Superflares on Solar Type Stars Observed with Kepler I. Statistical Properties of Superflares (1308.1480v2)

Abstract: By extending our previous study by Maehara et al. (2012), we searched for superflares on G-type dwarfs (solar type stars) using Kepler data for a longer period (500 days) than that (120 days) in our previous study. As a result, we found 1547 superflares on 279 G-type dwarfs, which are much more than previous 365 superflares on 148 stars. Using these new data, we studied the statistical properties of occurrence frequency of superflares, and basically confirmed the previous results, i.e., the occurrence frequency (dN/dE) of superflares vs flare energy (E) shows power-law distribution with dN/dE \propto E^{-\alpha}, where \alpha ~ 2. It is interesting that this distribution is roughly on the same line as that for solar flares. In the case of the Sun-like stars (with surface temperature 5600-6000K and slowly rotating with a period longer than 10 days), the occurrence frequency of superflares with energy of 10^34 -10^35 erg is once in 800-5000 years. We also studied long term (500 days) stellar brightness variation of these superflare stars, and found that in some G-type dwarfs the occurrence frequency of superflares was extremely high, ~ 57 superflares in 500 days (i.e., once in 10 days). In the case of Sun-like stars, the most active stars show the frequency of one superflares (with 10^34 erg) in 100 days. There is evidence that these superflares have extremely large starspots with a size about 10 times larger than that of the largest sunspot. We argue that the physical origin of extremely high occurrence frequency of superflares in these stars may be attributed to the existence of extremely large starspots.

• The paper analyzes 500 days of Kepler data, identifying 1547 superflares on 279 solar-type stars to establish their occurrence frequency.
• It finds that the superflare energy distribution follows a power-law with an index of about 2, suggesting a physical mechanism similar to solar flares.
• The study highlights that superflares mainly occur on rapidly rotating stars and are independent of the influence of hot Jupiters.

Analysis of Superflares on Solar-Type Stars Using Kepler Data

In the paper "Superflares on Solar Type Stars Observed with Kepler: I. Statistical Properties of Superflares," Shibayama et al. present a comprehensive examination of superflares on G-type dwarfs, extending their previous work through the analysis of 500 days of Kepler observational data. The expanded dataset reveals 1547 superflares across 279 G-type stars, offering stronger statistical evidence for the characteristics and frequency of these powerful stellar events.

The team observed that the occurrence frequency of these superflares adheres to a power-law distribution analogous to solar flares, described by $dN/dE \propto E^{-\alpha}$ with $\alpha \sim 2$. This consistency across flare energies from solar to superflares suggests a similar underlying physical mechanism driving flare activity across different energy scales. Notably, they found that superflare activity on Sun-like stars is infrequent, with those having energies of $10^{34}-10^{35}$ erg occurring approximately once every 800 to 5000 years.

The implications of these findings are notable, particularly in the context of solar flare analogues and their potential threat to Earth-like planets. Superflare events, significantly more energetic than any solar flares observed in recorded history, raise questions about the upper limits of solar activity and its potential impact on Earth. The analysis of rotational periods further highlights that rapidly rotating G-type stars are more likely to exhibit superflares, indicative of higher magnetic activity levels conducive to such events. This supports dynamo theory, whereby stellar rotation influences magnetic activity.

Importantly, the research challenges the previously proposed hypothesis linking superflares to the presence of hot Jupiters. None of the observed superflare stars showed evidence of such planets, suggesting that large stellar activity does not require their presence. This finding shifts the focus from orbital characteristics to intrinsic stellar properties.

For future research, the paper emphasizes the necessity of prolonged monitoring, increasing the number of observed superflares on slowly rotating, Sun-like stars. This would refine the statistical understanding of superflare occurrences and establish more robust connections to solar activity. Additionally, obtaining higher cadence data would help bridge the gap between solar flares and superflares.

In conclusion, Shibayama et al. provide robust statistical analysis confirming the occurrence and energy distributions of superflares on G-type stars, establishing a critical foundation for understanding stellar magnetic activity and its potential parallels to our own Sun.