Winds and Chromospheres of Cool (Super-) Giants

Wind Acceleration, Structure and Energy BalanceWhat we know, don’t know, and what we can do

about it

Graham Harper, CASA, University of Colorado

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 2

What we know

wind acceleration is mostly gradual in K & early-M stars

very good discriminator for theoretical models mass-loss rates are a weak discriminator

wind emissivity is lower than the chromosphere wind base filling factor <1

wind ionization declines with Teff Winds do not start at Mg II dividing-line (e.g. HR 6902 G9

IIb) eclipsing binary systems still yielding new results strong evidence for magnetic fields (O VI, SiO masers) stellar properties becoming better constrained

new Hipparcos reduction, angular diameters, SED

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 3

What we don’t know

We don’t know [1] nature of wind acceleration close to star [2] geometry of chromosphere and wind [3] wave energy spectrum [4] nature of acceleration in metal poor stars

Why is this important [1] acceleration is good discriminator of theory

empirically we need to get it right [2] geometry controls

wave damping thermodynamics

[2,3] boundary conditions for theoretical models [4] test new extreme of parameter space

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 4

Magnetohydrodynamic Models

Falceta-Gonçalves et al. 2006, MNRAS, 368, 1145

Carpenter et al. 1999, ApJ, 521, 382

Baade et al. 1996, ApJ, 466, 979

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 5

Magnetohydrodynamic Models

Time dependent Alfvén waves in super-radial flux tubes Suzuki (2007 ApJ, 659, 1592)

Symbiotics: Vogel/Crowley (2006 PhD), γ Cru Carpenter et al.

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 6

What do we need? Spatial Resolution – any way you can! Gradients -> physics High Spectral Resolution: resolve smallest scales possible Include geometric structure in wind acceleration models Improved model ionization and therefore energy balance Inputs from convection, granulation, and dynamo models

e.g. AGB input physics Freytag & Hofner (2008, A&A, 483, 571) Continue to learn from solar physics (spicule Alfvén waves)

Hinode: De Pontieu et al. 2007 Science, 318, 1574

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 7

Spectro-Interferometry: the 3rd spectrum

Provide excellent test of theoretical models

Left: Betelgeuse VTLI/MIDI R=30 (Perrin et al. 2007 A&A, 474, 599)

Right: HST/STIS R=15,000 (CS15)

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 8

Future Potential of Eclipsing Binaries

Similar radial excitation temperatures: Eaton (2007, AJ, 133, 2669) & Crowley (2006 PhD)

Different radial density scale heights: Schröder et al. (1988, A&A, 202, 136)

[Analytic solar magnetic field model: Banaszkiewicz et al. 1998, A&A, 337, 940]

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 9

High Spectral Resolution …

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Bennett

Line bifurcation (stable extended fine-structure)

Ca II H & K (R. Elizabeth Griffin 2005 CS13, p. 595)

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 10

A Way Forward

To make progress we DO require access to significant resources dedicated multi-wavelength studies @ multiple sites often on oversubscribed telescope facilities need to demonstrate to community that we really mean

business Form interdisciplinary teams working toward common goals

observers, modellers, theoreticians (all flavours) Identify key projects with specific goals and outcomes, e.g.,

theoretical model grids for specific stars identifying spectral signatures of super-radial flows differential study of wind acceleration in Pop II and Pop I

stars constraining the filling factors of Molspheres

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 11

A Way Forward We cannot choose stars to match the theoretical models –

the models need to move to the stars (anon, 2008) help theoreticians make relevant model grids

Single Stars Select stars that are to be the focus of mass-loss studies Identify single stars that are proxies for eclipsing binary

primaries Make database of stellar parameters and atmospheric

properties parameters to be critically evaluated by Teams identify missing crucial data (e.g., vsini, g*, isotopic

abundances) make observations to remedy missing data make database open

Eclipsing Binaries obtain good time-series during ingress & egress deconstruct the chromosphere and wind, establish

symmetries

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 12

Matching spectral-types

Eclipse spectra: K4 Ib star + scattered light ~ tau 0.4-0.5 : Bennett (2006 ASP Conf. Ser., 348, 254)

ζ Aur chromospheric heating rates same as spectral-type proxies: Eaton (1992, MNRAS, 258, 473)

Tuesday, April 18, 2023 Cool Stars XV - Splinter Session 13

Specific Challenges

For Cool Stars16 Theoretical models + spectrum of Mult. C II] 2325 Ang for

real stars. low mass K-M giants (EG And, αTau, αBoo) intermediate mass (ζAur systems, λVel K4 Ib-II)

Accurately measure vsini(i) for primaries of eclipsing systems clues to structure from where the wind decouples from the

star? St. Andrews: What kind of magnetic fields required for line

splitting? Large scale prominences? Projection + ΔV both ➱ R*