Coronal mass ejections (CMEs) are explosive events that occur basically daily
on the Sun. It is thought that these events play a crucial role in the angular
momentum and mass loss of late-type stars, and also shape the environment in
which planets form and live. Stellar CMEs can be detected in optical spectra in
the Balmer lines, especially in Halpha, as blue-shifted extra
emission/absorption. To increase the detection probability one can monitor
young open clusters, in which the stars are due to their youth still rapid
rotators, and thus magnetically active and likely to exhibit a large number of
CMEs. Using ESO facilities and the Nordic Optical Telescope we have obtained
time series of multi-object spectroscopic observations of late-type stars in
six open clusters with ages ranging from 15 Myrs to 300 Myrs. Additionally, we
have studied archival data of numerous active stars. These observations will
allow us to obtain information on the occurrence rate of CMEs in late-type
stars with different ages and spectral types. Here we report on the preliminary
outcome of our studies.Comment: 6 pages, submitted to the proceedings of IAU Symposium 328 'Living
  Around Active Stars

Korhonen, Heidi

Kovacs, Orsolya Eszter

Leitzinger, Martin

Odert, Petra

Vida, Krisztian

English

arXiv

AbstractCoronal mass ejections (CMEs) are explosive events that occur basically daily on the Sun. It is thought that these events play a crucial role in the angular momentum and mass loss of late-type stars, and also shape the environment in which planets form and live. Stellar CMEs can be detected in optical spectra in the Balmer lines, especially in Hα, as blue-shifted extra emission/absorption. To increase the detection probability one can monitor young open clusters, in which the stars are due to their youth still rapid rotators, and thus magnetically active and likely to exhibit a large number of CMEs. Using ESO facilities and the Nordic Optical Telescope we have obtained time series of multi-object spectroscopic observations of late-type stars in six open clusters with ages ranging from 15 Myrs to 300 Myrs. Additionally, we have studied archival data of numerous active stars. These observations will allow us to obtain information on the occurrence rate of CMEs in late-type stars with different ages and spectral types. Here we report on the preliminary outcome of our studies.</jats:p

