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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 LETTERS https://doi.org/10.1038/s41550-019-0781-4 A stellar flare−coronal mass ejection event revealed by X-ray plasma motions C. Argiroffi 1,2*, F. Reale1,2, J. J. Drake3, A. Ciaravella2, P. Testa3, R. Bonito2, M. Miceli 1,2, S. Orlando 2 and G. Peres1,2 Coronal mass ejections (CMEs), often associated with flares1–3, even though active stellar coronae differ profoundly from the Sun’s are the most powerful magnetic phenomena occurring on the corona. This extrapolation suggests that stellar CMEs should cause Sun. Stars show magnetic activity levels up to ten thousand enormous amounts of mass and kinetic energy loss6–9 (up to about 4 −9 −1 times higher , and CME effects on stellar physics and cir- 10 M⊙ yr and about 0.1Lbol, respectively, where M⊙ is the solar cumstellar environments are predicted to be substantial5–9. mass), and could strongly influence exoplanets5. However, stellar CMEs remain observationally unexplored. Thus far, there have been a few claims of stellar CME observa- Using time-resolved high-resolution X-ray spectroscopy of tions. Blueshifted components of chromospheric lines have some- a stellar flare on the active star HR 9024 observed with the times been attributed to CMEs13–15, but these could also be explained High Energy Transmission Grating Spectrometer onboard by chromospheric brightenings or evaporation events16,17. CMEs the Chandra X-ray Observatory space telescope, we dis- have been invoked to explain increased X-ray absorption observed tinctly detected Doppler shifts in S XVI, Si XIV and Mg XII lines during flares18. However, the simultaneous variations of flare tem- that indicate upward and downward motions of hot plasmas perature and emission measure18 (EM) imply that the increased (around 10–25 MK) within the flaring loop, with velocities of absorption may be a spurious result, coming from the limited diag- 100–400 km s−1, in agreement with a model of a flaring mag- nostic power of low-resolution X-ray spectroscopy, combined with netic tube. Most notably, we also detected a later blueshift the oversimplified assumptions of an isothermal flaring plasma in the O VIII line that reveals an upward motion, with velocity and a constant quiescent corona. CMEs have also been invoked to 90 ± 30 km s−1, of cool plasma (about 4 MK), that we ascribe explain transient ultraviolet and X-ray absorptions observed in the to a CME coupled to the flare. From this evidence we were eclipsing precataclysmic binary system V471 Tau19. However, such +2.6 21 20 able to derive a CME mass of 1.2−0.8 ×10 g and a CME kinetic absorptions can equally be produced by a stellar wind . In addition +27.734 energy of 5.2−3.6 ×10 erg. These values provide clues in the to their ambiguous interpretations, none of these candidate detec- extrapolation of the solar case to higher activity levels in other tions provide the required CME physical properties, unless substan- stars, suggesting that CMEs could indeed be a major cause of tial assumptions are made. mass and angular momentum loss. We present here strong evidence for the detection and identi- Intense stellar magnetic fields are responsible for the so-called fication of a stellar CME, an estimate of its properties, and the stellar magnetic activity4,10, and for the associated highly energetic simultaneous monitoring of the associated flare energetics. In the phenomena occurring in the outer stellar atmosphere. CMEs, the Sun, a flare−CME event produces hot plasma moving upward and most energetic coronal phenomena, are observed only on the Sun, downward within the flaring loop, and, after the flare onset, cool because their detection and identification needs spatial resolution. plasma in the CME moving upward. Therefore, monitoring the CMEs are closely linked to flares3. In the standard scenario11, plasma velocity at different temperatures during a stellar flare is a flares are driven by impulsive magnetic reconnections in the corona. potentially powerful method of searching for CMEs. The unrivalled The released energy is transported along the magnetic field lines X-ray spectral resolution of the Chandra High Energy Transmission and heats the underlying chromosphere, which expands upward at Grating Spectrometer (HETGS), combined with detailed hydro- hundreds of kilometres per second, filling the overlying magnetic dynamic modelling, allowed us to investigate the strong flare21 structure (flare rising phase). Then this plasma gradually cools observed on the active star HR 9024. 