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A Stellar Flare-Coronal Mass Ejection Event Revealed by X-Ray Plasma Motions

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A Stellar Flare-Coronal Mass Ejection Event Revealed by X-Ray Plasma Motions A stellar flare-coronal mass ejection event revealed by X-ray plasma motions C. Argiroffi1,2⋆, F. Reale1,2, J. J. Drake3, A. Ciaravella2, P. Testa3, R. Bonito2, M. Miceli1,2, S. Orlando2, and G. Peres1,2 1 University of Palermo, Department of Physics and Chemistry, Piazza del Parlamento 1, 90134, Palermo, Italy. 2 INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy. 3 Smithsonian Astrophysical Observatory, MS-3, 60 Garden Street, Cambridge, MA 02138, USA. ⋆ costanza.argiroffi@unipa.it May 28, 2019 Coronal mass ejections (CMEs), often associ- transported along the magnetic field lines and heats ated with flares 1,2,3, are the most powerful mag- the underlying chromosphere, that expands upward netic phenomena occurring on the Sun. Stars at hundreds of kms−1, filling the overlying magnetic show magnetic activity levels up to 104 times structure (flare rising phase). Then this plasma gradu- higher 4, and CME effects on stellar physics and ally cools down radiatively and conductively (flare de- circumstellar environments are predicted to be cay). The flare magnetic drivers often cause also large- significant 5,6,7,8,9. However, stellar CMEs re- scale expulsions of previously confined plasma, CMEs, main observationally unexplored. Using time- that carry away large amounts of mass and energy. resolved high-resolution X-ray spectroscopy of a Solar observations demonstrate that CME occurrence, stellar flare on the active star HR 9024 observed mass, and kinetic energy increase with increasing flare with Chandra/HETGS, we distinctly detected energy 1,2, corroborating the flare-CME link. Doppler shifts in S xvi, Si xiv, and Mg xii lines Active stars have stronger magnetic fields, higher that indicate upward and downward motions of flare energies, hotter and denser coronal plasma 12. hot plasmas (∼ 10 − 25 MK) within the flaring Their activity level, measured by the X-ray to bolo- ∼ − −1 4 loop, with velocity v 100 400kms , in agree- metric luminosity ratio, LX/Lbol, can be up to 10 ment with a model of flaring magnetic tube. times higher than the solar one. Currently, the prop- Most notably, we also detected a later blueshift erties of stellar CMEs can only be presumed extrap- in the O viii line which reveals an upward mo- olating the solar flare-CME relation up to several or- −1 tion, with v = 90 ± 30kms , of cool plasma ders of magnitude, even though active stellar coronae (∼ 4 MK), that we ascribe to a CME coupled differ profoundly from the solar one. This extrapo- to the flare. From this evidence we were able lation suggests that stellar CMEs should cause enor- +2.6 21 6,7,8,9 to derive a CME mass of 1.2−0.8 × 10 g and a mous amounts of mass and kinetic energy loss +27.7 34 −9 −1 CME kinetic energy of 5.2−3.6 × 10 erg. These (up to ∼ 10 M⊙ yr and ∼ 0.1Lbol, respectively), values provide clues in the extrapolation of the and could significantly influence exoplanets 5. solar case to higher activity levels, suggesting Thus far, there have been a few claims of stellar that CMEs could indeed be a major cause of CMEs. Blueshifted components of chromospheric lines arXiv:1905.11325v1 [astro-ph.SR] 27 May 2019 mass and angular momentum loss. were sometimes attributed to CMEs 13,14,15, but they could also be explained by chromospheric brighten- ings or evaporation events 16,17. CMEs were invoked Intense stellar magnetic fields are responsible for the to explain increased X-ray absorption observed dur- so-called stellar magnetic activity 10,4, and for the as- ing flares 18. However, the simultaneous variations of sociated highly energetic phenomena occurring in the flare temperature and emission measure 18 (EM) in- outer stellar atmosphere. CMEs, the most energetic sinuate that the increased absorption can be a spuri- coronal phenomena, are observed only on the Sun, be- ous result coming from the limited diagnostic power cause their detection and identification needs spatial of low-resolution X-ray spectroscopy, combined with resolution. the oversimplified assumptions of an isothermal flar- CMEs are closely linked to flares 3. In the stan- ing plasma and a constant quiescent corona. CMEs dard scenario 11 flares are driven by impulsive magnetic were candidates to explain transient UV/X-rays ab- reconnections in the corona. The released energy is sorptions observed in the eclipsing precataclysmic bi- 1 21 31 −1 HR 9024 indeed shows coronal (LX ∼ 10 ergs , with T ∼ 1 − 100 MK) and magnetic field properties 22 2 (a dominant poloidal field with Bmax ∼ 10 G) anal- ogous to that of other active stars. Therefore, irre- spective