Heidi Korhonen

Krisztián Vida

Martin Leitzinger

Petra Odert

Orsolya Eszter Kovács

Crossref

Proceedings of the International Astronomical Union

Hunting for Stellar Coronal Mass Ejections

Vida, Krisztián

Kovács, Orsolya Eszter

Repository of the Academy's Library

arXiv:1612.06643v1  [astro-ph.SR]  20 Dec 2016Title of your IAU SymposiumProceedings IAU Symposium No. xxx, 2015A.C. Editor, B.D. Editor & C.E. Editor, eds.c© 2015 International Astronomical UnionDOI: 00.0000/X000000000000000XHunting for Stellar Coronal Mass EjectionsHeidi Korhonen1, Krisztia´n Vida2, Martin Leitzinger3, PetraOdert4,3, and Orsolya Eszter Kova´cs2,51Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane MariesVej 30, DK-2100 Copenhagen Ø, Denmarke-mail: heidi.korhonen@nbi.ku.dk2Konkoly Observatory, MTA CSFK, Konkoly Thege M. u´t 15-17, 1121, Budapest, Hungary3University of Graz, Institute of Physics, Department for Geophysics, Astrophysics andMeteorology, NAWI Graz, Universita¨tsplatz 5, 8010, Graz, Austria4 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6,8042 Graz, Austria5 Department of Astronomy, Eo¨tvo¨s Lora´nd University, Pa´zma´ny Pe´ter se´ta´ny 1/A, 1117,Budapest, HungaryAbstract. Coronal mass ejections (CMEs) are explosive events that occur basically daily on theSun. It is thought that these events play a crucial role in the angular momentum and mass lossof late-type stars, and also shape the environment in which planets form and live. Stellar CMEscan be detected in optical spectra in the Balmer lines, especially in Hα, as blue-shifted extraemission/absorption. To increase the detection probability one can monitor young open clusters,in which the stars are due to their youth still rapid rotators, and thus magnetically active andlikely to exhibit a large number of CMEs. Using ESO facilities and the Nordic Optical Telescopewe have obtained time series of multi-object spectroscopic observations of late-type stars insix open clusters with ages ranging from 15 Myrs to 300 Myrs. Additionally, we have studiedarchival data of numerous active stars. These observations will allow us to obtain informationon the occurrence rate of CMEs in late-type stars with different ages and spectral types. Herewe report on the preliminary outcome of our studies.Keywords. stars: activity, corona, late-type1. IntroductionDue to the strong radiation coming from hot stars, the cooler stars having later spectraltypes (F–M) are thought to be more benign hosts for habitable planets. Still, also thecool stars show strong flaring activity and probably numerous coronal mass ejections(CMEs). Magnetic activity of the planet host star, especially the strong magnetic activityexhibited by many rapidly rotating young stars, has profound effects on the planets andtheir habitability; strong flares and CMEs are thought to even be able to strip a close-byplanet of its atmosphere (e.g., Lammer et al. 2007). Additionally, these processes are alsorelevant for stellar evolution, as stars lose mass and angular momentum via stellar windsand CMEs.In the Sun CMEs are seen regularly, the average daily occurrence rate during theactivity minimum being 0.5 and during the maximum 6, with one solar CME containingon average 1011 kg of material (see, e.g., Gopalswamy et al. 2009). On the Sun, flaresand CMEs are often closely correlated, their association increasing with flare energy (e.g.Yashiro et al. 2006). A few studies have therefore aimed to estimate stellar CME ratesfrom flares, and relations between flare and CME parameters known from the Sun (Aarnioet al. 2012; Drake et al. 2013; Leitzinger et al. 2014; Osten & Wolk 2015). Many studies12 Korhonen et al.have shown that young stars have very energetic Ultra Violet flares (see, e.g., Osten &Wolk 2009), but very few systematic studies of the stellar coronal mass ejections exists.There is a handful of detections of stellar CMEs, but they are very rare (e.g., Houdebineet al. 1990; Fuhrmeister & Schmitt 2004; Gu¨nther & Emerson 1997; Leitzinger et al.2011). The observations indicate velocities ranging from about twice the value for veryfast solar CMEs (5800km/s seen in AD Leo; Houdebine et al. 1990) to slightly slowerthan the values seen in the slowest solar CMEs (84 km/s also seen in AD Leo; Leitzingeret al. 2011). The slow velocities observed at times can be explained by the CME beingseen in projection.For their large impact on the environments of planets, and also on the stellar angularmomentum evolution, it is crucial to decipher the CME occurrence rate with stellar ageand spectral type.2. Detecting CMEs using optical spectraThe Doppler signal of a mass ejection is always seen in projection. It is strongest whenplasma is moving towards the observer and weaker for other directions. Doppler shifts andline asymmetries with enhanced blue wings of stellar emission lines can be interpretedas plasma ejected from