic cycles. This gives crucial information of the underlying dynamo process. Comparisons with the solar cycle enable us to infer knowledge about how the solar dynamo has evolved during the Sun’s lifetime. Aims. Here we study the correlation between photometric brightness variations, spottedness, and mean temperature in V889 Her, a young solar analogue. Our data covers 18 years of spectroscopic and 25 years of photometric observations. Methods. We use Doppler imaging to derive temperature maps from high-resolution spectra. We use the Continuous Period Search method to retrieve mean V-magnitudes from photometric data. Results. Our Doppler imaging maps show a persistent polar spot structure varying in strength. This structure is centred slightly off the rotational pole. The mean temperature derived from the maps shows an overall decreasing trend, as does the photometric mean brightness, until it reaches its minimum around 2017. The filling factor of cool spots, however, shows only a weak tendency to anti-correlate with the decreasing mean brightness. Conclusions. We interpret V889 Her to have entered into a grand maximum in its activity. The clear relation between the mean temperature of the Doppler imaging surface maps and the mean magnitude supports the reliability of the Doppler images. The lack of correlation between the mean magnitude and the spottedness may indicate that bright features in the Doppler images are real.
Highlights
The single, late-type star V889 Her is a young solar analogue of spectral class G2 V, classified as a BY Draconis variable (Kazarovets & Samus 1997), with an estimated age of 30–50 Myr (Strassmeier et al 2003)
We study the correlation between photometric brightness variations, spottedness, and mean temperature in V889 Her, a young solar analogue
We analysed V-band photometry of V889 Her obtained between the years 1994 and 2018 with the Tennessee State University T3 0.4m Automatic Photoelectric Telescope (APT) at Fairborn Observatory in southern Arizona
Summary
The single, late-type star V889 Her (or HD 171488) is a young solar analogue of spectral class G2 V, classified as a BY Draconis variable (Kazarovets & Samus 1997), with an estimated age of 30–50 Myr (Strassmeier et al 2003). Solar-type stars are important for the study of stellar activity and evolution since they provide us a look into what the Sun might have been like in the past. In this early evolutionary stage the stars are much more magnetically active than the Sun today. Marsden et al (2006) studied the surface magnetic field of V889 Her using Zeeman-Doppler imaging (ZDI) with Stokes I&V spectra They found no latitude dependence of the radial field, as opposed to the azimuthal field forming ring-like structures around the pole. From the spectroscopic observations we calculate surface temperature maps of the star, using the Doppler imaging method, and study the evolution of the mean temperature, and the spot coverage and distribution. A typical exposure time for V889 Her is about 40 min, achieving a signal-to-noise ratio (S/N) of about 270 depending on the seeing and sky conditions, as well as air mass
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