Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/institute/conference/iausps13/presentations/Maehara.pdf Дата изменения: Unknown Дата индексирования: Tue Feb 5 20:30:53 2013 Кодировка: Поисковые слова: starspots

Superflares on solar-type stars

Hiroyuki Maehara (Kyoto University)
Collaborators: Takuya Shibayama, Shota Notsu, Yuta Notsu, Takashi Nagao, Satoshi Kusaba, Satoshi Honda, Daisaku Nogami & Kazunari Shibata


Energy-frequency distribution of solar flares


Large flares are less frequent than smaller ones. Power-law distribution: dN/dE E-1




.5 -1.9

Flare energy: 1024 ~ 1032 ergs Frequency of solar flares with energy >1032 ergs: unknown



Aschwanden et al., ApJ, 535, 1047 (2000)


Superflares


Larger flares (energy 1033 ­ 1038 ergs) are observed on a variety of stars.


close binary systems YSOs (e.g. T Tauri stars) young, rapidly rotating stars
V773 Tau (T Tauri binary) Tsuboi et al., ApJ, 503, 894 (1998)



Schaefer et al. (2000) reported 9 superflares on ordinary solartype stars.


frequency of superflares ? relation between stellar proparties and superflares ?
­

e.g. rotation period


Flares on solar-type stars


Supreflares on solar-type stars are difficult to observe because...


The frequency of superflares may be extremely low.
­

Flares with energy 1,000 times larger than solar flares frequency: ~1/1000



The change in the stellar brightness due to flares on solar-type stars is very small.
­

X17-class solar flare: ~10-4 X1000-class flare: ~10-

2

0.027%

Kopp et al., Solar Phys. 230, 129 (2005)


Kepler Data


Kepler is the best space telescope for detecting superflares.


High photometric precision (<10-4) Large number of targets (~160,000 stars)



We searched for flare-like events (sudden brigetenings) from the Kepler data (Q0-Q2)


83,000 solar-type stars
­ ­

Teff: 5,100-6,000 K Log g: 4.0


Light curve of superflares


We found 365 superflares on 148 solar-type stars.



14 flares on 10 Sun-like stars (Teff: 5600-6000; P>10days)
Amplitude: 0.1-10% Duration: ~0.1 days Total bolometric energy of superflares: 1033-10
­
36

ergs

10-10,000 times larger than the largest solar flares (1032 ergs)


Energy-frequency distribution


Power-law distribution with the index of -2.3+/- 0.3


The frequency distribution is similar to those of solar flares and stellar flares on red dwarfs.
All G-type main sequence stars
Teff: 5100-6000K

1 in 800 years

1 in 5000 years
Sun-like stars
Teff: 5600-6000K Period: >10 days


Energy-frequency distribution


Long-term brightness variations


Rotation of the star with starspots


Period of brightness variation rotation period Amplitude of brightness variation size of starspots Period of brightness variation orbital period



Orbital modulation of binary system



Period vs. Flare frequency


Stars with short-period variations show more frequent superflares.


Superflraes frequently occur on rapidly rotating stars. E >5в1034 ergs

flare


Period vs. Flare energy


The maximum energy of superflares does not depend on the period of long-term light variations.


Superflares would occur on slowly rotating stars like our Sun.


Amplitude vs. superflare frequency


Most of superflares occur on the stars with large-amplitude light variations.


Superflare stars have larger starspots than does our Sun.

All G-type main sequence stars

without P<10d


superflare!


Hot jupters and superflares


It has been proposed that hot jupiters have an important effect on stellar magnetic activity (e.g. Rubenstein & Schaefer 2000).


Superflares on our Sun are extremly unlikely.



If all superflare stars have hot Jupiters, Kepler should detect ~10 of them from transits. No hot jupiters have been found around the 148 superflare stars. superflares caused by hot Jupiters are rare.




Summary


We found 365 superflares on 148 solar-type stars


14 superflares on 10 Sun-like stars



The energy-frequency distribution of superflares on solar-type stars is similar to that of solar flares. The frequency of superflares depens on the period and the amplitude of long-term light variations.




Superflares occur more frequently on the stars with short-period and large-amplitude light variations. Superflare stars may have large starspots





Flare energy does not depend on the period of light variations. Superflares caused by hot Jupiters are rare.




We could not exclude the possibility of superflares on our Sun.


Thank you for your kind attention!