Statistical Properties of Superflares on Solar-type Stars: Results Using All of the Kepler Primary Mission Data (2011.02117v2)

Abstract: We report the latest statistical analyses of superflares on solar-type (G-type main-sequence; effective temperature is 5100 - 6000 K) stars using all of the $Kepler$ primary mission data, and $Gaia$-DR2 (Data Release 2) catalog. We updated the flare detection method from our previous studies by using high-pass filter to remove rotational variations caused by starspots. We also examined the sample biases on the frequency of superflares, taking into account gyrochronology and flare detection completeness. The sample size of solar-type stars and Sun-like stars (effective temperature is 5600 - 6000 K and rotation period is over 20 days in solar-type stars) are $\sim$4 and $\sim$12 times, respectively, compared with Notsu et al. (2019, ApJ, 876, 58). As a result, we found 2341 superflares on 265 solar-type stars, and 26 superflares on 15 Sun-like stars: the former increased from 527 to 2341 and the latter from 3 to 26 events compared with our previous study. This enabled us to have a more well-established view on the statistical properties of superflares. The observed upper limit of the flare energy decreases as the rotation period increases in solar-type stars. The frequency of superflares decreases as the stellar rotation period increases. The maximum energy we found on Sun-like stars is $4 \times 10^{34}$ erg. Our analysis of Sun-like stars suggest that the Sun can cause superflares with energies of $\sim 7 \times 10^{33}$ erg ($\sim$X700-class flares) and $\sim 1 \times 10^{34}$ erg ($\sim$X1000-class flares) once every $\sim$3,000 years and $\sim$6,000 years, respectively.

• The paper identifies 2341 superflares on 265 solar-type stars, including 26 events on Sun-like stars, using high-pass filtering to enhance flare detection.
• It establishes an inverse correlation between stellar rotation periods and flare energy, reinforcing statistical robustness with a quadrupled sample size.
• The analysis implies that while solar superflares are rare, the Sun’s magnetic environment could produce events up to 7×10^33 erg every 3,000–6,000 years.

Superflares on Solar-Type Stars: An Analysis Using Kepler Data

The paper conducted by Okamoto et al. focusses on examining the properties of superflares on solar-type stars. By leveraging data from the primary mission of the Kepler Space Telescope alongside Gaia Data Release 2 (DR2), it updates previous methodologies and results to offer a comprehensive statistical analysis of these superflares. This research is pivotal in understanding the potential for such events to occur on our Sun, given their potential impact on life and technology on Earth.

Methodological Innovations

One of the major updates in this paper is the use of high-pass filtering to refine the flare detection techniques. This filtering reduces the noise caused by stellar rotation, which is critical for an accurate identification of superflares. This paper utilizes a larger sample size, approximately four times that used in previous studies (like Notsu et al., 2019), leading to more robust statistical conclusions.

Key Findings

• The research identified 2341 superflares on 265 solar-type stars, significantly increasing the known occurrences compared to earlier studies. Of these, 26 superflares were detected on 15 Sun-like stars, which are slowly rotating solar-type stars with effective temperatures similar to the Sun.
• An inverse correlation between rotation period and flare energy is established. As rotation periods increase, superflare frequency and energy diminish. The largest superflares are found in stars with rapid rotation.
• Statistically, the analysis suggests that the Sun could potentially produce superflares, although at a much lower frequency compared to younger, more rapidly rotating stars.

Implications for Stellar and Solar Physics

The observational data confirm that superflares can occur due to the release of magnetic energy stored around starspots. The magnetic environment, deduced from the starspot size and energy, is sufficient to facilitate these high-energy events.

For Sun-like stars, superflare events can reach energies up to $4 \times 10^{34}$ erg. By extrapolating the analyzed data for our own solar context, it implies that superflares on the Sun, capable of releasing upwards of $7 \times 10^{33}$ erg, could occur once every 3,000 to 6,000 years.

Future Directions and Theoretical Considerations

The paper's results have profound implications for astrophysical models, particularly those related to stellar magnetic activity and the solar-stellar connection. The potential for solar superflares posits questions for planetary safety and climate models, which will benefit from incorporating such frequency estimates. Furthermore, the consideration of geometrical and physical properties of sunspots on both a stellar and solar scale enriches theoretical frameworks that tackle stellar dynamo processes.

Given the implications for solar and stellar activity modelling, future observations could prioritize high-resolution spectroscopy to better resolve stellar surfaces and validate starspot metrics further. The additional insights into starspot lifetimes and decay processes can aid in refining our understanding of stellar magnetic field evolution and energy buildup in active regions.

Conclusions and Broader Impact

Through a detailed analysis of Kepler data, Okamoto et al. provide substantial evidence and refined statistical properties of superflares on solar-type stars, broadening the astrophysical understanding of stellar activity and its potential parallels to solar behavior. This research enhances the predictive models concerning solar activity and aids in assessing the broader impact of such stellar phenomena on systems orbiting active stars, including our own.

In conclusion, this work marks a significant advancement in the field of stellar astrophysics, presenting verifiable insights that align observational data with contemporary theoretical models, offering a robust pathway to decoding the complexities of superflare activities in sun-like stars.