Do Kepler Superflare Stars Really Include Slowly Rotating Sun-like Stars?—Results Using APO 3.5 m 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) 3.5 m telescope spectroscopic observations and Gaia-DR2 data. First, we newly conducted APO 3.5 m spectroscopic observations of 18 superflare stars found from Kepler 1-minute 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 APO 3.5 m and our previous Subaru/HDS observations. The measurements of v sin i (projected rotational velocity) and chromospheric lines (Ca ii H and K and Ca ii λ8542) support that 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 rot increases. Superflares with energies ≲5 × 1034 erg occur on old, slowly rotating Sun-like stars (P rot ∼ 25 days) approximately once every 2000–3000 yr, while young, rapidly rotating stars with P rot ∼ a few days have superflares up to 1036 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 rot ≳ 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.

Rotation-activity relations and flares of M dwarfs with K2 long- and short-cadence data

“…One result of the analysis of the photometric data collected in the main Kepler mission was the detection of superflares on solar-type (G-type main-sequence) stars (e.g., Maehara et al 2012. Superflares with energy up to 10 35 erg also occur on slowly rotating stars (e.g., Kitze et al 2014, Notsu et al 2019). However, the frequency for superflares on these slow rotators is very low compared with rapidly rotating stars.…”

“…This finding confirms the results of S16, which was based on long-cadence LCs. Notsu et al (2019) found a correlation between amplitude of spot modulation and the occurrence rate of superflares in solarlike stars. They concluded that the majority of these strongest flares occur on stars with large spots.…”

“…This anticorrelation requires spots that cover >50% of the stellar surface (e.g., Newton et al 2017). We estimated the spot coverage of our targets using the relation given in Notsu et al (2019) and found a maximum spot filling factor of ∼9%. This shows that the amplitude of the spot modulation is a reliable activity indicator for our sample.…”

Superflares and variability in solar-type stars with TESS in the Southern hemisphere

“…In order to determine the starspot area we will look to use the method described by Notsu et al (2019). Firstly we must estimate the temperature of the spot which can be done by applying a relation on the difference between the photosphere and the spot.…”

“…More recently, Notsu et al (2019) presented a complete review of Kepler solar-type superflares including updates on a new sample using the Apache Point Observatory (APO) and Gaia DR2. The results from Gaia DR2 revealed the possibility of contamination of subgiant stars within the classification of Kepler solar-type stars.…”