Probable detection of an eruptive filament from a superflare on a solar-type star

Solar flares are often accompanied by filament/prominence eruptions ($\sim10^{4}$ K and $\sim 10^{10-11}$ 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 $2.0\times10^{33}$ 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 $1.1\times10^{18}$ 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.

tohydrodynamic process, though all of them are not necessarily observed in the same event. Magnetic reconnection is a key energy release mechanism for flares, which are thought to be sometimes be triggered by the instability of cool filaments in active regions 1 . Recently, it has been discussed that much larger "superflares" that release the energy of more than 10 33 erg (10 times the largest solar flares ∼ 10 32 erg) can occur -or have occurred relatively recently -even on the Sun [3][4][5]8 .
Superflares may produce much larger CMEs than the largest solar flares, which can significantly affect the environment, habitability, and development of life around young and intermediate age stars 6 . However, superflares on solar-type stars have been mainly detected by optical photometry (e.g., Kepler space telescope) 3 . Therefore, no observational indication of filament eruptions/CMEs has been reported for solar-type stars. Optical spectroscopic observations are a promising way to detect stellar filament eruptions, which can be indirect evidence of CMEs. However, for solar-type stars, optical spectra of superflares have never been obtained.
EK Draconis (EK Dra) is known to be an active young solar-type star (a G-type, zero-age main-sequence star with an effective temperature of 5560-5700 K and age of 50-125 million years 9 ) that exhibits frequent UV stellar flares 10,11 and gigantic starspots at low-high latitudes 9 .
We conducted optical spectroscopic monitoring of EK Dra for 19 nights between 21 January 2020 and 15 April 2020, simultaneously with optical photometry from the Transiting Exoplanet Survey Satellite (TESS) 12 . Time-resolved neutral-hydrogen Hα-line spectra at 6562.8Å (radiation from cool plasma of a few times 10,000 K) were spectroscopically observed at the 3.8-m Seimei Telescope 13 and the 2-m Nayuta Telescope. In this campaign, we succeeded in obtaining optical spectra of large superflares on a solar-type star. The superflare that occurred on 5 April 2020 was simultaneously observed using TESS photometry in white light (∼6,000-10,000Å) and groundbased spectroscopy in Hα line ( Fig. 1a-b and Extended Data Fig. 1). The Hα brightening was associated with the TESS white-light flare, which lasted 16 ± 2 min. The radiated bolometric energy of the TESS white-light flare is estimated to be 2.0±0.1×10 33 erg (20 times the most energetic solar flares), and the radiated Hα-line energy was 1.7±0.1×10 31 erg; thus, the flare is classified as a superflare.
After the impulsive phase, the TESS white-light intensity returned to its pre-flare level. However, the equivalent width (hereafter E.W.) of Hα (the wavelength-integrated Hα emission normalized by the continuum level) became lower than the pre-flare level (i.e., it displayed enhanced absorption), returning to the pre-flare level in approximately 2 hours (Fig. 1b). The blue-shift Hα absorption component with a maximum central velocity of about −510 km s −1 and a half-width of ±220 km s −1 appeared soon after the superflare. The velocity gradually slowed down with time, and a red-shifted absorption component appeared at a few times 10 km s −1 (Fig. 1c-e, Extended Data Fig. 2a, 3a). Both ground-based spectroscopic observations simultaneously recorded the same spectral change, demonstrating that low-temperature and high-density neutral plasma above the stellar disk moves at high speed toward the observer before some parts finally start to fall back to the surface. In addition, the deceleration is not monotonic: it was 0.34±0.04 km s −2 in the initial phase, dropping to 0.016±0.008 km s −2 in the later phase ( Fig. 1c-d and Extended Data   Fig. 3b). This is interpreted in terms of changes in the height of the ejected mass. The observed deceleration is in good agreement with that due to the surface gravity of approximately 0.30±0.05 km s −2 (ref. 9 ), although the initial value is slightly larger.
