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Testing Models of Coronal Heating, X-Ray Emission, and Winds . . . . . . From Classical T Tauri Stars Steven R. Cranmer Harvard-Smithsonian Center for Astrophysics Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Testing Models of Coronal Heating, X-Ray Emission, and Winds . . . Outline: 1. Brief overview of T Tauri star & solar activity 2. Impact-driven turbulence: a plausible chain of events? 3. Testing the hypothesis: • Accretion shocks • Coronal loops • Stellar winds . . . From Classical T Tauri Stars Steven R. Cranmer Harvard-Smithsonian Center for Astrophysics Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 T Tauri stars: complex geometry & activity • T Tauri stars show signatures of disk accretion, “magnetospheric accretion streams,” an X-ray corona, and polar (?) outflows from some combination of star & disk. • Nearly every observational diagnostic varies in time, sometimes with stellar rotation, but often more irregularly. (Rucinski et al. 2008) (Romanova et al. 2007) (Matt & Pudritz 2005, 2008) Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Context from the Sun’s corona & wind • Photospheric flux tubes are shaken by an observed spectrum of convective motions. • Alfvén waves propagate along the field, and partly reflect back down (non-WKB). • Nonlinear couplings allow MHD turbulence to occur: cascade produces dissipation. Closed field lines experience strong turbulent heating Open field lines see weaker turbulent heating & “wave pressure” acceleration Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Ansatz: accretion stream impacts make waves • The impact of inhomogeneous “clumps” on the stellar surface can generate MHD waves that propagate out horizontally and enhance existing surface turbulence. • Scheurwater & Kuijpers (1988) computed the fraction of a blob’s kinetic energy that is released in the form of far-field wave energy. • Cranmer (2008, 2009) estimated wave power emitted by a steady stream of blobs. similar to solar flare generated Moreton/EUV waves? Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Testing the ansatz… with real stars • Classical T Tauri stars in the Taurus-Auriga star forming region are well-observed: AA Tau BP Tau CY Tau DE Tau DF Tau DK Tau DN Tau DO Tau DS Tau GG Tau GI Tau GM Aur HN Tau UY Aur • Cranmer (2009) used two independent sets of M*, L*, R*, ages, & accretion rates, from Hartigan et al. (1995) and Hartmann et al. (1998). • Accretion spot “filling factors” δ taken from Calvet & Gullbring (1998) measurements of Balmer & Paschen continua → accretion energy fluxes & areas. • Surface magnetic field strengths B* for 10/14 stars taken from Johns-Krull (2007) measurements of Ti-line Zeeman broadening; other 4 from empirical <B* / Bequi>. Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Start with the simplest geometry • Königl (1991) showed how inner-disk edge can scale with stellar parameters: • Measured filling factor δ gives router, as well as size of blobs at stellar surface. • Assume ballistic (free-fall) velocity to compute ram pressure; this gives ρshock/ρphoto. The streams are inhomogeneous: L. Hartmann, lecture notes • Need to assume “contrast:” ρblob / <ρ> ≈ 3. • This allows us to compute: N (number of flux tubes impacting the star) Δt (inter-blob intermittency time) Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Accretion shock models • Temporarily ignoring the existence of “blobs” allows a straightforward 1D calculation of time-steady shock conditions & the post-shock cooling zone. • Typical post-shock conditions: log Te ~ 5–6, log ne ~ 13.5–15 • Cranmer (2009) synthesized X-ray luminosities: ROSAT (PSPC), XMM (EPIC-pn). Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Results: accretion shock X-rays • Blah… Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Coronal loops: MHD turbulent heating • Cranmer (2009) modeled equatorial zones of T Tauri stars as a collection of closed loops, energized by “footpoint shaking” (via blob-impact surface turbulence). • Coronal loops are always in motion, with waves & bulk flows propagating back and forth along the field lines. • Traditional Kolmogorov (1941) dissipation must be modified because counter-propagating Alfvén waves aren’t simple “eddies.” n = 0 (Kolmogorov), 3/2 (Gomez), 5/3 (Kraichnan), 2 (van Ballegooijen), f (VA/veddy) (Rappazzo) • T, ρ along loops computed via Martens (2010) scaling laws: log Tmax ~ 6.6–7. Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Results: coronal loop X-rays Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Stellar winds from polar regions • The Scheurwater & Kuijpers (1988) wave generation mechanism allows us to compute the Alfvén wave velocity amplitude on the “polar cap” photosphere . . . • Waves propagate up the flux tubes & photosph. sound speed accelerate the flow via “wave pressure.” • If densities are low, waves cascade and dissipate, giving rise to T > 106 K. • If densities are high, radiative cooling is too strong to allow coronal heating. • Cranmer (2009) used the “cold” wavedriven wind theory of Holzer et al. (1983) to solve for stellar mass loss rates. v┴ from accretion v┴ from interior impacts Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds convection ( ) 1 solar mass model S. R. Cranmer, July 14, 2010 Results: wind mass loss rates O O II 6300 6300 blueshifts blueshifts (yellow) (yellow) (Hartigan (Hartigan et et al. al. 1995) 1995) Model Model predictions predictions Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010 Conclusions • Insights from solar MHD have led to models that demonstrate how the accretion energy can contribute significantly to driving T Tauri outflows & X-ray emission. . • Is M enough to solve the T Tauri angular momentum problem? • Why do (non-accreting) weak-lined T Tauri stars show stronger X-rays? wind • More realistic models must include: (1) more complex magnetic fields, and (2) the effects of rapid rotation on convective dynamo “activity.” Cohen et al. (2010) Brown et al. (2010) For more information: http://www.cfa.harvard.edu/~scranmer/ Testing Models of CTTS Coronal Heating, X-Ray Emission, & Winds S. R. Cranmer, July 14, 2010