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Sun-as-a-star Analysis of Hα Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening  

Kosuke Namekata   Submitted: 2022-06-05 19:21

Stellar flares sometimes show red/blue asymmetries of Hα line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not completely understood. For a deeper understanding of stellar data, we performed a Sun-as-a-star analysis of Hα line profiles of an M4.2-class solar flare showing dominant emissions from flare ribbons by using the data of the Solar Dynamics Doppler Imager onboard the Solar Magnetic Activity Research Telescope at Hida Observatory. The Sun-as-a-star Hα spectra of the flare show red asymmetry of up to ∼95 km s-1 and line broadening of up to ∼7.5 . The Sun-as-a-star Hα profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of ∼2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the Hα equivalent widths, which is consistent with chromospheric condensation model and spatially-resolved flare spectra. This suggests that as a result of superposition, the nature of chromospheric condensation is observable even from stellar flare spectra. The time evolution of redshift velocities is found to be similar to that of luminosities of near-ultraviolet rays (1600 ), while the time evolution of line broadening is similar to that of optical white lights. These Hα spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of Hα red asymmetry of stellar flares is a flare ribbon or other phenomena.

Authors: Kosuke Namekata, Kiyoshi Ichimoto, Takako T. Ishii, Kazunari Shibata
Projects: SDO-AIA

Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2022-06-06 15:48
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Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for Hα and White-Light Emissions  

Kosuke Namekata   Submitted: 2022-02-03 15:16

Young solar-type stars are known to show frequent "superflares", which may severely influence the habitable worlds on young planets via intense radiations and coronal mass ejections. Here we report an optical spectroscopic and photometric observation of a long-duration superflare on the young solar-type star EK Draconis (50-120 Myr age) with the Seimei telescope and Transiting Exoplanet Survey Satellite (TESS). The flare energy 2.61034 erg and white-light flare duration 2.2 hr are much larger than those of the largest solar flares, and this is the largest superflare on a solar-type star ever detected by optical spectroscopy. The Hα emission profile shows no significant line asymmetry, meaning no signature of a filament eruption, unlike the only previous detection of a superflare on this star (Namekata et al. 2021, Nat.Astron). Also, it did not show significant line broadening, indicating that the non-thermal heating at the flare footpoints are not essential or that the footpoints are behind the limb. The time evolution and duration of the Hα flare are surprisingly almost the same as those of the white-light flare, which is different from general M-dwarf (super-)flares and solar flares. This unexpected time evolution may suggest that different radiation mechanisms than general solar flares are predominant, as follows: (1) radiation from (off-limb) flare loops, and (2) re-radiation via radiative backwarming, in both of which the cooling timescales of flare loops could determine the timescales of Hα and white light.

Authors: Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda, Yuta Notsu, Soshi Okamoto, Jun Takahashi, Masaki Takayama, Tomohito Ohshima, Tomoki Saito, Noriyuki Katoh, Miyako Tozuka, Katsuhiro L. Murata, Futa Ogawa, Masafumi Niwano, Ryo Adachi, Motoki Oeda, Kazuki Shiraishi, Keisuke Isogai, Daisaku Nogami, Kazunari Shibata
Projects: None

Publication Status: Accepted for publication in The Astrophysical Journal Letters
Last Modified: 2022-02-07 15:28
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Probable detection of an eruptive filament from a superflare on a solar-type star  

Kosuke Namekata   Submitted: 2021-12-09 22:50

Solar flares are often accompanied by filament/prominence eruptions (∼104 K and ∼1010-11 cm-3), sometimes leading to coronal mass ejections (CMEs) that directly affect the Earth's environment. `Superflares' are found on some active solar-type (G-type main-sequence) stars, but the association of filament eruptions/CMEs has not been established. Here we show that our optical spectroscopic observation of the young solar-type star EK Draconis reveals the evidence for a stellar filament eruption associated with a superflare. This superflare emitted a radiated energy of 2.01033 erg, and blue-shifted hydrogen absorption component with a large velocity of -510 km s-1 was observed shortly after. The temporal changes in the spectra greatly resemble those of solar filament eruptions. Comparing this eruption with solar filament eruptions in terms of the length scale and velocity strongly suggests that a stellar CME occurred. The erupted filament mass of 1.11018 g is 10 times larger than those of the largest solar CMEs. The massive filament eruption and an associated CME provide the opportunity to evaluate how they affect the environment of young exoplanets/young Earth and stellar mass/angular-momentum evolution.