How much do the stellar spectral changes obtained here actually resemble those of solar filament eruptions? Blue-shifted Hα absorption profiles are often observed from solar filament eruptions 1,14 . As in Fig. 2, we generated spatially-integrated Hα spectra of a solar flare/filament eruption that occurred on the solar disk using the SMART data 15 (Extended Data Fig. 4, Supplementary Movie 1). We converted to the full-disk pre-flare subtracted spectra by multiplying by the partial-region/full-disk ratio (i.e., virtual Sun-as-a-star spectra). We found that the blue-shifted absorption component at approximately 100 km s −1 was predominant soon after the solar flare, and the spatially integrated Hα E.W. showed enhanced absorption (Fig. 2a). These blue-shifted profiles are unequivocally due to the filament eruption. Later, the blue-shifted component decelerated and gradually turned into slow, red-shifted absorption (Fig. 2b-c). The Hα E.W. returned to the pre-flare level in approximately 40 min (Fig. 2a). Although the energy scales and velocities are different, the solar data greatly resembles the spectral changes in the superflare on EK Dra (see Supplementary Information for another event). This similarity suggests that the stellar phenomenon is the same as the simply magnified picture of the solar filament eruption.
A filament eruption is the only explanation for the blue-shifted absorption component on EK Dra by solar analogy 1 . The hypothesis that the blue-shifted absorption on EK Dra might come from up-/down-flow in flare kernels must be rejected because they never show Hα absorption 16,17 . Also, down-flow in cooled magnetic loops (known as post-flare loops) 14 show red-shifted absorption, so they cannot explain the blue-shifted absorption. (However, the red-shifted absorption in EK Dra in the later phase might be caused by post-flare loops 14 .) Rotational visibilities of prominences or spots also are not adequate to explain it since the rotation speed of EK Dra is only 16.4±0.1 km s −1 (ref. 9 ). Thus, we concluded that we detected a stellar filament eruption on the solar-type star. Some observational signatures for stellar filament eruptions or CMEs have been reported previously for cooler K-M dwarfs [18][19][20][21]23 and evolved giant stars 22 (see Methods and ref. 6,24 for review). The observation of a giant star shows a blue-shifted X-ray emission line of 90 km s −1 in the post-flare phase and hotter-CME is proposed as a possible explanation 22 . Recently, Xray/EUV dimmings are reported as an indirect evidence of stellar CMEs on K-M dwarfs 23 . In Mdwarf flares, many blue-shifted Balmer/UV line emission components have been reported 18-21, 24 , which are interpreted as filament eruptions. Some M-dwarf flares share properties similar to the eruption on EK Dra: the blue-shift emissions have high velocities of hundreds of km s −1 , and some exhibit velocity changes and appear after the impulsive phase 20,21 . For M-dwarf events, the number of studies reporting highly-time-resolved velocity variations of blue-shift components is still insignificant (∼5-min cadence), and simultaneous white-light flare has never been detected.
Our detection of a stellar filament eruption is reliable because we provided solar counterparts, highly time-resolved spectra (∼50-sec cadence), and simultaneous TESS white-light flare.
What properties does the filament eruption on EK Dra have? The maximum observed velocity of the blue-shifted component was ∼ −510 km s −1 with a width of 220 km s −1 . This is larger than the typical velocities of solar filament eruptions (10-400 km s −1 ) associated with CMEs 2 , although it is a little smaller than the escape velocity at the surface on EK Dra (∼670 km s −1 ).
The cool plasma reached at least ∼1.0 stellar radii from the stellar surface (or the initial height) as derived by integrating the velocity over time (or ∼3.2 stellar radii from the stellar surface based on the deceleration rates). In this case, the projection angle can be allowed at most 45 • when we assume the event occurs on the disk center. On this projection angle, the velocity can be up to ∼ −720 km s −1 , so there is a possibility that the velocities of some components of the EK Dra eruption could exceed the escape velocity. However, it should be noted that there are weak redshifted components with a velocity of a few 10 km s −1 in the late phase, indicating some materials fell back to the star. This is often observed in the case of solar filament eruptions with CMEs 25 .
