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Physical processes in cool-star activity

Physical processes in cool-star activity

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Physical processes in cool-star activity

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  1. CSW12: Focus on the future of cool-star astrophysics: Physical processes in cool-star activity Karel Schrijver “I propose to adopt such rules as will ensure the testability of scientific statements; which is to say, their falsifiability.” Karl Popper (1902-1994) “Everything you know is wrong.” Fred Walter, in 2001 CSW12, Boulder 2 August 2001

  2. Many unknowns Status of our “understanding” Empirical understanding Physical understanding • Internal structure • Flux dispersal • Spectral irradiance Numerical understanding • (p,f) mode generation and damping • Flux “tube” properties and evolution • Field geometry & dynamics • Dynamo(s) and cycles • Instabilities & plasma physics • Compressible convection • Dynamo(s) and cycles • Stellar wind & rotational braking • Radiative coupling • Atmospheric heating • Near-surface field topology • Flux life cycle (appearance to disappearance) • Large-scale flows • “Weak-field” origin and role • Starspots Inadequate or no understanding • Dynamic chemistry CSW12, Boulder

  3. To simplify or not to simplify? • Fundamental difficulties: non-linear, non-local, non-stationary system with disjoint interfaces in a 3D geometry Non Non b < 1 t < 1 Non t > 1 b > 1 CSW12, Boulder

  4. What do we “understand”? • Physical: “Do we understand all in detail?” • Empirical, pragmatic: “Can we test (i.e., attempt to falsify) a composite model based on sets of known phenomena for situations other than those used to establish the ‘rules’?” • How do we extrapolate to other systems? • Falsification strategy: Assume no differences from the baseline system, or scale properties with activity level only where undeniably needed, to find where observations indicate unacceptable deviations from the model. CSW12, Boulder

  5. Modeling cool-star activity:Physical processes (I) • Knowledge deduced from observations(I: Ness) • Rotation as function of activity (05.13: Pizzolato et al.: what is the Rossby number really?; 05.19 Ambruster et al. on ZAMS rotation and activity) • Activity and flux-source pattern as a function of time (cycles are rare: seen 1 in 3 Sun-like stars) • Flux emergence (06.06: Berdyugina et al. on persistence of active longitudes) • Photospheric to chromospheric structure and dynamics(I: Morossi) • Flux dispersal: convection and large-scale circulation(I: Reiners; 01.02 Hurlburt, on magnetoconvection; 06.05 Hackman+Jetsu and 06.03 Weber et al. on possible anti-solar differential rotation in giants) CSW12, Boulder

  6. A magnetic cycle CSW12, Boulder

  7. Fluxemergence Flux frag- mentation Collision & cancelation Random stepping Diff. Rot. & Merid. flow Simulating photospheric activity Intrinsically NON-LINEAR with CONTINUOUS distributions (ApJ v547, p475; v551, p1099) • Model builds on work by • Leighton, Sheeley, Wang.New: • statistical sampling for source function • ephemeral-region population • magnetoconvective coupling • magneto-chemistry: “atomic,” fragmentation & collision CSW12, Boulder

  8.  /= 1/30  /=30 Simulating other stars Or: the Sun at different ages! • Hypothesis: Stellar dynamos are exactly like that of the Sun, except for the frequency of active-region emergence CSW12, Boulder

  9. Effects of large-scale flows • Differential rotation and meridional flow only Stellar differential rotation appears to be similar to the solar case. Stellar meridional flow remains essentially unknown CSW12, Boulder

  10. The Sun through the cycle Simulated “Sun” viewed from 40 latitude, in a corotating reference frame: CSW12, Boulder

  11. Polar-cap flux (>60º ) Absolute Net Positive only The Sun through the cycle Total flux Flux in activity belt CSW12, Boulder

  12. No sunspots above ~50º Sunspots: location and frequency CSW12, Boulder

  13. Present Sun Young Sun at ~500 Myr? Simulations of activity Simulated “Sun” from 40N: Active star (30x higher rate of flux injection), from 40N: CSW12, Boulder

