Starspot activity and superflares on solar-type stars (1702.07141v1)

Abstract: We analyze the correlation between starspots and superflares on solar-type stars using observations from the Kepler mission. The analysis shows that the observed fraction of stars with superflares decreases as the rotation period increases and as the amplitude of photometric variability associated with rotation decreases. We found that the fraction of stars with superflares among the stars showing large-amplitude rotational variations, which are thought to be the signature of the large starspots, also decreases as the rotation period increases. The small fraction of superflare stars among the stars with large starspots in the longer-period regime suggests that some of the stars with large starspots show a much lower flare activity than the superflare stars with the same spot area. Assuming simple relations between spot area and lifetime and between spot temperature and photospheric temperature, we compared the size distribution of large starspot groups on slowly-rotating solar-type stars with that of sunspot groups. The size distribution of starspots shows the power-law distribution and the size distribution of larger sunspots lies on this power-law line. We also found that frequency-energy distributions for flares originating from spots with different sizes are the same for solar-type stars with superflares and the Sun. These results suggest that the magnetic activity we observe on solar-type stars with superflares and that on the Sun is caused by the same physical processes.

Starspot Activity and Superflares on Solar-type Stars

This paper presents a detailed analysis of the correlation between starspot activity and superflare occurrences on solar-type stars, utilizing observational data from the Kepler mission. The focus is on understanding the frequency and energy distribution of these superflares, alongside the statistical properties of large starspots.

Key Findings

1. Correlation With Rotation Period: The paper found that the frequency of superflares decreases as the stellar rotation period increases. This decrease in superflare activity is similarly mirrored in the amplitude of photometric variability linked to rotation, suggesting a strong coupling between rotation rate, spot activity, and flare occurrence.
2. Spot Area and Lifetime: The paper proposes a simple model connecting starspot area to lifetime, positing that larger starspots tend to have longer lifetimes. This model supports the idea that slowly-rotating stars, with larger spots, may still exhibit lower flare activity. The size distribution of starspots adheres to a power-law, suggesting that the processes underpinning starspot formation and evolution are similar to those of sunspots.
3. Energetic Comparison: Superflares on solar-type stars exhibit bolometric energies of $10^{33}$ to $10^{35}$ erg, vastly outstripping the energy produced by major solar flares. Despite their difference in scale, the fundamental physical processes are postulated to be similar due to observed parallels in the frequency-energy distributions of both solar and stellar flares.
4. Magnetic Activity: The paper details how chromospheric activity, as indicated by Ca II line intensities, is correlated with photometric variations in brightness on superflare stars. This correlation underscores the significant role of starspots and associated magnetic fields in the occurrence of superflares.

Implications

• Astrophysical Insights: The statistically significant decline in superflare frequency with increasing rotation period gives crucial insights into dynamo activity on solar-type stars and supports theories linking magnetic activity to rotational dynamics.
• Starspot Dynamics: The distribution of starspot sizes following a power-law distribution similar to sunspots supports the hypothesis that magnetic processes, such as flux tube emergence and spot formation, could be universally applicable across different stellar environments.
• Future Research Directions: Long-term studies comparing magnetic structures of stars with and without superflares are necessary to elucidate the conditions necessary for large-scale flare production. Research utilizing high-precision photometry and spectral imaging will be instrumental in advancing this field.

Theoretical and Practical Considerations

The research forms a bridge between our understanding of solar flares and stellar flares, informing models of stellar magnetic field generation and stability. Practically, understanding superflares is critical for studies on the impacts of such energetic events on planetary atmospheres and potential habitability.

In summary, this paper eloquently expands our knowledge of starspot activity and its integral role in superflare generation on solar-type stars, serving as a definitive reference point for understanding magnetic variability across the cosmos.