The filament area is estimated to be 1.6×10 21 cm 2 (5.6 % of the stellar disk), and the erupted mass is calculated to be 1.1 +4.2 −0.9 ×10 18 g based on the absorption components. The mass is more than 10 times larger than those of the largest solar CMEs 28,29 (it should be noted that the mass can be somewhat under-/over-estimated, see Methods). This mass estimate is in reasonable agreement with those predicted from empirical 28,29 , and theoretical 30 solar scaling relations between CME mass and flare energy within the error bars (∼ 9.4 +3.2 −2.4 × 10 16 and 3.1 +1.6 −1.1 × 10 17 g for ref. 29 and ref. 28 , respectively) ( Fig. 3a). This suggests that the stellar filament eruption can share the common underlying mechanism with smaller-scale filament eruptions/CMEs (i.e., magnetic energy release 1, 30 ) although the absolute values of most physical quantities are very different.
Moreover, the kinetic energy is calculated to be 3.5 +14.0 −3.0 ×10 32 erg, which is 16 % of radiation energy in white light. The magnetic energy stored around the starspots on EK Dra can be at least 8.0×10 35 erg, which is enough to produce superflares and filament eruptions with energy of ∼10 33 erg. In addition, this value is slightly smaller than those extrapolated from the solar CME scaling law (4.8 +1.1 −0.9 ×10 33 erg; ref. 29 ) (Fig. 3b), which is similar to the filament eruption/CME candidates on other stars 24 . In previous studies, it has been argued that kinetic energy can be reduced by overlaying magnetic fields 24,27 . The deceleration of our events was a few times 10 % larger than the stellar gravity (Extended Data Fig. 3b). The strong magnetic fields on EK Dra were reported before 9 and may support the above explanations. However, its small kinetic energy can also be understood through a solar analogy: The velocities of (lower-lying) filament eruptions are usually 4-8 times slower than those of the corresponding (higher-lying) CMEs 2 , and therefore the kinetic energies of filament eruptions are typically smaller (green symbols in Fig. 3b).
Did a CME occur in this event? Obviously, the line-of-sight velocity ∼510 km s −1 was slower than the escape velocity and some masses fell back, which may indicate a so-called "failed" filament eruption 27 . However, this does not necessarily mean that a CME did not occur, again by solar analogy. In fact, the erupted filaments often fall back to the Sun while CMEs happen. For example, a well-studied solar event on 2011 June 7 involved a 200-600 km/s filament eruption where lots of filamentary material fell back to the Sun, but some mass clearly escapes as a CME with velocities of ∼1000 km s −1 (see ref. 25 and Supplementary Information). The event on EK Dra may correspond to this solar event. In addition, ref. 26 showed that whether a solar filament eruption leads to a CME can be simply distinguished by a parameter of (V r max /100 km s −1 ) (L/100 Mm) 0.96 , where V r max is the maximum radial velocity and L is the length scale (Fig. 4).
When the parameter is more than ∼ 0.8, the probability that a filament eruption lead to a CME is more than 90% 26 . The value of the parameter of eruption on EK Dra is ∼18, meaning that our detection of the fast and sizeable stellar filament eruption is indirect evidence that mass escapes into interplanetary space as a CME.
Finally, we summarize future directions of our findings (see Supplementary Information for details): It is speculated that the filament eruptions/CMEs associated with superflares can severely affect planetary atmospheres 6 . Our findings can therefore provide a proxy for the possible enormous filament eruptions on young solar-type stars and the Sun, which would enable us to evaluate the effects on the ancient, young Solar-System planets and the Earth, respectively. Further, it is also speculated that stellar mass loss due to filament eruptions/CMEs can more significantly affect the evolutionary theory of stellar mass, angular momentum, and luminosity 7, 28 , than stellar winds.
