Abstract:We present and analyze long-term optical photometric measurements of the three active stars V2253 Oph, IT Com and IS Vir. All three systems are single-lined spectroscopic binaries with an early K giant as primary component but in different stages of orbital-rotational synchronization. Our photometry is supplemented by 2MASS and WISE near-IR and mid-IR magnitudes and then used to obtain more accurate effective temperatures and extinctions. For V2253 Oph and IT Com, we found their spectral energy distributions c… Show more
“…The origin of their magnetic field and activity is thought to be generated either through dynamo mechanisms resulting from planet engulfment (Siess & Livio 1999), angular dredgeup from the interior for the fastest rotators (Simon & Drake 1989), or from remnant rotation for massive stars (Stepien 1993). Active RGs were also identified in rather close detached binary systems that usually rotate fast, with orbits and rotations in phase or close to a resonance, and where spots are detected with ZDI ( Özdarcan et al 2010;Oláh et al 2013).…”
Among 19 red-giant stars belonging to eclipsing binary systems that have been identified in Kepler data, 15 display solar-like oscillations. We study whether the absence of mode detection in the remaining 4 is an observational bias or possibly evidence of mode damping that originates from tidal interactions. A careful analysis of the corresponding Kepler light curves shows that modes with amplitudes that are usually observed in red giants would have been detected if they were present. We observe that mode depletion is strongly associated with short-period systems, in which stellar radii account for 16-24 % of the semi-major axis, and where red-giant surface activity is detected. We suggest that when the rotational and orbital periods synchronize in close binaries, the red-giant component is spun up, so that a dynamo mechanism starts and generates a magnetic field, leading to observable stellar activity. Pressure modes would then be damped as acoustic waves dissipate in these fields.
“…The origin of their magnetic field and activity is thought to be generated either through dynamo mechanisms resulting from planet engulfment (Siess & Livio 1999), angular dredgeup from the interior for the fastest rotators (Simon & Drake 1989), or from remnant rotation for massive stars (Stepien 1993). Active RGs were also identified in rather close detached binary systems that usually rotate fast, with orbits and rotations in phase or close to a resonance, and where spots are detected with ZDI ( Özdarcan et al 2010;Oláh et al 2013).…”
Among 19 red-giant stars belonging to eclipsing binary systems that have been identified in Kepler data, 15 display solar-like oscillations. We study whether the absence of mode detection in the remaining 4 is an observational bias or possibly evidence of mode damping that originates from tidal interactions. A careful analysis of the corresponding Kepler light curves shows that modes with amplitudes that are usually observed in red giants would have been detected if they were present. We observe that mode depletion is strongly associated with short-period systems, in which stellar radii account for 16-24 % of the semi-major axis, and where red-giant surface activity is detected. We suggest that when the rotational and orbital periods synchronize in close binaries, the red-giant component is spun up, so that a dynamo mechanism starts and generates a magnetic field, leading to observable stellar activity. Pressure modes would then be damped as acoustic waves dissipate in these fields.
“…On the other hand, long-term photometric observations exist for a much wider sample of spotted stars in binary systems (e.g., Strassmeier et al 1997Strassmeier et al , 2011Oláh & Strassmeier 2002;Oláh et al 2003Oláh et al , 2013Lindborg et al 2013, etc. ), which can also be used for investigating the role of binarity in the formation of magnetic activity.…”
Context. Surface differential rotation and other global surface flows on magnetically active stars are among the observable manifestations of the underlying stellar dynamo. Therefore, these types of observations are important for stellar dynamo theory and useful constraints for solar dynamo studies as well. Aims. We revisit the active K1-giant component of the long-period RS CVn-type binary system σ Gem and its global surface flow pattern. Methods. We refine the differential rotation law from recovering the spot migration pattern. We apply a detailed cross-correlation technique to a unique set of 34 time-series Doppler images recovered using data from 1996−97. By increasing the number of the available cross-correlation function maps, we expect a more robust determination of the differential surface rotation law. In addition, we present a new time-series Doppler imaging study of σ Gem using our advanced surface reconstruction code iMap for a data set collected in 2006−07. Results. Results from the reprocessed cross-correlation study confirm that the star performs antisolar-type differential rotation with a surface shear α of −0.04 ± 0.01, i.e., almost a factor of two larger compared to the previously claimed value. We also confirm the evidence of a global poleward spot migration, with an average velocity of 0.21 ± 0.03 km s −1 , in accordance with theoretical predictions. From the new observations, we obtain three subsequent Doppler images. The time evolution of these images confirms the antisolar-type differential rotation of the same amount.
“…Detected long-period cycles are overplotted with red continuous line. Figure 6. Relation between and for giant and subgiant stars. Filled circles are stars listed in Table 3, open circles show BD+13 5000, TYC 5163-1764-1 and BD+11 3024 (Özdarcan & Dal 2018), filled stars denote HD 208472 (Özdarcan et al 2010) and HD 89546 (Özdarcan et al 2012), filled triangles are for IM Peg, HK Lac, XX Tri, IL Hya, HU Vir, UZ Lib, V711 Tau, and FK Com (Oláh et al 2009), open squares show IS Vir and V2253 Oph (Oláh et al 2013) open triangle is BM Cam (Zboril & Messina 2009), filled square is DM UMa (Taş & Evren 2012), and open star is σ Gem (Kajatkari et al 2014). Equation of the linear fit is indicated in the insert where numbers in parentheses show statistical errors for the last digit.…”
Section: Photometrymentioning
confidence: 99%
“…Efficient and precise broad-band V observations obtained from APTs allowed studying photometric activity cycles for main sequence and giant stars (Oláh & Strassmeier 2002; Messina & Guinan 2003; Oláh et al 2013). Furthermore, these observations enabled studies on photometric period variations and long-term tracing of the light curve properties of giant and subgiant stars (Jetsu, Henry, & Lehtinen 2017).…”
We investigate magnetic activity properties of 21 stars via medium resolution optical spectra and long-term photometry. Applying synthetic spectrum fitting method, we find that all targets are cool giant or sub-giant stars possessing overall [M/H] abundances between 0 and
$-0.5$
. We find that six of these targets exhibit only linear trend in mean brightness, while eight of them clearly shows cyclic mean brightness variation. Remaining seven target appear to exhibit cyclic mean brightness variation, but this cannot be confirmed due to the long timescales of the predicted cycle compared to the current time range of the photometric data. We further determine seasonal photometric periods and compute average photometric period of each target. Analysed sample in this study provides a quantitative representation of a positive linear correlation between the inverse of the rotation period and the cycle period normalised to the rotation period, on the log-log scale. We also observe no correlation between the activity cycle length and the relative surface shear, indicating that the activity cycle must be driven by a parameter rather than the differential rotation. Our analyses show that the relative surface shear is positively correlated with the rotation period and there is a noticeable separation between main sequence stars and our sample. Compared to our sample, the relative surface shear of a main sequence star is larger for a given rotation period. However, dependence of the relative surface shear on the rotation period appears stronger for our sample. Analysis of the current photometric data indicates that the photometric properties of the observed activity cycles in eight targets seem dissimilar to the sunspot cycle.
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