35 research outputs found
Statistical properties of superflares on solar-type stars based on 1-min cadence data
We searched for superflares on solar-type stars using Kepler data with 1 min
sampling in order to detect superflares with short duration. We found 187
superflares on 23 solar-type stars whose bolometric energy ranges from the
order of erg to erg. Some superflares show multiple peaks
with the peak separation of the order of - seconds which is
comparable to the periods of quasi-periodic pulsations in solar and stellar
flares. Using these new data combined with the results from the data with 30
min sampling, we found the occurrence frequency () of superflares as a
function of flare energy () shows the power-law distribution () with for erg which is
consistent with the previous results. The average occurrence rate of
superflares with the energy of erg which is equivalent to X100 solar
flares is about once in 500-600 years. The upper limit of energy released by
superflares is basically comparable to a fraction of the magnetic energy stored
near starspots which is estimated from the photometry. We also found that the
duration of superflares () increases with the flare energy () as . This can be explained if we assume the time-scale of
flares is determined by the Alfvn time.Comment: Accepted for for publication in Earth, Planets and Spac
Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
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 (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 increases. Superflares with energies
erg occur on old, slowly-rotating Sun-like stars
(25 days) approximately once every 2000--3000 years,
while young rapidly-rotating stars with a few days have
superflares up to 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 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.Comment: 71 pages, 31 figures, 10 tables. Accepted for publication in The
Astrophysical Journal (on March 29, 2019