  14. Polar-cap flux (>60º ) Absolute Positive only Simulations of an active star Total flux Flux in activity belt CSW12, Boulder

  15. Possible causes of polar spots Possibility II: • In a rapidly-rotating sun-like star, the Coriolis force may deflect rising flux to “high” latitudes. CSW12, Boulder

  16. Possible causes of polar spots Possibility III: • In a rapidly-rotating cooler star, magnetic tension may cause the entire loop to rise, sometimes to high latitudes. CSW12, Boulder

  17. Models of flux emergence • Flux-emergence simulations for different stars (Granzer et al., 2000, A&A 355, 1087): Increasing rotation CSW12, Boulder

  18.  /=30 Activity, rotation, and saturation CSW12, Boulder

  19. CSW12, Boulder

  20. Modeling cool-star activity:Physical processes (II) • Atmospheric field geometry • Time-dependent (non-)magnetic energy dissipation (06.08 Jardine et al., and Hussain, on AB Dor modelling) • Radiative losses and transfer from loop ensemble (05.01: Jordan et al.: To resolve or not to resolve?) • Coronal structure and dynamics (II: Audard, Osten, Guedel) • Comparing observation and expectation(III: Raassen) CSW12, Boulder

  21. A cool-star corona CSW12, Boulder

  22. Average Sun The Sun among the Stars • Flux-flux relationships are power laws over a factor 100,000 in soft X-rays • The average Sun lies on those relationships , and moves along them through the cycle FXDFCaII1.6 CSW12, Boulder

  23. The coronal magnetic field • Hypothesis: The coronal magnetic configuration can be adequately approximated by a potential field. CSW12, Boulder

  24. The solar coronal magnetic field Near cycle maximum CSW12, Boulder

  25. A stellar coronal magnetic field Near cycle maximum CSW12, Boulder

  26. Field strength with height scale height for active-region fields: ~15 Mm Potential fields over active regions: Sun-like 10x solar CSW12, Boulder

  27. Constraints on coronal heating Many loops are nearly isothermal along their length: coronal loops are probably heated primarily in the lower 10-20Mm. E.g., Aschwanden et al., ApJ 551,1036. Compatible with SXT X-ray observations: MacKay et al., SPh 193, 93. TRACE 171Å (~1MK) CSW12, Boulder

  28. Loop atmospheres Quasi-static, hydrostatic loops, uniform cross section: Uniformly heated Heating concentrated in lower 20 Mm CSW12, Boulder

  29. Modeling coronal loops • Potential-field skeleton, quasi-static loops, with finite heating scale length. Heating flux density: PH = 2 107(B/100)b (l/24)-l (v/0.4)u ergs/cm2/s, (normalized to active-region loops). Theoretical/numerical models predict values of • b [ -1, 2.1], l [-3, 1.1], u [ 0, 2]. • Combine with solar and stellar constraints on Tcor and FX throughout the cycle to find a best fit. CSW12, Boulder

  30. AR: 01.03 Fludra and Ireland (2001) Coronal heating • Best fit (for all field lines): PH  B(1.00.5) l-(0.70.3) • Compatible with (slow or fast) driving of • magnetic reconnection(Parker, 1983; Galsgaard & Nordlund, 1996; Mandrini et al. 2000) movie • turbulence(Einaudi et al. 1996; Dmitruk and Gomez, 1997; Inverarity & Priest, 1995). valid for Sun and stars Active regions: Loop length exponent QS: Winebarger and Warren (2001) Winebarger & Warren (2001) Magnetic field exponent CSW12, Boulder

  31. Direct measurements (Saar, 2001) Indirect relationship expected from solar & stellar measurements (Schrijver et al., 19..) Stellar coronal activity Simulated results agree with observations: stellar radiative and magnetic flux densities related through power laws 30x solar 10x solar Sun-like CSW12, Boulder

  32. The hot solar atmosphere The million-degree solar corona. Seen by the Transition Region and Coronal Explorer. CSW12, Boulder

