STELLAR “PROMINENCES”

1. STELLAR “PROMINENCES” • Mapping techniques • Mechanical support • Short- and long-term evolution • Implications for coronal structure and evolution Andrew Collier Cameron University of St Andrews, Scotland.

2. -v sin i +v sin i -v sin i +v sin i Starspot signatures in photospheric lines Absorption transients in H alpha Spots and prominences: signatures • AB Dor, AAT/UCLES, 1996 Dec 29 • Donati et al 1998

3. Goals • Neutral gas condenstions as probes of coronal structure • Radial distribution • Inclination dependence • Determine physical properties of prominences at various distances from star. • Measure timescales for • Prominence formationdifferential rotation • Surface • changes in coronal structure • Does flux emergence or surface differential rotation drive coronal evolution? • Potential field extrapolations from magnetic maps • Surface distribution of open field lines (coronal holes) • Potential minima as prominence formation sites?

4. Radial accelerations • Radial acceleration of co-rotating cloud -> axial distance • Most transients have similar drift rates across Ha profile

5. Axial distances of absorbing clouds • Clouds congregate mainly near or just outside co-rotation radius ( ). • AB Dor: Corotation radius is 2.7 R* from rotation axis.

6. Coronal condensations: single stars • Detected in 90% of young (pre-) main sequence stars with Prot<1 day • AB Dor (K0V): Collier Cameron &Robinson 1989 • HD 197890 =“Speedy Mic” (K0V): Jeffries 1993 • 4 G dwarfs in Per cluster: Collier Cameron & Woods 1992 • HK Aqr = Gl 890 (M1V): Byrne, Eibe & Rolleston 1996 • RE J1816+541: Eibe 1998 • PZ Tel: Barnes et al 2000 (right) Prot = 1 day (slowest yet) • Pre-main sequence G star RX J1508.6-4423 (Donati et al 2000) --prominences in emission!

7. Physical properties: • Areas: 3 x 1021 cm2(up to 0.3 A*) • Column densities: NH ~ 1020cm-2 • Masses: 2-6 x 1017 g (cf solar quiescent prominences M ~ 1015 g) • Temperatures: 8000-9000K • Number: about 6-8 in observable slice of corona • Co-rotation enforced out to about 8R* in AB Dor • Ambientcoronal temperature T ~ 1.5 x107 K • (Physical data from simultaneous Ha + Ca IIK absorption studies, Cameron et al 1990)

8. Emission signatures • Seen only in the most rapidly-rotating, early G dwarfs, e.g. RX J1508.6 -4423 (Donati et al 2000): Star is viewed at low inclination; uneclipsed Ha-emitting clouds trace out sinusoids

9. Tomographic back-projection • Clouds congregate near co-rotation radius (dotted). • Little evidence of material inside co-rotation radius. • Substantial evolution of gas distribution over 4 nights.

10. T What’s holding them down? • Radial accelerations (2r sin i) show that most of the prominences lie at cylindrical radii near (but some inside and and some substantially outside) the equatorial co-rotation radius. • Outside co-rotation radius, the gravitational force on the plasma isn’t enough to keep the clouds in a synchronous orbit. • So we need an extra inward force to keep them in co-rotation with the star. • Can use the magnetic tension of a closed magnetic loop to anchor the cloud to the surface.

11. Condensations within equatorial co-rotation radius • Byrne, Eibe & Rolleston (1996) found clouds substantially below co-rotation radius in single M1V rapid rotator HK Aqr. • Eibe (1998) mapped condensations in M1V rapid rotator RE J1816+541, also found clouds within corotation radius.

12. Latitude dependence • AB Dor prominences need to be anchored at high latitude to cross stellar disk, since i = 60 degrees. • What about other stars with different inclinations? • BD+22 4409: Low inclination, no transients found: Jeffries et al 1994 

13. High latitude downflows in BD+22 4409 • Eibe, Byrne, Jeffries & Gunn (1999): No absorption transients seen in 2 nights of time-resolved echelle data from 1993 August. • Narrow emission profile: FWHM(Ha) < FWHM(v sin i) • Persistent red-shifted absorption at all phases • Low inclination i~50o • Walter & Byrne (CS10 1998): inflowing material in unsupported high latitude regions well within co-rotation surface? 1993 Aug 5 1993 Aug 4

14. Evolution of absorption transients • Evolution of absorbing clouds around AB Doradus, 1996 December 23, 25, 27 & 29:

15. AB Dor: starspot distribution 1996 Dec 23 - 29

16. AB Dor: Radial magnetic field 1996 Dec 23 - 29

17. Surface shear: AB Dor, 1996 Dec 23 - 29 • CCF for surface-brightness images • CCF for magnetic images: Donati et al (1998)

18. Back projections sliced at 2.5 stellar radii 0.020 0.010 0.000 -0.010 Aa Ab E -0.020 D C B -0.030 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Rotation phase Phase drift of prominences AB Dor 1995 Dec 7 to 11 • Prominence rotation lags equator. • Rotation rate matches surface at latitude 60o to 70o. • cf. east-west alternating magnetic polarity pattern at same latitude. Donati & Cameron (1997)

19. Support in complex field structures • Ferreira (1997): component of effective gravity along the field must be in stable balance. • Stable locations exist inside corotation even for a dipole field (left) or quadrupole-sextupole (right) R K R K

20. Open field topology from ZDI • AB Dor, 1995 December 7-12. • Zeeman Doppler image derived from echelle circular spectropolarimetry at Anglo-Australian Telescope (Donati et al 1997) • Open field lines traced from Zeeman-Doppler image assuming potential field with source surface at 5 stellar radii.

21. Stable gravitational-centrifugal minima • Potential-field models from Zeeman-Doppler images (ZDI) show stable potential minima along closed field lines satisfying: • Here geff is the effective gravitational potential gradient including centrifugal terms. • Condensations can be supported stably in these locations. Image derived from AAT+UCLES+Semel polarimeter data, 1996 Dec 23+25 Jardine et al 2000, in preparation

22. Summary and conclusions • Coronal condensations probe extent of closed-field region in rapidly rotating late-type stars. • Prominences within corotation radius require complex field topologies for support. • Can form up to 30o or so out of equatorial plane at intermediate axial inclinations. • Downflows seen in BD+22 4409 suggest coronal condensations form in unsupported regions too. • Prominence system evolves faster than surface structure: coronal field continually destabilised by surface shear? • Where are the open field lines? Need to combine ZDI with prominence studies to obtain self-consistent picture of 3D coronal structure.