Abstract
AbstractWe 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.
Highlights
Solar irradiance measurements performed by Nimbus-7 satellite revealed that the solar constant coherently varied with daily sunspot number (Hickey et al 1988)
Baliunas et al (1996) used a sample from HK project targets and discovered a correlation between the cycle length normalised to the rotation period (Pcyc/Prot) and the dynamo number, D ∼ 1/Prot, which was expected to provide clues about the type of the dynamo operating in these stars
local thermodynamic equilibrium (LTE) spectrum synthesis shows that the target stars are cool red giant or subgiant stars with an overall metallicity, [M/H], between 0 and −0.5
Summary
In addition to the Strömgren photometry, some other projects on the long-term photometric observations of chromospherically active stars from various luminosity classes have started by different groups (Henry & Eaton 1995; Strassmeier et al 1997; Rodonò et al 2001) with automated photoelectric telescopes (APTs). Evren, & Henry (2012) re-analysed the long-term photometry of HD 89546, which was doubled as the time coverage of the data compared to the former study and showed that the distribution of the mean brightness with respect to the photometric period is similar to the one observed in the sunspot cycle (see Figure 6 in their study) Besides these APT observations, photometric sky surveys, such as The All Sky Automated Survey (ASAS3, Pojmanski 1997; 2002; Pojmanski, Pilecki, & Szczygiel 2005) and All-Sky Automated Survey for Supernovae Sky Patrol (ASAS-SN, Shappee et al 2014; Kochanek et al 2017) provided long-term photometric data for a given position on the sky plane, which is useful for long-term photometric analyses of chromospherically active stars (see, e.g. Özdarcan & Dal 2018). We summarise and discuss our findings in the last section
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