of the origin of some magnetic components, and bearing in mind that active stars may show di- verse magnetic configurations, the coronal phenomena occurring on HR 9024 can be considered as represen- tative of those of active stars. HR 9024 X-ray spectrum was collected during a 98 ks-long Chandra/HETGS observation (Fig. 1a), during which a strong flare (peak luminosity ∼ 1032 ergs−1 and X-ray fluence ∼ 1036 erg) was regis- tered (Fig. 1b). The high energy of this flare maximizes the proba- bility to have an associated CME 1. The flaring loop located near the stellar disk center, as hinted by the Fe fluorescence 25, maximizes the possibility to detect radial velocities of both flaring and CME plasmas. We measured time-resolved individual line positions, considering only strong and isolated lines that probe plasma with T ranging from 2 to 25 MK (Fig. 2 and Methods and Supplementary Table 1). We found: Figure 1: Observed X-ray spectra and light curve of HR 9024. a, X-ray spectra collected with the Medium Energy Grating (MEG) - significant blueshifts during the rising phase of and High Energy Grating (HEG) during the 98 ks long Chandra the flare in the S xvi line at 4.73 Å (−400 ± observation, with the strongest emission lines labeled. MEG and 180kms−1) and in the Si xiv line at 6.18 Å HEG bin size are 5 and 2.5mÅ. b, X-ray light curve of registered (−270 ± 120kms−1), with a 99.99% combined sig- during the Chandra observation, obtained from the ±1 order spectra of HEG and MEG. nificance of the two line shifts. - significant redshifts in the Si xiv line at 6.18 Å (140±80kms−1) and Mg xii line at 8.42 Å (70±50 nary V471 Tau 19. However, such absorptions can and 90±40kms−1), during the maximum and de- equally be produced by a stellar wind 20. In addition cay phases of the flare, with a 99.997% combined to their ambiguous interpretation, all these candidate significance of the three line shifts. detections never provide the CME physical properties, unless substantial assumptions are made. - a significant blueshift in the O viii line at 18.97 Å We present here strong evidence for the detection (−90 ± 30kms−1), after the flare, significant at and identification of a stellar CME, the estimate of the 99.9% level. its properties, and the simultaneous monitoring of the associated flare energetics. The Sun indicates that a The first two Doppler shifts tell us that hot flaring flare-CME event should produce hot plasma moving plasma moves upward at the beginning of the flare, and upward and downward within the flaring loop, and, then settles back down to the chromosphere, as pre- after the flare onset, cool plasma in the CME mov- dicted 11. We compared the observed velocities with ing upward. Therefore, monitoring the plasma veloc- predictions based on the flaring loop model 21 (Meth- ity at different temperatures during a stellar flare is ods and Supplementary Fig. 1). For each line and each a potentially powerful method of searching for CMEs. time interval, we computed the radial velocity corre- The unrivalled X-ray spectral resolution of the Chan- sponding to the predicted line centroid, for different dra/HETGS jointly with detailed hydrodynamic (HD) possible inclinations of the flaring loop (identified by modeling allowed us to investigate a very favorable the φ and θ angles defined in Fig. 3a). case: the strong flare 21 observed on the active star The best agreement between observed and predicted HR 9024. velocities (Fig. 3b-f) is obtained assuming a loop ob- 22 ◦ ◦ HR 9024 is a G1 III single giant , with M⋆ ∼ served from above (φ = 0 and θ = 90 , see Meth- 25 2.85 M⊙ and R⋆ ∼ 9.45 R⊙, located at 139.5pc. Its ods), confirming previous independent results . The convective envelope and rotational period 23 (24.2 d) agreement for the Si xiv and Mg xii lines is striking indicate that an efficient dynamo is at work 22, as ex- (Fig. 3c-f). The observed S xvi blueshift, associated pected in single G-type giants 24. Even if some contri- with chromospheric plasma upflows, is of the same or- bution to its magnetic field could have fossil origin 22, der but even more extended in time than expected. 2 Figure 2: Time-resolved line fits. a, Analysis of the S xvi line at 4.73 Å, as registered with the MEG ±1 orders. In all plots vertical bars indicate errors at 1σ. a1, count rate detected in a 0.1 Å interval centered on the line. Letters and colors indicate the different time intervals considered to collect the line profile. a2, Observed line profile, in different time intervals (black), with superimposed (in different colors, following the same color-code used for the different time intervals) the corresponding best fit function (with dashed curves corresponding to the two transitions of the Lyα doublet, solid curve to their sum).
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