[Paper Review] On cyclic activity of the Sun and solar-type stars

Е. А. Бруевич, G. S. Ivanov-Kholodnyj|arXiv (Cornell University)|Aug 27, 2011

Solar and Space Plasma Dynamics8 references3 citations

TL;DR

This study analyzes 20 years of simultaneous photospheric and chromospheric flux data from the HK project for 33 solar-type stars and the Sun, using Fast Fourier Transform to detect cyclic activity. It confirms widespread quasi-biennial variations (2.2–3.8 years) in chromospheric emission, with about 75% of stars showing such cycles—more common than 11-year cycles—suggesting a fundamental link between subphotospheric convective zone structure and magnetic activity cycles in solar-type stars.

ABSTRACT

The cyclicity of 33 solar-type stars that are similar to 11-year and to the quasi-biennial variations of solar radiation have studied. Our calculation were based on new simultaneous observations of the flux variations of the photospheric and chromospheric emissions of 33 solar-type stars and the Sun during the HK project that were conducted over the last 20 years. The method of Fast Fourier Transform was applied to these observed data. In addition to the known cyclic chromospheric emission variations of stars at the 11-year time scale, which were discovered at the Mount Wilson Observatory, we found a recurrences on the quasi-biennial time scale. The results of calculations of periods of the star's fluxes variations at the 11-year and quasi-biennial time scales are presented

Motivation & Objective

• To investigate the presence and characteristics of cyclic activity in solar-type stars beyond the well-known 11-year cycle.
• To determine whether quasi-biennial variations in chromospheric emission, similar to those in the Sun, are common among solar-type stars.
• To examine the dependence of 11-year and quasi-biennial cycle periods on stellar spectral type and effective temperature.
• To assess the relationship between cycle periods and subphotospheric convective zone structure in solar-type stars.

Proposed method

• Utilized 20 years of simultaneous observations from the HK project, including photospheric fluxes (ubvy system) and chromospheric Ca II H and K line emissions.
• Applied Fast Fourier Transform (FFT) to extract periodicities in the flux variation data of the Sun and 33 solar-type stars.
• Calculated and compared the 11-year cycle period ($T_{11}$) and the quasi-biennial cycle period ($T_2$) for each star.
• Correlated $T_{11}$ and $T_2$ with stellar effective temperature ($T_{\text{eff}}$) and spectral class to assess structural dependencies.
• Validated results by comparing $T_{11}$ values with previously reported values from Mount Wilson observations (Lockwood et al. 2007).
• Analyzed the stability of $T_2$ over the 11-year cycle, noting variations similar to those observed in the Sun.

Experimental results

Research questions

• RQ1 Do solar-type stars exhibit quasi-biennial cycles in chromospheric emission similar to those observed in the Sun?
• RQ2 How frequently do 11-year and quasi-biennial cycles appear among solar-type stars in the HK project sample?
• RQ3 Is there a correlation between the 11-year cycle period ($T_{11}$) and the effective temperature or spectral class of the stars?
• RQ4 To what extent do the periods of quasi-biennial cycles ($T_2$) vary across the sample, and is there a dependence on stellar parameters?
• RQ5 How do the observed cycle periods compare with theoretical expectations based on subphotospheric convective zone size?

Key findings

• Approximately 75% of the 33 solar-type stars in the sample exhibit well-pronounced quasi-biennial flux variations, indicating this cycle is common among solar-type stars.
• The quasi-biennial cycle periods ($T_2$) range from 2.2 to 3.8 years, with most values falling between 2.0 and 3.2 years.
• The 11-year cycle periods ($T_{11}$) increase by about 30–40% from F5-type stars to K7-type stars, correlating with increasing subphotospheric convective zone size.
• The $T_{11}$ values derived from FFT analysis agree well with previously reported values from Mount Wilson observations, validating the method.
• The duration of the quasi-biennial cycle ($T_2$) is not constant over the 11-year cycle, varying from ~25 to ~39 months—mirroring the Sun’s behavior.
• The study confirms that quasi-biennial variations are more frequently observed than 11-year cycles in the sample, suggesting a fundamental role in stellar magnetic activity.