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STELLAR “PROMINENCES” Mapping techniquesMapping techniques Mechanical supportMechanical support Short- and long-term evolutionShort- and long-term evolution.

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Presentation on theme: "STELLAR “PROMINENCES” Mapping techniquesMapping techniques Mechanical supportMechanical support Short- and long-term evolutionShort- and long-term evolution."— Presentation transcript:

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

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

3 Goals Neutral gas condenstions as probes of coronal structureNeutral gas condenstions as probes of coronal structure –Radial distribution –Inclination dependence Determine physical properties of prominences at various distances from star.Determine physical properties of prominences at various distances from star. Measure timescales forMeasure timescales for –Prominence formationdifferential rotation –Surface –changes in coronal structure Does flux emergence or surface differential rotation drive coronal evolution?Does flux emergence or surface differential rotation drive coronal evolution? Potential field extrapolations from magnetic mapsPotential 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 distanceRadial acceleration of co-rotating cloud -> axial distance Most transients have similar drift rates across H  profileMost transients have similar drift rates across H  profile

5 Axial distances of absorbing clouds Clouds congregate mainly near or just outside co-rotation radius ( ).Clouds congregate mainly near or just outside co-rotation radius ( ). AB Dor: Corotation radius is 2.7 R* from rotation axis.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 P rot <1 dayDetected in 90% of young (pre-) main sequence stars with P rot <1 day –AB Dor (K0V): Collier Cameron &Robinson 1989 –HD =“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 J : Eibe 1998 –PZ Tel: Barnes et al 2000 (right) P rot = 1 day (slowest yet) –Pre-main sequence G star RX J (Donati et al 2000) --prominences in emission!

7 Physical properties: Areas: 3 x cm 2 (up to 0.3 A * )Areas: 3 x cm 2 (up to 0.3 A * ) Column densities: N H ~ cm -2Column densities: N H ~ cm -2 Masses: 2-6 x g (cf solar quiescent prominences M ~ g)Masses: 2-6 x g (cf solar quiescent prominences M ~ g) Temperatures: KTemperatures: K Number: about 6-8 in observable slice of coronaNumber: about 6-8 in observable slice of corona Co-rotation enforced out to about 8R * in AB DorCo-rotation enforced out to about 8R * in AB Dor Ambient coronal temperature T ~ 1.5 x10 7 KAmbient coronal temperature T ~ 1.5 x10 7 K (Physical data from simultaneous H  + Ca IIK absorption studies, Cameron et al 1990)(Physical data from simultaneous H  + Ca IIK absorption studies, Cameron et al 1990)

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

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

10 What’s holding them down? Radial accelerations (  2 r 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.Radial accelerations (  2 r 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.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.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.Can use the magnetic tension of a closed magnetic loop to anchor the cloud to the surface. T

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.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 J , also found clouds within corotation radius.Eibe (1998) mapped condensations in M1V rapid rotator RE J , 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.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?What about other stars with different inclinations? –BD : Low inclination, no transients found: Jeffries et al 1994 

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

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

15 AB Dor: starspot distribution 1996 Dec

16 AB Dor: Radial magnetic field 1996 Dec

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

18 Phase drift of prominences AB Dor 1995 Dec 7 to 11 Prominence rotation lags equator.Prominence rotation lags equator. Rotation rate matches surface at latitude 60 o to 70 o.Rotation rate matches surface at latitude 60 o to 70 o. cf. east-west alternating magnetic polarity pattern at same latitude.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.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)Stable locations exist inside corotation even for a dipole field (left) or quadrupole-sextupole (right) R K

20 Open field topology from ZDI AB Dor, 1995 December 7-12.AB Dor, 1995 December Zeeman Doppler image derived from echelle circular spectropolarimetry at Anglo-Australian Telescope (Donati et al 1997)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.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:Potential-field models from Zeeman-Doppler images (ZDI) show stable potential minima along closed field lines satisfying: Here g eff is the effective gravitational potential gradient including centrifugal terms.Here g eff is the effective gravitational potential gradient including centrifugal terms. Condensations can be supported stably in these locations.Condensations can be supported stably in these locations. Image derived from AAT+UCLES+Semel polarimeter data, 1996 Dec Jardine et al 2000, in preparation

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

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