Takashi Nagao

Superflares on solar-type stars

Solar flares are caused by the sudden release of magnetic energy stored near sunspots. They release 1029 to 1032u2009ergs of energy on a timescale of hours. Similar flares have been observed on many stars, with larger ‘superflares’ seen on a variety of stars, some of which are rapidly rotating and some of which are of ordinary solar type. The small number of superflares observed on solar-type stars has hitherto precluded a detailed study of them. Here we report observations of 365 superflares, including some from slowly rotating solar-type stars, from about 83,000 stars observed over 120 days. Quasi-periodic brightness modulations observed in the solar-type stars suggest that they have much larger starspots than does the Sun. The maximum energy of the flare is not correlated with the stellar rotation period, but the data suggest that superflares occur more frequently on rapidly rotating stars. It has been proposed that hot Jupiters may be important in the generation of superflares on solar-type stars, but none have been discovered around the stars that we have studied, indicating that hot Jupiters associated with superflares are rare.

SUPERFLARES ON SOLAR-TYPE STARS OBSERVED WITH KEPLER. I. STATISTICAL PROPERTIES OF SUPERFLARES

By extending our previous study by Maehara et al., we searched for superflares on G-type dwarfs (solar-type stars) using Kepler data for a longer period (500xa0days) than that (120xa0days) in our previous study. As a result, we found 1547 superflares on 279xa0G-type dwarfs, which is much more than the previous 365 superflares on 148 stars. Using these new data, we studied the statistical properties of the occurrence rate of superflares, and confirmed the previous results, i.e., the occurrence rate (dN/dE) of superflares versus flare energy (E) shows a power-law distribution with dN/dEE –α, where α ~ 2. It is interesting that this distribution is roughly similar to that for solar flares. In the case of the Sun-like stars (with surface temperature 5600-6000xa0K and slowly rotating with a period longer than 10xa0days), the occurrence rate of superflares with an energy of 1034-1035xa0erg is once in 800-5000xa0yr. We also studied long-term (500xa0days) stellar brightness variation of these superflare stars and found that in some G-type dwarfs the occurrence rate of superflares was extremely high, ~57 superflares in 500xa0days (i.e., once in 10xa0days). In the case of Sun-like stars, the most active stars show a frequency of one superflare (with 1034xa0erg) in 100xa0days. There is evidence that these superflare stars have extremely large starspots with a size about 10xa0times larger than that of the largest sunspot. We argue that the physical origin of the extremely high occurrence rate of superflares in these stars may be attributed to the existence of extremely large starspots.

Can Superflares Occur on Our Sun

Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy 10^34 erg occur once in 800 years and superflares with 10^35 erg occur once in 5000 years on Sun-like stars. 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 relating to the mechanisms of the solar dynamo.

Superflares on Solar-type Stars Observed with Kepler II. Photometric Variability of Superflare-generating Stars: A Signature of Stellar Rotation and Starspots

We performed simple spot-model calculations for quasi-periodic brightness variations of solar-type stars showing superflares using Kepler photometric data. Most of the superflare stars show quasi-periodic brightness modulations with a typical period of one to a few tens of days. Our results indicate that these brightness variations can be explained by the rotation of a star with fairly large starspots. Using the results of the period analysis, we investigated the relation between the energy and frequency of superflares and the rotation period. Stars with relatively slower rotation rates can still produce flares that are as energetic as those of more rapidly rotating stars although the average flare frequency is lower for more slowly rotating stars. We found that the energy of superflares is related to the total coverage of the starspot. The correlation between the spot coverage and the flare energy in superflares is similar to that in solar flares. These results suggest that the energy of superflares can be explained by the magnetic energy stored around the starspots.

High-Dispersion Spectroscopy of the Superflare Star KIC 6934317

We conducted the high-resolution spectroscopic observation with Subaru/HDS for a G-type star (KIC6934317). We selected this star from the data of the Kepler spacecraft. This star produces a lot of superflares, and the total energy of the largest superflare on this star is � 10 3 times larger (� 2.2×10 35 erg) than that of the most energetic flare on the Sun (� 10 32 erg). The core depth and emission flux of Ca II infrared triplet lines and Hline show high chromospheric activity in this star, in spite of its low lithium abundance and the small amplitude of the rotational modulation. Using the empirical relations between emission flux of chromospheric lines and X-ray flux, this star is considered to show much higher coronal activity than that of the Sun. It probably has large starspots which can store a large amount of magnetic energy enough to give rise to superflares. We also estimated the stellar parameters, such as effective temperature, surface gravity, metallicity, projected rotational velocity (vsini), and radial velocity. KIC6934317 is then confirmed to be an early G-type main sequence star. The value of vsini is estimated to be �1.91 km s −1 . In contrast, the rotational velocity is calculated to be �20 km s −1 by using the period of the brightness variation as the rotation period. This difference can be explained by its small inclination angle (nearly pole-on). The small inclination angle is also supported by the contrast between the large superflare amplitude and the small stellar brightness variation amplitude. The lithium abundance and isochrones implies that the age of this star is more than about a few Gyr, though a problem why this star with such an age has a strong activity remains unsolved.

EXTINCTION LAWS TOWARD STELLAR SOURCES WITHIN A DUSTY CIRCUMSTELLAR MEDIUM AND IMPLICATIONS FOR TYPE IA SUPERNOVAE

Many astronomical objects are surrounded by dusty environments. In such dusty objects, multiple scattering processes of photons by circumstellar (CS) dust grains can effectively alter extinction properties. In this paper, we systematically investigate the effects of multiple scattering on extinction laws for steady-emission sources surrounded by the dusty CS medium using a radiation transfer simulation based on the Monte Carlo technique. In particular, we focus on whether and how the extinction properties are affected by properties of CS dust grains by adopting various dust grain models. We confirm that behaviors of the (effective) extinction laws are highly dependent on the properties of CS grains, especially the total-to-selective extinction ratio R V , which characterizes the extinction law and can be either increased or decreased and compared with the case without multiple scattering. We find that the criterion for this behavior is given by a ratio of albedos in the B and V bands. We also find that either small silicate grains or polycyclic aromatic hydrocarbons are necessary for realizing a low value of R V as often measured toward SNe Ia if the multiple scattering by CS dust is responsible for their non-standard extinction laws. Using the derived relations between the properties of dust grains and the resulting effective extinction laws, we propose that the extinction laws toward dusty objects could be used to constrain the properties of dust grains in CS environments.

OISTER optical and near-infrared observations of the super-Chandrasekhar supernova candidate SN 2012dn: Dust emission from the circumstellar shell

We present extensively dense observations of the super-Chandrasekhar supernova (SC SN) candidate SN 2012dn from

The Origin of the Near-infrared Excess in SN Ia 2012dn: Circumstellar Dust around the Super-Chandrasekhar Supernova Candidate

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Polarization as a probe of dusty environments around Type Ia supernovae: radiative transfer models for SN 2012dn

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Circumstellar Light Echo as a Possible Origin of the Polarization of Type IIP Supernovae

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