Physical processes in cool star activity
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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).

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Physical processes in cool-star activity

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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

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

To simplify or not to simplify?

  • Fundamental difficulties:

    non-linear, non-local, non-stationary system

    with disjoint interfaces in a 3D geometry



b < 1

t < 1


t > 1

b > 1

CSW12, Boulder

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

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)

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A magnetic cycle

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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

 /= 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

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

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The Sun through the cycle

Simulated “Sun” viewed from 40 latitude, in a corotating reference frame:

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Polar-cap flux (>60º )



Positive only

The Sun through the cycle

Total flux

Flux in activity belt

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No sunspots above ~50º

Sunspots: location and frequency

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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:

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Polar-cap flux (>60º )


Positive only

Simulations of an active star

Total flux

Flux in activity belt

CSW12, Boulder

Possible causes of polar spots

Possibility II:

  • In a rapidly-rotating sun-like star, the Coriolis force may deflect rising flux to “high” latitudes.

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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.

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Models of flux emergence

  • Flux-emergence simulations for different stars (Granzer et al., 2000, A&A 355, 1087):

Increasing rotation

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 /=30

Activity, rotation, and saturation

CSW12, Boulder

CSW12, Boulder

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)

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A cool-star corona

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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


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The coronal magnetic field

  • Hypothesis:

    The coronal magnetic configuration can be adequately approximated by a potential field.

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The solar coronal magnetic field

Near cycle maximum

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A stellar coronal magnetic field

Near cycle maximum

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Field strength with height

scale height for active-region fields: ~15 Mm

Potential fields over active regions:


10x solar

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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

Loop atmospheres

Quasi-static, hydrostatic loops,

uniform cross section:

Uniformly heated

Heating concentrated

in lower 20 Mm

CSW12, Boulder

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

    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

    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


    CSW12, Boulder

    The hot solar atmosphere

    The million-degree solar corona.

    Seen by the Transition Region and Coronal Explorer.

    CSW12, Boulder

    CSW12, Boulder

    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

    Solar coronal structure

    Solar observations (element dependent)

    Best-fit simulations

    CSW12, Boulder

    Stellar coronal structure

    1 Ori; G0V; 5.5d

    Stellar observations

     Boo A; G8V+k4V; 6.4d

    Best-fit simulations

    CSW12, Boulder

    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

    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

    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

    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

    The evolution of the Sun

    The evolution of a star like the Sun

    CSW12, Boulder

    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

    Betelgeuse’s appearance

    HST/FOC UV image

    From Gilliland and Dupree

    (1996; ApJL 463, 29)

    CSW12, Boulder

    CSW12, Boulder

    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

    Properties of convection

    CSW12, Boulder

    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


    Two-component model

    SOHO/MDI high-res.

    “Power spectrum” of convection

    SOHO/MDI full-disk

    Hathaway et al. (2000)

    CSW12, Boulder

    Solar mass loss

    CSW12, Boulder

    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

    Rotation and age

    CSW12, Boulder

    The extended stellar atmosphere

    CSW12, Boulder


    Current estimate of the distance to the termination shock: 85  5 AU

    CSW12, Boulder


    Current estimate of the distance to the termination shock: 85  5 AU

    CSW12, Boulder

    CSW12, Boulder

    The future

    • Where are the most significant gaps in our knowledge? Where can we expect the most significant advances? Where do we need them?

      • Dynamo and (magneto-)convection

      • Field dynamics (reconnection, waves, …)

      • Energy dissipation, heating

      • Realistic field-matter interaction, including radiation and chemistry

      • Plasma physics observable diagnostics

    • What do we need?

      • Multi-wavelength, coordinated observations

      • Long runs, revisiting targets (imagine a movie of Betelgeuse)

      • Imaging

      • ...

    CSW12, Boulder

    This presentation can be found at:



      … movies yet to be included.

    CSW12, Boulder

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