  33. CSW12, Boulder

  34. Modeling cool-star activity: Physical processes (III) • Coronal structure and dynamics (II: Audard, Osten, Guedel; III: Raassen) • Different environments(III: McMurry, Lobel; IV Gray) CSW12, Boulder

  35. Solar coronal structure Solar observations (element dependent) Best-fit simulations CSW12, Boulder

  36. Stellar coronal structure 1 Ori; G0V; 5.5d Stellar observations  Boo A; G8V+k4V; 6.4d Best-fit simulations CSW12, Boulder

  37. What determines low-T DEM(T)? • Possible causes for the steepness of the observed DEM(T): • Loop expansion • Base heating • Dynamics • Obscuration by “chromosphere” • Abundances, ...? CSW12, Boulder

  38. What determines high-T DEM(T)? • Possible causes for the DEM(T) for T>5MK: • Flares/post-flare systems (05.06: Redfield et al.: forbidden lines at 10 MK not broadened above thermal width) • Very compact loops in increasingly confused photospheric field • ? CSW12, Boulder

  39. Coronal physics • Elemental fractionation: • Solar corona: Low-FIP enhanced over high-FIP by a factor of four. Position of hydrogen remains under discussion (perhaps as high-FIP, perhaps intermediate) • Solar wind: Low-FIP enhancement also seen in the fast solar wind, so fractionation mechanism must also work in coronal holes. Consequences for plasma physical and MHD models? • Stellar observations: FIP effect reverses with increasing activity. • Cause(s)? Different competing mechanisms? (II: Guedel;05.07 Linsky et al: “Crazy abundances”) CSW12, Boulder

  40. Where is the chromosphere? • “Extended chromosphere:” filaments, spicules, coronal rain, plus: • Extinction of long-wavelength EUV emission in solar observations (Kanno & Suematsu, 1982); but no excess 20cm microwave emission (Brosius et al., 1997)? • Differential extinction for EUVE/ORFEUS observations of Capella (Brickhouse et al., 1996) • Centrifugally supported coronal rain and filament material in rapidly rotating stars (Ayres et al. 1998; Collier-Cameron, 2000) • Giant-star winds ... CSW12, Boulder

  41. The evolution of the Sun The evolution of a star like the Sun CSW12, Boulder

  42. Convection simulations Movie: 20 years in the life of a simulated supergiant (~600 R):  Orionis in the computer of Bernd Freytag (09.01) CSW12, Boulder

  43. Betelgeuse’s appearance HST/FOC UV image From Gilliland and Dupree (1996; ApJL 463, 29) CSW12, Boulder

  44. CSW12, Boulder

  45. Modeling cool-star activity:Physical processes (IV) • Different environments(III: McMurry, Lobel; IV Gray) • Asterospheric field, extended atmosphere, stellar wind(IV Wood), mass ejections CSW12, Boulder

  46. Properties of convection CSW12, Boulder

  47. Scales of convection • Problem: numerical simulations cannot cover the entire range of convective scales throughout the convective envelope of a star. • Are granulation, mesogranulation, supergranulation, and giant-cell convection distinct phenomena? Do their origins reflect ionization processes of hydrogen and helium? • Do all these scales exist? CSW12, Boulder

  48. Simulation Two-component model SOHO/MDI high-res. “Power spectrum” of convection SOHO/MDI full-disk Hathaway et al. (2000) CSW12, Boulder

  49. Solar mass loss CSW12, Boulder

  50. Stellar mass loss • Direct detection • Hot winds of main-sequence stars: • Radio measurements (mostly upper limits, or ambiguous results) • Charge-exchange induced X-ray emission (proposed) • UV spectral signatures at the interaction region between wind and interstellar medium (IV: Wood) • Cool winds of evolved stars: • UV spectral signatures from the wind itself (09.02 Schröder et al. on dusty wind in giants at tip of AGB; 09.08 Böger et al. on spectral diagnostics of turbulent winds in K and M giants) • Indirect detection through angular momentum loss CSW12, Boulder