the star. The material released in CMEs is mainly hydrogen asthe CME core is often built by a filament, and they can be detected in the Hα line.The blue enhancement caused by a CME is an absorption feature when seen againstthe stellar disc, and an emission feature when seen outside the disc. The time that theejecta can be seen against the stellar disc is quite short for many projections, meaningthat emission features would be more common than absorption features. On the otherhand, the material would need to be quite dense to be seen in emission.The velocities of CMEs span from few hundreds of km/s to some thousands of km/s.The exact value naturally depends on the energetics of the event itself, but also onthe projection effects. The maximum velocity is only achieved if the material is ejecteddirectly towards the observer. These high speeds also mean that detecting CMEs doesnot require very high spectral resolution.3. Results on single starsWe have gone through the archival data of more than 40 single active stars observedwith Narval and ESPaDOnS, and also obtained new observations for a handful of targets.Even with a careful analysis no CMEs were found, except on V374 Peg.V374 Peg is a young, active M4 dwarf which has been extensively studied (e.g., Donatiet al. 2006; Morin et al. 2008). We investigated the long-term photometric and spec-troscopic variability of this star, and also studied archival ESPaDOnS data of the Hαline-profiles (Vida et al. 2016). The spectra obtained in 2005 showed a CME event: onereal CME that was preceded by two failed events (see Fig. 1). We estimated that theminimum CME mass for the real event was 1016g. In addition, we predicted the CMErate using the formalism by Leitzinger et al. (2014), and found out that the star shouldhave 15-60 CMEs per day. We detected only one in 10 hours of observations – clearlyless than predicted.4. Monitoring of open clustersAn efficient way for getting a better handle on the CME rate in young stars is tomonitor open clusters with different ages using multi-object spectroscopy. In this wayIAUS328. Hunting for Stellar Coronal Mass Ejections 3Figure 1. Dynamic spectrum of Hα line of V374 Peg. The arrows show the complex CMEevent with two failed eruptions and finally one real CME. (from Vida et al. 2016)Table 1. Overview of the clusters observed for this project. Table gives the name of the cluster,age, number of observed targets, instrument used, spectral resolution, typical signal-to-noiseratio (S/N), and the observing time allocated to the cluster.Cluster Age [Myrs] Targets Instrument Resolution Cadence S/N Obs timeBlanco 1 65 28 VIMOS, ESO 2500 3 min 40 11hIC 2391 40 7 EFOSC, ESO 2500 10 min 40 1nNGC 2516 110 8 EFOSC, ESO 2500 6 min 40 1nNGC 3532 300 9 EFOSC, ESO 2500 6 min 40 1nh Per 15 17 ALFOSC, NOT 2500 10 min 30 3nIC 348 45 29 ALFOSC, NOT 2500 10 min 30 3nmany cool stars of known age can be simultaneously observed, increasing the chances ofdetecting CMEs.Our first attempt was done at ESO VLT using VIMOS spectrograph of a young opencluster Blanco 1 (PI Leitzinger). The results from these observations have been published,but unfortunately no CMEs were detected (Leitzinger et al. 2014). Still, we estimated anupper limit of four CMEs per day per star, and that we should have detected at least oneCME per star with a mass of 1−15×1016g. After the Blanco 1 campaign we have observedalso other young open clusters. IC 2391, NGC 2516, and NGC 3532 were observed usingEFOSC2 at ESO’s New Technology Telescope (PI Leitzinger). Three further clustersh Per, IC 348, and NGC 1662 were targeted with the ALFOSC instrument on the NordicTelescope (PI Korhonen). Unfortunately, many of the NOT observing campaigns werehampered by bad weather, and therefore we decided not to observe the oldest cluster,NGC 1662, which anyway only had few observable cluster members. In total the ALFOSCobservations consist of some 2.5 nights worth of good quality data on h Per and IC 348(out of 9 allocated nights). More details on the observed clusters is given in Table 1. Inthis table the observing time is the total time allocated for that cluster.4 Korhonen et al.Figure 2. Examples of Hα line profiles of a star in IC 348.From the EFOSC observations of IC 2391, NGC 2516, and HGC 3532 we only detectedflares. No CME signatures were seen. Also the h Per observations obtained with ALFOSConly show some flare events, but IC 348 looks more promising. Our targets in this clusterinclude stars ranging spectral types G–M. Most of the targets with earlier spectral typeshave Hα in absorption, and the later ones in emission.An example of Hα line-profiles for star number 9 in our multi-object spectroscopy ofIC 348 is given in Fig. 2. This star is of spectral type late K – early M. As can be seen, theline-profile shows clear variability. Also, note that the persistent extra emission bump inthe blue