Authors: Kosuke Namekata, Hiroyuki Maehara, Satoshi Honda, Yuta Notsu, Soshi Okamoto, Jun Takahashi, Masaki Takayama, Tomohito Ohshima, Tomoki Saito, Noriyuki Katoh, Miyako Tozuka, Katsuhiro L. Murata, Futa Ogawa, Masafumi Niwano, Ryo Adachi, Motoki Oeda, Kazuki Shiraishi, Keisuke Isogai, Daikichi Seki, Takako T. Ishii, Kiyoshi Ichimoto, Daisaku Nogami, Kazunari Shibata
Projects: None

Publication Status: Published in Nature Astronomy (2021)
Last Modified: 2021-12-10 15:08
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Lifetimes and Emergence/Decay Rates of Star Spots on Solar-type Stars Estimated by Kepler Data in Comparison with Those of Sunspots  

Kosuke Namekata   Submitted: 2018-11-28 21:03

Active solar-type stars show large quasi-periodic brightness variations caused by stellar rotations with star spots, and the amplitude changes as the spots emerge and decay. The Kepler data are suitable for investigations on the emergence and decay processes of star spots, which are important to understand underlying stellar dynamo and stellar flares. In this study, we measured temporal evolutions of star spot area with Kepler data by tracing local minima of the light curves. In this analysis, we extracted temporal evolutions of star spots showing clear emergence and decay without being disturbed by stellar differential rotations. We applied this method to 5356 active solar-type stars observed by Kepler and obtained temporal evolutions of 56 individual star spots. We calculated lifetimes, emergence and decay rates of the star spots from the obtained temporal evolutions of spot area. As a result, we found that lifetimes (T) of star spots are ranging from 10 to 350 days when spot areas (A) are 0.1-2.3 percent of the solar hemisphere. We also compared them with sunspot lifetimes, and found that the lifetimes of star spots are much shorter than those extrapolated from an empirical relation of sunspots (T∝ A), while being consistent with other researches on star spot lifetimes. The emerging and decay rates of star spots are typically 5 x 1020 Mx · h-1 (8 MSH · h-1) with the area of 0.1-2.3 percent of the solar hemisphere and are mostly consistent with those expected from sunspots, which may indicate the same underlying processes.

Authors: Kosuke Namekata, Hiroyuki Maehara, Yuta Notsu, Shin Toriumi, Hisashi Hayakawa, Kai Ikuta, Shota Notsu, Satoshi Honda, Daisaku Nogami, and Kazunari Shibata
Projects: None

Publication Status: accepted for publication in ApJ
Last Modified: 2018-11-29 10:38
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Statistical Study of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars  

Kosuke Namekata   Submitted: 2017-11-04 04:30

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies (E) and durations (τ): τ ∝ E0.39 (Maehara et al. 2015, EP& S, 67, 59), similar to those of solar hard/soft X-ray flares: τ ∝ E0.2-0.33. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out a statistical research on 50 solar WLFs observed with SDO/HMI and examined the correlation between the energies and durations. As a result, the E-τ relation on solar WLFs (τ ∝ E0.38) is quite similar to that on stellar superflares (τ ∝ E0.39). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy. (1) In solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect. (2) The distribution can be understood by applying a scaling law (τ ∝ E1/3B-5/3) derived from the magnetic reconnection theory. In this case, the observed superflares are expected to have 2-4 times stronger magnetic field strength than solar flares.

Authors: Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe, Ayumi Asai, Hiroyuki Maehara, Yuta Notsu, Shota Notsu, Satoshi Honda, Takako Ishii, Kai Ikuta, Daisaku Nogami, Kazunari Shibata
Projects: GOES X-rays,Hinode/SOT,RHESSI,SDO-AIA,SDO-HMI

Publication Status: accepted for publication in ApJ
Last Modified: 2017-11-06 11:34
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Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares  

Kosuke Namekata   Submitted: 2016-10-31 19:51

Shibata & Yokoyama (1999, 2002) proposed a method of estimating the coronal magnetic field strengths (B) and magnetic loop lengths (L) of solar and stellar flares, on the basis of magnetohydrodynamic simulations of the magnetic reconnection model. Using the scaling law provided by Shibata & Yokoyama (1999, 2002), B and L are obtained as functions of the emission measure (EM=n^2L^3) and temperature (T) at the flare peak. Here, n is the coronal electron density of the flares. This scaling law enables the estimation of B and L for unresolved stellar flares from the observable physical quantities EM and T, which is helpful for studying stellar surface activities. To apply this scaling law to stellar flares, we discuss its validity for spatially resolved solar flares. EM and T were calculated from GOES soft X-ray flux data, and B and L are theoretically estimated using the scaling law. For the same flare events, B and L were also observationally estimated with images taken by Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic Imager (HMI) Magnetogram and Atmospheric Imaging Assembly (AIA) 94Å pass band. As expected, a positive correlation was found between the theoretically and observationally estimated values. We interpret this result as indirect evidence that flares are caused by magnetic reconnection. Moreover, this analysis makes us confident in the validity of applying this scaling law to stellar flares as well as solar flares.

Authors: Kosuke Namekata, Takahito Sakaue, Kyoko Watanabe, Ayumi Asai, Kazunari Shibata
Projects: GOES X-rays,SDO-AIA,SDO-HMI

Publication Status: Accepted for publication in PASJ
Last Modified: 2016-11-02 08:41
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Abstracts by Author
Sun-as-a-star Analysis of Hα Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening
Discovery of a Long-Duration Superflare on a Young Solar-Type Star EK Draconis with Nearly Similar Time Evolution for Hα and White-Light Emissions
Probable detection of an eruptive filament from a superflare on a solar-type star
Lifetimes and Emergence/Decay Rates of Star Spots on Solar-type Stars Estimated by Kepler Data in Comparison with Those of Sunspots
Statistical Study of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars
Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares

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