Conference Proceeding© International Astronomical Union 2013In our own solar system, the necessity of understanding space weather is readily evident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting activity levels several orders of magnitude lower than young, solar-type stars. In protoplanetary systems, stellar magnetic phenomena observed are analogous to the solar case, but dramatically enhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejections (CMEs) could play a significant role in the evolution of protoplanets, they could also affect the evolution of the central star itself. To assess the consequences of prominence eruption/CMEs, we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how frequently stellar CMEs may occur and their attendant mass and angular momentum loss rates. We will demonstrate the necessary conditions under which CMEs could slow stellar rotation. © 2013 International Astronomical Union

Aarnio, Alicia N.

Matt, Sean P.

Stassun, Keivan G.

Proceedings of the International Astronomical Union

Alicia N. Aarnio

Keivan G. Stassun

Sean P. Matt

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Coronal Mass Ejections and Angular Momentum Loss in Young Stars

© International Astronomical Union 2013In our own solar system, the necessity of understanding space weather is readily evident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting activity levels several orders of magnitude lower than young, solar-type stars. In protoplanetary systems, stellar magnetic phenomena observed are analogous to the solar case, but dramatically enhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejections (CMEs) could play a significant role in the evolution of protoplanets, they could also affect the evolution of the central star itself. To assess the consequences of prominence eruption/CMEs, we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how frequently stellar CMEs may occur and their attendant mass and angular momentum loss rates. We will demonstrate the necessary conditions under which CMEs could slow stellar rotation. © 2013 International Astronomical Union