At present, frequency and statical properties of CMEs on solar-type star is unknown, but important insights into these points are obtained by increasing the samples in the future.    Comparison between bolometric flare energy and ejected mass. The red square represents the superflare on EK Dra, the black crosses denote for solar CME data, the green triangles are data for solar prominence/filament eruptions and surges taken from previous studies, and the green plus sign signifies the solar filament eruption/surges displayed in Fig. 2 and Supplementary Fig. 9 (Supplementary Information "Velocity, mass, and kinetic energy: solar data"), respectively (see Table 1). Note that solar "surges" are jet-like filament eruption phenomena (see Supplementary   Solar filament eruption with CME Solar filament eruption without CME Filament eruption on EK Dra    MALLS is optical spectroscopy with a spectral resolution of R ∼ 10,000 at the Hα line covering a wavelength range from 6350 to 6800Å; it is also equipped with Fe, Ne, and Ar gas emission lines for wavelength calibration and instrument characterization. The sky spectrum was subtracted using a nearby region along the slit direction for each observation. The exposure time was set to be 3 min for this night. The MALLS data reduction follows the prescription in ref. 19 . The signal-to-noise ratio (S/N) for one frame is typically 86±8 during this observation. For the MALLS data, the wavelength corrections are also performed for each spectrum by using the Earth's atmospheric absorption lines.  15,42 . The spectra from the event are integrated over a spatial region that is large enough to cover the visible phenomena (the magenta region in Extended Data Fig. 4a-b). The spectra are reconstructed by using the template solar Hα spectrum convolved with SDDI instrumental profile.
Here, we define L(λ, t, A) as a luminosity at a wavelength of λ and time of t which is integrated for the region A (i.e., L(λ, t, A) = A I(t) dA, I(t) is intensity). We now define A local as the integration region (magenta region in Extended Data Fig. 4a-b), and A full−disk as the solar full disk. We first obtained the local (partial-image) pre-flare subtracted spectra ∆S local which are normalized by local (partial-image) total continuum level (L(6570.8Å, t, A local )): where t 0 is a given time of the pre-flare period. Then, the (virtual) full-disk pre-flare-subtracted spectra ∆S full−disk are obtained by multiplying the ratio of the partial-image continuum to full-disk continuum (total continuum ratio: then we obtained a virtual pre-flare-subtracted spectrum of this phenomena as if we observed the Sun as a star. The E.W. of the Hα is also calculated by using the full-disk-normalized and pre-flare-subtracted spectra (∆S full−disk ), and we obtained the virtual Sun-as-a-star ∆Hα E.W. (i.e., differential Hα flux normalized by the full-disk continuum level).
Velocity, mass, and kinetic energy: stellar data For the stellar filament eruption, the velocity is derived by fitting the absorption spectra obtained by Seimei telescope with the normal distribution N (λ, µ, σ 2 ) where µ is the mean wavelength and σ 2 is the variance. In Extended Data Fig. 3a, we plotted the temporal evolution of the velocity ((µ − λ)/λ × c, where λ is 6562.8Å, c is light speed) for the fitted absorption feature with the width of σ. We only plotted the data whose absorption features are clear enough to fit the shape with the threshold of the fitted absorption amplitude > 0.01 and fitted velocity dispersion of < 500 km s −1 and > 100 km s −1 . The threshold was determined by trial and error, and we find that many missed detections of absorption features occur when we select threshold values other than this one. The amplitude value of 0.01 corresponds to the detection limit when considering the typical S/N∼170 of the Seimei Telescope/KOOLS-IFU, and the lower limit of 100 km s −1 is determined to avoid detecting the sharp noisy signals. About 27% of data points were discarded due to this threshold from the initial points (22 min) to final points (110 min), especially for the latter decaying phase. Here, the maximum observed velocity and its errors are calculated as 510±120 km s −1 with its width of 220±90 km s −1 from the mean values of the µ and σ of the first five points (t = 22-26 min in Fig. 1), respectively. The mean values of the velocity when the absorption becomes strong (t = 25-50 min in Fig. 1) is estimated as 258 km s −1 .
