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.
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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. Spots and prominences: signatures • AB Dor, AAT/UCLES, 1996 Dec 29 • Donati et al 1998 -v sin i +v sin i Starspot signatures in photospheric lines -v sin i +v sin i Absorption transients in H alpha 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? Radial accelerations • Radial acceleration of co-rotating cloud -> axial distance • Most transients have similar drift rates across Ha profile 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. 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 aPer 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! 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 • Ambient coronal temperature T ~ 1.5 x107 K • (Physical data from simultaneous Ha + Ca IIK absorption studies, Cameron et al 1990) 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 Haemitting clouds trace out sinusoids 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. 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. T 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. 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 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 4 1993 Aug 5 Evolution of absorption transients • Evolution of absorbing clouds around AB Doradus, 1996 December 23, 25, 27 & 29: AB Dor: starspot distribution 1996 Dec 23 - 29 QuickTime™ and a Video decompressor are needed to see this picture. AB Dor: Radial magnetic field 1996 Dec 23 - 29 QuickTime™ and a Video decompressor are needed to see this picture. Surface shear: AB Dor, 1996 Dec 23 - 29 • CCF for surfacebrightness images Donati et al (1998) • CCF for magnetic images: Phase drift of prominences AB Dor 1995 Dec 7 to 11 Back projections sliced at 2.5 stellar radii 0.020 0.010 0.000 -0.010 Aa Ab E -0.020 D -0.030 0.2 C B 0.4 0.6 0.8 1.0 Rotation phase 1.2 1.4 Donati & Cameron (1997) • Prominence rotation lags equator. • Rotation rate matches surface at latitude 60o to 70o. • cf. east-west alternating magnetic polarity pattern at same latitude. Support in complex field structures RK RK • 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) 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. QuickTime™ and a GIF decompressor are needed to see this picture. Stable gravitational-centrifugal minima • Potential-field models from Zeeman-Doppler images (ZDI) show stable potential minima along closed field lines satisfying: g eff .B 0 and Image derived from AAT+UCLES+Semel polarimeter data, 1996 Dec 23+25 QuickTime™ and a GIF decompressor are needed to see this picture. ( B. )( g eff .B ) 0 • Here geff is the effective gravitational potential gradient including centrifugal terms. • Condensations can be supported stably in these locations. Jardine et al 2000, in preparation 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 selfconsistent picture of 3D coronal structure.