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arXiv:1403.0006v1  [astro-ph.SR]  28 Feb 2014Nature of prominences and their role in space weatherProceedings IAU Symposium No. 300, 2013A.C. Editor, B.D. Editor & C.E. Editor, eds.c© 2013 International Astronomical UnionDOI: 00.0000/X000000000000000XCoronal Mass Ejections and AngularMomentum Loss in Young StarsAlicia N. Aarnio1 and Keivan G. Stassun 2,3 and Sean P. Matt41Dept. of Astronomy, University of Michigan, Ann Arbor, MI, 48109, USAemail: aarnio@umich.edu2Dept. of Physics & Astronomy, Vanderbilt University, Nashville, TN, 37235, USA3Dept. of Physics, Fisk University, Nashville, TN, 37208 USA4School of Physics, University of Exeter,Stocker Road, Exeter, EX4 4QL, UKAbstract. In our own solar system, the necessity of understanding space weather is readilyevident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting ac-tivity levels several orders of magnitude lower than young, solar-type stars. In protoplanetarysystems, stellar magnetic phenomena observed are analogous to the solar case, but dramaticallyenhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejec-tions (CMEs) could play a significant role in the evolution of protoplanets, they could also affectthe evolution of the central star itself. To assess the consequences of prominence eruption/CMEs,we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how fre-quently stellar CMEs may occur and their attendant mass and angular momentum loss rates.We will demonstrate the necessary conditions under which CMEs could slow stellar rotation.Keywords. Sun: flares, Sun: coronal mass ejections, stars: activity, stars: flare, stars: evolution,stars: rotation, stars: planetary systems: protoplanetary disks1. IntroductionOn young stars, we observe flares hundreds to ten thousand times more energetic andfrequent than solar flares. Along with energy scales greater by orders of magnitude, wealso observe physical scales far greater than in the solar case: while solar prominencessoar around 1 R⊙ above the solar surface and CMEs launch from similar radii in theSun’s atmosphere, magnetic structures on T Tauri Stars (TTS, young solar analogs)–post-flare loops and prominences–can extend tens of stellar radii from the star’s surface.The discovery of such large magnetic structures arose from solar-stellar analogy, applyingsolar flare models to the X-ray light curve data from young stars (e.g., Reale et al. 1998).In characterizing the solar-stellar connection, overwhelming evidence has been foundin support of the idea that the fundamental physics of magnetic reconnection is the same,despite differences in stellar parameters (e.g., mass, radius, B, age). As such, we approachanalysis of young stars’ flares and CMEs under this supposition, and aim to assess howthe physical properties of these events–and their frequency–may scale accordingly withstellar parameters. Ultimately, we seek to understand the consequences of exoplanetaryspace weather on protostellar systems and their forming planets.2. Estimating Angular Momentum Loss via Stellar CMEsThe inference of magnetic loops many stellar radii in extent (Favata et al. 2005; here-after F05) inspired three questions: one, are these loops interacting with circumstellar12 Alicia Aarnio, Keivan Stassun, and Sean Mattdisks? In Aarnio et al. (2010), we did not find evidence for this. Second, if there is nota star-disk link, how do the loops remain stable for the multiple rotation periods overwhich the X-ray flares are observed to decay? We showed in Aarnio et al. (2012) thatwhen modeled as hot prominences, the addition of a scaled-up wind consistent with TTSobservations provided sufficient support for the loops to be stable. Finally, here (and inAarnio, Matt, & Stassun, 2012; hereafter AA12) we address the third question of whathappens when stability is lost: at many stellar radii, is the specific angular momentumshed significant enough to slow stellar rotation?In order to estimate the effects of eruptive prominences and stellar CMEs on therotation of young stars, we must procure two ingredients: the mass lost via these events,and their frequency of occurrence. Despite ongoing and historical efforts to observe stellarCMEs, we lack definitive detections and thus frequency distributions. It is known thatat times, magnetic reconnection on the Sun will produce both a flare and an associatedCME; for stars, the flare is the observable quantity, and so we characterize stellar CMEfrequency by using stellar flare frequency as a proxy.In Fig. 1, we show our solar flare energy/CME mass relationship (Aarnio et al. 2011,hereafter AA11) extrapolated up to the energies of young stellar flares. We calculateloop masses for the 32 “superflaring” stars from the Chandra Orion Ultradeep Project(Getman et al. 2005) from the parameters reported by F05. Interestingly, these loopmasses are close in parameter space to the extrapolated solar relationship. This is perhapsunsurprising, as the plasma confined in a post-flare loop has properties which relate tothe energy of the flare. In AA11, we found that for associated flares and CMEs, that is tosay, flares and CMEs which likely originated from a shared magnetic reconnection event,the CME mass and flare energy were related. As such, the post-flare loop mass and massof an associated CME should then also be related.From Shibata & Yokoyama (2002), we can estimate the total energy released by a flareis related to the total magnetic energy in a flare loop:Emag =B2L38pi. (2.1)If we substitute the loop volume expressed in terms of mass (i.e., L3 ∼ V = mloop/ρ), wefind a relationship between the total flare energy and the mass confined in the magneticloop (dashed, parallel lines in Fig. 1). Here, to be consistent with the analysis of F05, weFigure 1. CME mass/flare energy re-lationship of AA11 shown with TTSpost-flare loop masses (black diamonds)derived in AA12 plotted as a function ofthe flare’s energy. Point size denotes themass of the star on which the flare wasobserved. Black dotted lines show pre-dicted post-flare stellar loop masses (Eqn.2.1 and discussion in text) for a range ofobserved densities from 1010-1012 cm−3and assuming a confining field strengthof 50G. Gray, solid-lined boxes denotethe range of observed X-ray flare energies(Maggio et al. 2000, Collier Cameron etal. 1988) and cool Hα prominence massestimates (Collier Cameron & Robinson,1989a,b) for AB Dor and Speedy Mic,20-50Myr K dwarfs.CMEs on Young Stars 3Figure 2. Flare frequencies forthe Sun, M