The plasma mass is simply calculated from the total Hα E.W.. We used the simple Becker's cloud model 49 where I 0λ is background intensity and I 0,Cont. is continuum intensity. This is the E.W. value for an extreme case when the full disk of the star is completely covered with absorbing, cool ejected plasma. By comparing the modeled E.W. (Eq. 3) with the lowest observed stellar E.W. value of −0.16Å (integrated for Hα -20Å ∼ Hα + 10Å; see Supplementary Fig. 4c), the coolplasma filling factor compared to the stellar disk is calculated to be 5.9 % of stellar disk (i.e., modeled E.W./observed E.W.; Area = 1.6×10 21 cm 2 ). Using the length scale of the ejected plasma 3.9×10 10 cm (= Area 0.5 ), the hydrogen column density is derived as 4.0×10 20 cm −2 from the assumed optical depth based on the plasma model 51 . In the model of ref. 51 , hydrogen/electron density is calculated by assuming an ionization equilibrium for a population of hydrogen atoms due to a balance between recombination and radiative photoionization through Balmer/Lyman continuum.
It should be noted that the ionization equilibrium of filaments on active stars may be somewhat different from the solar observations due to their high UV radiations, which may affect the evaluation of the mass of the ejecta. By multiplying the hydrogen column density by the filament area, we then obtained the plasma mass of 1. this mass estimate could be either a significant overestimate of the mass of an affiliated CME due to most of the filament falling back to the star, or it could be a significant underestimate due to most of the CME actually being hot coronal material rather than cool filament. The plasma kinetic energy is then calculated as 3.5 +14.0 −3.0 ×10 32 erg by using the velocity of 258 km s −1 . The observed maximum velocity was 510 km s −1 in the early phase, so the kinetic energy can be larger by a factor of 4 although the absorption component was weak at that time.
Related works on candidates of stellar filament eruptions/CMEs on other types of stars Here, we discuss potential stellar filament eruptions/CMEs reported in the previous studies (see 6 Other signatures of kinematic characteristics of the ejected plasma are also inferred from continuous X-ray absorption during stellar flares, which can be caused by neutral material above the flaring region, such as filament eruptions 24,[63][64][65][66][67][68][69] . However, on the Sun, X-ray absorption by promi-nences is uncommon 50,85 , and instrumental calibration effects at low energy have been pointed out 86 .
In some cases of binary stars, eclipses of the white dwarf component have been interpreted as obscuration by stellar mass ejected from the late-type companion star 71,72 . Other than this, pre-flare dips have been reported in stellar flares, suggesting potential prominence eruptions/CMEs 87,88 . Radio observations have recently investigated the type-II radio bursts associated with shocks in front of CMEs as possible indirect evidence of CMEs, but no significant signature has been obtained so far [74][75][76][77][78][79][80]86 . Recently, a stellar type-IV burst event from the M-type star Proxima Centauri was reported and may be evidence for a stellar CME 61 .
Data availability In addition to the figure data available, all raw spectroscopic data are available either in the associated observatory archive (https://smoka.nao.ac.jp/index.jsp for KOOLS-IFU data in Figure 1 (available after Jan 2022); https://www.hida.kyoto-u.ac.jp/SMART/T1. html for a part of SDDI data in Figure 2) or upon request from the corresponding author (for MALLS data in Figure 1 and full raw data of SDDI  Competing Interests The authors declare that they have no competing financial interests.
Correspondence Correspondence and requests for materials should be addressed to Kosuke Namekata