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 Data ...Release 2 catalog. We updated the flare detection method from our previous studies by using a 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 sizes of solar-type and Sun-like stars (effective temperature is 5600-6000 K and rotation period is over 20 days in solar-type stars) are ∼4 and ∼12 times, respectively, compared with Notsu et al. 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 three 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 × 1034 erg. Our analysis of Sun-like stars suggests that the Sun can cause superflares with energies of ∼7 × 1033 erg (∼X700-class flares) and ∼1 × 1034 erg (∼X1000-class flares) once every ∼3000 and ∼6000 yr, respectively.
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 Prot increases. Superflares with energies 5 × 1034 erg occur on old, slowly rotating Sun-like stars (Prot ∼ 25 days) approximately once every 2000-3000 yr, while young, rapidly rotating stars with Prot ∼ 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 Prot 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.
We searched for superflares on solar-type stars using Kepler data with 1-min sampling in order to detect superflares with a short duration. We found 187 superflares on 23 solar-type stars whose ...bolometric energy ranges from the order of 10
32
to 10
36
erg. Some superflares show multiple peaks with the peak separation of the order of 100 to 1,000 s 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 that the occurrence frequency (dN/dE) of superflares as a function of flare energy (
E
) shows the power-law distribution (dN/dE∝
E
−
α
) with
α
∼−1.5 for 10
33
<
E
<10
36
erg which is consistent with the previous results. The average occurrence rate of superflares with the energy of 10
33
erg which is equivalent to X100 solar flares is about once in 500 to 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 (
τ
) increases with the flare energy (
E
) as
τ
∝
E
0.39 ± 0.03
. This can be explained if we assume the time scale of flares is determined by the Alfvén time.
Abstract
We analyze the correlation between starspots and superflares on solar-type stars using observations from the Kepler mission. The analysis shows that the observed fraction of stars with ...superflares decreases as the rotation period increases and as the amplitude of photometric variability associated with rotation decreases. We found that the fraction of stars with superflares among the stars showing large-amplitude rotational variations, which are thought to be the signature of the large starspots, also decreases as the rotation period increases. The small fraction of superflare stars among the stars with large starspots in the longer-period regime suggests that some of the stars with large starspots show a much lower flare activity than the superflare stars with the same spot area. Assuming simple relations between spot area and lifetime and between spot temperature and photospheric temperature, we compared the size distribution of large starspot groups on slowly rotating solar-type stars with that of sunspot groups. The size distribution of starspots shows the power-law distribution and the size distribution of larger sunspots lies on this power-law line. We also found that frequency–energy distributions for flares originating from spots with different sizes are the same for solar-type stars with superflares and the Sun. These results suggest that the magnetic activity we observe on solar-type stars with superflares and on the Sun is caused by the same physical processes.
Abstract
Active M-type stars are known to often produce superflares on the surface. Radiation from stellar (super)flares is important for exoplanet habitability, but the mechanisms are not well ...understood. In this paper, we report simultaneous optical spectroscopic and photometric observations of a stellar superflare on an active M dwarf, YZ Canis Minoris, with the 3.8 m Seimei telescope and the Transiting Exoplanet Survey Satellite. The flare bolometric energy is
1.3
−
0.6
+
1.6
×
10
34
erg
and the H
α
energy is
3.0
−
0.1
+
0.1
×
10
32
erg
. The H
α
emission line profile shows red asymmetry throughout the flare, with a duration of 4.6–5.1 hr. The velocity of the red asymmetry is ∼200–500 km s
–1
and the line width of H
α
broadens up to 34 ± 14 Å. The redshifted velocity and line width of H
α
line decay more rapidly than the equivalent width, and their time evolutions are correlated with that of the white-light emission. This indicates the possibility of the white light, the H
α
red asymmetry, and the H
α
line broadening originating from nearly the same site, i.e., the dense chromospheric condensation region, heated by nonthermal electrons. On the other hand, the flux ratio of the redshifted excess components to the central components is enhanced one hr after the flare’s onset. This may be due to the main source of the red asymmetry changing to post-flare loops in the later phase of the flare.
ABSTRACT
An overview of the Seimei telescope, a 3.8 m optical infrared telescope located on Mt. Chikurinji in the Okayama prefecture of Japan, is presented. Seimei is a segmented-mirror telescope ...whose primary mirror consists of 18 petal-shaped segments. The telescope tube supporting the thin segmented mirrors is structurally incorporated within large arc-rails providing the elevation axis. The tube has a light-weight homologous structure designed with a genetic algorithm. The total weight of the telescope tube, including 1.4-ton optics, is only 8 tons. By virtue of its light weight, the telescope is able to point at an object anywhere in the observable sky within one minute. The telescope is operated by Kyoto University in collaboration with the National Astronomical Observatory of Japan (NAOJ). Half of the telescope time is used by Kyoto University. The remaining time is open to the Japanese astronomical community. NAOJ is responsible for the management of the open-use time, including handling of the observation proposals. The telescope is now regularly performing scientific observations on the basis of a variety of proposals.