dwarfs (Hilton et al.2011), TTS in the ONC (AlbaceteColombo et al. 2007), and active,main sequence G stars from Kepler(Maehara et al. 2012). Interestingly,the active G stars are almost indis-tinguishable from the TTS. Note thesolar and TTS frequencies are de-rived from X-ray flare data, whilethe M dwarf and G stars are opti-cal flare frequencies.assume an Euclidian loop filling but do note that recent solar X-ray flare imaging hasindicated that a fractal scaling of V(L)∝L2.4 is likely a more accurate characterization(e.g., Aschwanden, Stern, & Gu¨del, 2008).It is remarkable that the post-flare loop masses even lie near the extrapolated so-lar CME mass/flare energy relationship, several orders of magnitude away in parameterspace. In Fig. 1, we have also shown representative ranges of X-ray flare energy andprominence mass for two K dwarfs intermediate in age to the TTS sample and the Sun;with an eruptive prominence thought to be the core of a CME, these mass ranges likelyrepresent lower limits on the range of CME masses on these stars. In the following calcu-lations, to represent a fiducial TTS case, we will simply extrapolate the solar relationshipto generate a stellar CME mass distribution.In Fig. 2, we show the frequency distributions for flares observed on the Sun, TTS,M dwarfs, and active, main sequence G stars. For this work, we use the TTS frequencydistribution. Clearly, not all CMEs are flare-associated, nor are all flares CME-associated;AA11 found, however, that the association fraction increases with increasing flare energy,so for young stars for which we observe flares several orders of magnitude more energeticthan in the solar case, we simply assume this association fraction to be of order unity.2.1. Angular momentum lossIn AA12, we extrapolate the solar CME mass/flare energy relationship (Fig. 1) to TTSflare energies and frequencies (Fig. 2) and construct a CME frequency distribution as afunction of CME mass. Given the observational completeness limits on the flare distri-butions that went into the CME distribution, we derive lower and upper limits on themass loss rate by empirical (integrating the distribution) and analytical (integrating a fitto the distribution) means. The range of mass loss rates we estimate for the TTS case is10−12-10−9 M⊙ yr−1.To assess the torque applied against stellar rotation by these CMEs, we apply stellarwind models with mass loss rates set as determined above. Given the episodic natureof CMEs, we included an efficiency parameter to account for the fact that steady-statewinds are more efficient at removing angular momentum than “clumpy” winds (cf. AA12and references therein). We adopt a dipolar field with strength 600G, consistent withobservations of TTS fields, and allow the stellar radius to contract as stellar evolutionmodels predict.In a protostellar system, multiple torques act simultaneously to spin up and spin downthe star. In this analysis, we compared spin up due to contraction and spin down due tomass loss from stellar CMEs to see if, at any point in the pre-main sequence our fiducialTTS could have its rotation slowed due to CMEs. Comparing parameters of efficiency4 Alicia Aarnio, Keivan Stassun, and Sean Mattand the range of mass loss rates we calculated, it became clear that only towards the endof the pre-main sequence phase (ages &6 Myr) could a very efficient, high CME massloss rate begin to counteract spin up from contraction. We have left out factors such asspin up from accretion and mass loss via stellar wind; Matt & Pudritz (2005) explorethese two torques in depth and the necessary conditions for an accretion powered stellarwind to slow stellar rotation.3. DiscussionWe have shown that for young, solar-type stars, spin down due to CMEs might playa significant role in stellar rotation evolution after the star has ceased accreting. OurFigs. 1 and 2 illustrate a critical selection effect in performing this kind of calculation:we only have data for the most active young stars, or the star conveniently located at 1AU. There is a dearth of data for older, less active stars, and we suggest that filling inthe gaps in flare X-ray energy could trace age evolution in these parameter spaces. Theaddition of data from the ∼20-50 Myr old K dwarfs in Fig. 1 hints at this, but more dataare needed to conclusively show age dependence. In both figures, we have taken care tospecify the masses of the stars involved: how would evolution with stellar age look inthese parameter spaces as a function of stellar mass? While the fundamental physics arethe same, the scaling could change, and the ramifications certainly would. For low-massstars in particular, high activity levels are observed for longer fractions of the stars’ lives;this could have grave implications for exoplanets as these stars’ habitable zones could bewithin range of extreme exo-weather.AcknowledgementsA. N. A. thanks K. Shibata for helpful discussion regarding stellar post-flare loops.ReferencesAarnio, A. N., Matt, S. P., & Stassun, K. G. 2012, ApJ, 760, 9Aarnio, A. N., Llama, J., Jardine, M., & Gregory, S. G. 2012, MNRAS, 421, 1797Aarnio, A. N., Stassun, K. G., Hughes, W. J., & McGregor, S. L. 2011, Solar Phys., 268, 195Aarnio, A. N., Stassun, K. G., & Matt, S. P. 2010, ApJ, 717, 93Albacete Colombo, J. F., Flaccomio, E., Micela, G., Sciortino, S., & Damiani, F. 2007, A&A,464, 211Aschwanden, M. J., Stern, R. A. & Gu¨del, M. 2008, ApJ 672, 659Collier Cameron, A. & Robinson, R. D. 1989, MNRAS, 238, 657Collier Cameron, A. & Robinson, R. D. 1989, MNRAS, 236, 57Collier Cameron, A., Bedford, D. K., Rucinski, S. M., Vilhu, O., & White, N. E. 1988, MNRAS,231, 131Dunstone, N. J., Collier Cameron, A., Barnes, J. R., & Jardine, M. 2006, MNRAS, 373, 1308Favata, F., Flaccomio, E., Reale, F., Micela, G., Sciortino, S., Shang, H., Stassun, K. G., &Feigelson, E. D. 2005, ApJS, 160, 469Getman, K. V. et al. 2005, ApJS, 160, 319Hilton, E. J., Hawley, S. L., Kowalski, A. F. & Holtzman, J. 2011, ASPC 16th CambridgeWorkshop on Cool Stars, Stellar Systems, and the Sun, 448, 197Maehara, H., Shibayama, T., Notsu, S., Notsu, Y., Nagao, T., Kusaba, S., Honda, S., Nogami,D., & Shibata, K. 2012, Nature, 485, 478Maggio, A., Pallavicini, R., Reale, F., & Tagliaferri, G. 2000, A&A, 356, 627Matt, S. P. & Pudritz, R. E. 2005, ApJL, 632, L135Reale, F., & Micela, G. 1998, A&A, 334, 1028Shibata, K., & Yokoyama, T. 2002, ApJ, 577, 422

Coronal mass ejections and angular momentum loss in young stars

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