Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -

Abstract: We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly conducted APO3.5m spectroscopic observations of 18 superflare stars found from Kepler 1-min time cadence data. More than half (43 stars) are confirmed to be "single" stars, among 64 superflare stars in total that have been spectroscopically investigated so far in this APO3.5m and our previous Subaru/HDS observations. The measurements of $v\sin i$ (projected rotational velocity) and chromospheric lines (Ca II H&K and Ca II 8542\AA) support the brightness variation of superflare stars is caused by the rotation of a star with large starspots. We then investigated the statistical properties of Kepler solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates. As a result, the maximum superflare energy continuously decreases as the rotation period $P_{\mathrm{rot}}$ increases. Superflares with energies $\lesssim 5\times10^{34}$ erg occur on old, slowly-rotating Sun-like stars ($P_{\mathrm{rot}}\sim$25 days) approximately once every 2000--3000 years, while young rapidly-rotating stars with $P_{\mathrm{rot}}\sim$ a few days have superflares up to $10^{36}$ erg. The maximum starspot area does not depend on the rotation period when the star is young, but as the rotation slows down, it starts to steeply decrease at $P_{\mathrm{rot}}\gtrsim$12 days for Sun-like stars. These two decreasing trends are consistent since the magnetic energy stored around starspots explains the flare energy, but other factors like spot magnetic structure should also be considered.

• The paper confirms that a majority of Kepler superflare stars are single, using spectroscopic data to rule out binary contamination.
• It establishes a clear correlation between rapid rotation, large starspot coverage, and heightened chromospheric activity via Ca II lines.
• Gaia-DR2 refinement reveals many solar-type candidates are subgiants, indicating that slowly rotating Sun-like stars rarely produce large superflares.

Analysis of Superflares on Solar-Type Stars: A Spectroscopic View

In the scholarly pursuit of understanding stellar flare phenomena akin to solar flares but of far greater energy, the paper "Do {\it Kepler} superflare stars really include slowly-rotating Sun-like stars?" by Notsu et al. presents a meticulous spectroscopic analysis of superflare stars detected by the {\it Kepler} space telescope. The study critically examines the characteristics of these stars, focusing on whether they include slowly-rotating G-type main-sequence stars akin to the Sun. By integrating data from the Apache Point Observatory (APO) 3.5m telescope and {\it Gaia} Data Release 2 (DR2), the research deepens our understanding of superflares and their implications for solar-type stars.

Methodology and Key Findings

The authors conducted detailed spectroscopic observations of 18 superflare stars initially identified in {\it Kepler} data with 1-minute time cadence. This observational effort complements earlier studies with Subaru/HDS, resulting in a broader dataset of 64 stars to refine the understanding of these extreme energy releases. The primary objectives included verifying single-star characteristics versus binary star contamination and examining chromospheric activities and rotational dynamics.

1. Binarity and Stellar Evolution: Among the 64 stars analyzed, 43 were confirmed as single stars. This distinction is critical as binary companions can introduce stellar activity noise, potentially skewing flare frequency and energy assessments.
2. Rotational Dynamics and Chromospheric Activity: The study revealed a significant correlation between the rotational velocity (determined via the $v\sin i$ parameter) and the periodic brightness variations indicative of large starspots. Moreover, the use of Ca II lines demonstrated heightened chromospheric activity, revealing that superflares are typically associated with rapidly rotating stars with substantial magnetic features.
3. Statistical Properties: Incorporating {\it Gaia}-DR2 data allowed for more accurate stellar radius estimation, affirming that a considerable portion of stars previously categorized as solar-type are indeed subgiants. With this refinement, the energy potential of superflares was characterized, showing a decrease in maximum flare energy as stellar rotation periods lengthened, supporting the inference that older, slowly rotating stars possess lesser potential for such energetic outputs.
4. Starspot Dynamics: The analysis emphasized the association between large starspot coverage and superflare potential. There is a demonstrable decrease in spot area with increasing rotation period for G-type stars, aligning with the theoretical framework where flare energy is proportionate to the magnetic energy stored around starspots.
5. Implications for Solar Analogs: Superflares in stars similar to the Sun ($T_{\mathrm{eff} = 5600 - 6000 \, \mathrm{K}$, $P_{\mathrm{rot} \sim 25 \, \mathrm{days}$) occur less frequently and with significantly lower energy (about $5 \times 10^{34}$ erg) compared to younger, more rapidly-rotating counterparts. This suggests that while possible, the occurrence of superflares on a Sun-like star is infrequent, providing context to historical extreme solar events.

Theoretical and Practical Implications

This research not only advances the theoretical understanding of stellar magnetic activity and flare mechanics but also provides pivotal insights into potential solar activity extremes, contributing to solar-terrestrial physics. The discernment of superflare characteristics supports solar physics by potentially analogizing large solar storm events through stellar comparisons, thus offering broader context for solar observation data interpreted across varying temporal frameworks.

Future Directions

While the study substantially clarifies the spectroscopic properties of superflare stars, the authors highlight the need for expanded datasets, particularly from future missions such as {\it TESS} and {\it PLATO}, to refine statistical analyses and stellar models. The continuous exploration of magnetic fields on stellar surfaces and the evolution of rotational dynamics remain essential for understanding not just superflare phenomena but wider stellar evolution processes.

In conclusion, the paper by Notsu et al. systematically integrates advanced spectroscopic analysis and cross-mission data synthesis to explore the properties of superflare-inducing stars, bridging key observational gaps and supporting theoretical models of stellar activity. This comprehensive approach underscores the intricate dynamics of stellar magnetic phenomena and positions this research as a cornerstone for evolving studies in stellar and solar physics.