22 down radiatively and conductively (flare decay). The flare magnetic HR 9024 is a G1 III single giant star , with stellar mass M⋆ ≈ 2.85 M⊙ drivers often also cause large-scale expulsions of previously con- and stellar radius R⋆ ≈ 9.45 R⊙, located at 139.5 pc. Its convective fined plasma, CMEs, that carry away large amounts of mass and envelope and rotational period23 (24.2 days) indicate that an effi- energy. Solar observations demonstrate that CME occurrence, mass cient dynamo is at work22, as is expected in single G-type giants24. and kinetic energy increase with increasing flare energy1,2, corrobo- Even if some contribution to its magnetic field may have a fossil 22 21 31 −1 rating the links between flares and CMEs. origin , HR 9024 shows coronal properties (LX ≈ 10 erg s , with Active stars have stronger magnetic fields, higher flare energies T ≈ 1–100 MK) and magnetic-field properties22 (a dominant poloi- 12 2 and hotter and denser coronal plasma . Their activity levels, mea- dal field with Bmax ≈ 10 G) analogous to that of other active stars. sured by the X-ray to bolometric luminosity ratio, LX/Lbol, can be Therefore, irrespective of the origin of some magnetic components, up to 10,000 times higher than the solar level. Currently, the prop- and bearing in mind that active stars may show diverse magnetic erties of stellar CMEs can only be presumed by extrapolating the configurations, the coronal phenomena occurring on HR 9024 can solar flare−CME relation up to several orders of magnitude higher, be considered as representative of those of active stars. 1Department of Physics and Chemistry, University of Palermo, Palermo, Italy. 2INAF - Osservatorio Astronomico di Palermo, Palermo, Italy. 3Smithsonian Astrophysical Observatory, Cambridge, MA, USA. *e-mail: [email protected] NatURE AstronomY | www.nature.com/natureastronomy LETTERS NATURE ASTRONOMY a and Supplementary Fig. 1). For each line and each time interval, XII XII XI XXV XXIV XXIV XXIII XXIV XXIII XXI XVIII XXII XVII XVII XVII XVII X XIII VIII VIII VII we computed the radial velocity corresponding to the predicted line VII XVI XIV Fe O O Si Si Mg Fe Mg Mg Fe Fe Fe Fe Ne Fe Fe O N S Fe Fe Fe Fe Fe centroid, for different possible inclinations of the flaring loop (iden- 200 HR 9024 tified by the ϕ and θ angles defined in Fig. 3a). MEG 150 The best agreement between observed and predicted veloci- ) ties (Fig. 3b–f) is obtained assuming a loop observed from above –1 100 (ϕ = 0° and θ = 90°; see Methods), confirming previous indepen- 50 dent results25. The agreement for the Si xiv and Mg xii lines is 0 striking (Fig. 3c–f). The observed S xvi blueshift, associated with HR 9024 60 chromospheric plasma upflows, is of the same order but even more HEG Flux (counts bin extended in time than expected. The agreement obtained for flaring 40 plasma velocities is an important validation of the standard flare 36 20 model for flare energies up to 10 erg. 0 Interestingly, we found that the coolest line inspected, O viii 0 5 10 15 20 25 Lyα, which forms at about 3 MK, is strongly blueshifted (with Wavelength (Å) v = −90 ± 30 km s−1) in the post-flare phase, whereas no shift is observed during the flare. The low rotational velocity of HR 9024 b 2.0 Bin = 3.0 ks (vsini ≈ 20 km s−1)23 excludes the possibility that this motion is due ) to structures fixed on the stellar surface. –1 1.5 We identify this motion as the signature of a CME (Fig. 3g): it involves only cool plasma, it occurs after a strong flare located 1.0 near the stellar disk centre, and the observed velocity is within the range of solar CME velocities3 (that is, 20–3,000 km s−1). Solar flares 0.5 are sometimes also followed by the formation of expanding giant 26 1 Count rate (counts s Count rate arches . However, this expansion is very slow (around 1–10 km s− ) Flare Post-flare and possibly only apparent because of the sequential brightening 0 27 0 20 40 60 80 100 of different loops , and hence cannot be related to Doppler shifts. Time (ks) Although the extrapolation of the magnetic configuration of the Sun to more active stars is not straightforward, a CME remains the Fig. 1 | Observed X-ray spectra and light curve of HR 9024. a, X-ray most logical and (at present) only explanation. spectra collected with the Medium Energy Grating (MEG) and High Hypothesizing that the CME moves exactly along the line of Energy Grating (HEG) during the 98-ks-long Chandra observation, with sight, the distance travelled by the CME in the post-flare phase is the strongest emission lines labelled. MEG and HEG bin sizes are 5 mÅ about 0.8R⋆. Most probably the CME started its motion at the same and 2.5 mÅ. b, X-ray light curve registered during the Chandra observation, time as the flare onset (solar flare and CME onsets differ by most obtained from the +1 and −1 diffraction order spectra of HEG and MEG. 1 ks, and the O viii profile during the flare appears to be broad, with some blue-shifted excess; see Fig.
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