Abstract
Starspots and stellar flares are indicators of stellar magnetic activity. The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely ...understood. To investigate the relation between spot locations deduced from light curves and the occurrence of flares therein, we perform starspot modeling for the TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables us to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We find that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that any spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons why there is no or low correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude.
Can Superflares Occur on Our Sun? Shibata, Kazunari; Isobe, Hiroaki; Hillier, Andrew ...
Publications of the Astronomical Society of Japan,
06/2013, Letnik:
65, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Recent observations of Sun-like stars, similar to our Sun in their surface temperature (5600–6000 K) and slow rotation (rotational period
$ >$
10 d), using the Kepler satellite by Maehara et al. ...(2012, Nature, 485, 478) have revealed the existence of superflares (with energy of 10
$ ^{33}$
–10
$ ^{35}$
erg). From statistical analyses of these superflares, it was found that superflares with energy of 10
$ ^{34}$
erg occur once in 800 yr, and superflares with 10
$ ^{35}$
erg occur once in 5000 yr. In this paper, we examine whether superflares with energy of 10
$ ^{33}$
–10
$ ^{35}$
erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas related to the mechanisms of the solar dynamo. If magnetic flux is generated by differential rotation at the base of the convection zone, as assumed in typical dynamo models, it is possible that the present Sun would generate a large sunspot with a total magnetic flux of
$ \sim$
2
$ \times$
10
$ ^{23}$
Mx (
$ =$
G cm
$ ^{2}$
) within one solar cycle period, and lead to superflares with an energy of 10
$ ^{34}$
erg. To store a total magnetic flux of
$ \sim$
10
$ ^{24}$
Mx, necessary for generating 10
$ ^{35}$
erg superflares, it would take
$ \sim$
40 yr. Hot Jupiters have often been argued to be a necessary ingredient for the generation of superflares, but we found that they do not play any essential role in the generation of magnetic flux in the star itself, if we consider only the magnetic interaction between the star and the hot Jupiter. This seems to be consistent with Maehara et al.'s finding of 148 superflare-generating solar-type stars that do not have a hot Jupiter-like companion. Altogether, our simple calculations, combined with Maehara et al.'s analysis of superflares on Sun-like stars, show that there is a possibility that superflares of 10
$ ^{34}$
erg would occur once in 800 yr on our present Sun.
We conducted high dispersion spectroscopic observations of 50 superflare stars with Subaru/HDS. These 50 stars were selected from the solar-type superflare stars that we had discovered from the ...Kepler data. More than half (34 stars) of these 50 target superflare stars show no evidence of binarity, and we estimated stellar parameters of these 34 stars in our previous study (Notsu et al. 2015, PASJ, 67, 32). According to our previous studies using Kepler data, superflare stars show quasi-periodic brightness variations whose amplitude (0.1%–10%) is much larger than that of the solar brightness variations (0.01%–0.1%) caused by the existence of sunspots on the rotating solar surface. In this study, we investigated whether these quasi-periodic brightness variations of superflare stars are explained by the rotation of a star with fairly large starspots, by using stellar parameters derived in Paper I. First, we confirmed that the value of the projected rotational velocity, v sin i, is consistent with the rotational velocity estimated from the period of the brightness variation. Next, we measured the intensity of Ca ii infrared triplet lines and Hα line, good indicators of the stellar chromospheric activity, and compared them with other stellar properties. The intensity of Ca ii infrared triplet lines indicates that the mean magnetic field strength (〈fB〉) of the target superflare stars can be higher than that of the Sun. A correlation between the amplitude of the brightness variation and the intensity of Ca ii triplet line was found. All the targets expected to have large starspots because of their large amplitude of the brightness variation show high chromospheric activities compared to the Sun. These results support the idea that the brightness variation of superflare stars is due to the rotation with large starspots.
Star spot evolution is visible evidence of the emergence/decay of the magnetic field on a stellar surface, and it is therefore important for the understanding of the underlying stellar dynamo and ...consequential stellar flares. In this paper, we report the temporal evolution of individual star spot areas on the hot-Jupiter-hosting, active solar-type star Kepler-17, whose transits occur every 1.5 days. The spot longitude and area evolution are estimated (1) from the stellar rotational modulations of Kepler data and (2) from the brightness enhancements during the exoplanet transits caused by existence of large star spots. As a result of the comparison, the number of spots, spot locations, and the temporal evolution derived from the rotational modulations are largely different from those of in-transit spots. We confirm that, although only two light-curve minima appear per rotation, there are clearly many spots present on the star. We find that the observed differential intensity changes are sometimes consistent with the spot pattern detected by transits, but at other times they do not match with each other. Although the temporal evolution derived from the rotational modulation differs from those of in-transit spots to a certain degree, the emergence/decay rates of in-transit spots are within an order of magnitude of those derived for sunspots as well as our previous research based only on rotational modulations. This supports the hypothesis that the emergence/decay of sunspots and extremely large star spots on solar-type stars occur through the same underlying processes.
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