part of the profile is not from a CME event, but is due to fringing in the CCD atthese wavelengths. The dynamic Hα spectra for the same target in November–December2015 are shown in Fig. 3. Both the dynamical spectra with original line-profiles (left side)and with the average line-profile subtracted (right side) are shown. Clear variability inthe Hα region is seen, and several events with blue shifted extra emission are discovered.More thorough analysis of these data are needed for firmly confirming the nature of theevents, and calculating their occurrence frequency.5. Conclusions and future prospectsWe are detecting stellar CMEs, but less than expected. The detection is limited bothby the intrinsic properties of the star and observing constraints. The intrinsic propertiesinclude the CME occurrence rate, which depends on the activity level/age of the star,and the typical CME parameters (velocity, mass, etc.). The observing constraints are onone hand related to the timing of the observations (time coverage and cadence), and onthe other to the spectral properties (resolution and S/N). One has to also keep in mindthat a fraction of the CMEs are also lost due to the projection effects.Typical S/N of our observations is around 30–40, which is not high for studying smallfeatures in the spectral line-profiles. With these observations we can detect only themassive CMEs, comparable to the most massive solar events. These events would be inthe range of 1016 − 1017g. If we increase the S/N then we should also see less massiveCMEs. To test this we have been granted UVES bad weather programme on Xi BooA and B, a 200Myrs old G and K dwarf system. The observations have a resolution of40000, S/N 600, and time resolution of less than a minute. With these observations wewill be able to test the detectability of smaller CMEs.IAUS328. Hunting for Stellar Coronal Mass Ejections 5Figure 3. Dynamic spectrum of Hα line of a star in IC 348 (the same star as in Fig. 2).The observations are from November–December 2015, and the title of the plot gives the exactobserving date. The crosses show the times from which the observations are (in the HJD minusHJD of that date at noon). The four leftmost plots show the dynamic spectra from the originalprofiles, and the four rightmost plots the residual dynamic spectra after the average profile hasbeen subtracted.The so-far detected stellar CMEs have all been on M dwarfs, and the ejecta werefound to be in emission. Possibly the CMEs on K and G stars are not dense enough to bedetected in emission, and we are not quick enough to detect them in absorption. Anotherintriguing question is, whether the very active stars really have numerous CMEs. Maybewe are not detecting many CMEs, because there actually are only few of them? It hasbeen hypothesised that the strong magnetic fields on young stars could actually preventa filament from erupting in analogy to solar failed eruptions (see Drake et al. 2016).Acknowledgments H.K. acknowledges the support from the Fonden Dr. N.P. Wieth-Knudsens Observatorium for the travel grant that made it possible for her to attendthe IAU Symposium 328. K.V. was supported by the Ja´nos Bolyai Research Scholarshipof the Hungarian Academy of Sciences. M.L. and P.O. acknowledge the support fromthe FWF project P22950-N16. P.O. acknowledges also support from the Austrian Sci-ence Fund (FWF): P27256-N27. The authors acknowledge support from the HungarianResearch Grants OTKA K-109276, OTKA K-113117, the Lendu¨let-2009 and Lendu¨let-2012 Program (LP2012-31) of the Hungarian Academy of Sciences, and the ESA PECSContract No. 4000110889/14/NL/NDe.ReferencesAarnio, A. N., Matt, S. P., & Stassun, K. G. 2012, ApJ, 760, 9Donati, J.-F., Forveille, T., Collier Cameron, A., et al. 2006, Science, 311, 633Drake, J. J., Cohen, O., Yashiro, S., & Gopalswamy, N. 2013, ApJ, 764, 170Drake, J. J., Cohen, O., Garraffo, C., & Kashyap, V. 2016, arXiv:1610.05185Fuhrmeister B., & Schmitt J. H. M. M. 2004, A&A, 420, 1079Gopalswamy, N., Akiyama, S., Yashiro, S., & Ma¨kela¨, P. 2010, in: S.S. Hasan & R.J. Rutten(eds) Astrophysics and Space Science Proceedings (Springer Berlin Heidelberg) p. 289Gu¨nther, E. W., & Emerson, J. P. 1997, A&A, 321, 803;Houdebine, E. R., Foing, B. H., & Rodono, M. 1990, A&A, 238, 249;6 Korhonen et al.Lammer, H., Lichtenegger, H. I. M., Kulikov, Y. N., et al. 2007, Astrobiology, p. 185Leitzinger, M., Odert, P., Ribas, I., et al. 2011, A&A, 536, A62;Leitzinger, M., Odert, P., Greimel, R., et al. 2014, MNRAS, 443, 898Morin, J., Donati, J.-F., Forveille, T., et al. 2008, MNRAS, 384, 77Osten, R. A., & Wolk, S. J. 2009, ApJ, 691, 1128Osten, R. A., & Wolk, S. J. 2015, ApJ, 809, 79Vida, K., Kriskovics, L., Ola´h, K., et al. 2016, A&A, 590, A11Yashiro, S., Akiyama, S., Gopalswamy, N., & Howard, R. A. 2006, ApJ (Letters), 650, L143

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