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Spatially resolved evolution of stellar active regions Outline Unveiling the stellar surface Introduction to Doppler imaging Short term changes of active regions: differential rotation Long term evolution: spotcycles Future prospects Potsdam im Oktober 2003, M. Weber http://www.aip.de/groups/activity Thanks to: K.G. Strassmeier J. Rice AIP - activity group
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Sunspots & differential rotation Equator rotates faster than the pole “Rigidity” changes throughout the solar cycle and between Odd & Even cycles Equatorial rotation faster in ONSC
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Stars exhibit periodic light variations (often rotationally modulated) Activity-related features found in starspots are present in stellar spectra (e.g. CaII H&K) Chromospheric emission lines very strong in such stars Starspots
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Direct imaging of starspots 'direct' image of Betelgeuse Gilliland & Dupree 1996, ApJ Faint Object Camera of HST Interferometric techniques Only very large & very near objects observable
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Photometric spot models e.g. HK Lac: Oláh et al. 1997 Positions and sizes of spots are optimized Several bandpasses (V,R,I,..) are used for inversion Only simple spot configurations can be retrieved Some assumptions have to be made
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Principle of Doppler imaging Missing flux (in case of a dark spot) leaves a characteristc bump in spectral line profile.
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Doppler imaging 1 Missing flux from spots produce line profile deformations 'bumps' move from blue to red wing of the profile due to the 'Doppler' effect. Position of spots correspond to spot longitudes
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Doppler imaging 2 Speed of spots give indication of the latitude (more uncertain than the longitude) 'bumps' from high latitude spots start out somewhere in the middle of the line wing, low latitude spots at the shoulder
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Short term variations: differential rotation Note the sign convention for (the solar case is positive) [ ]=degr/day (x 0.202 = µrad/s) B & C not independent or: Artificial star (see Rice & Strassmeier 2000) = -0.05, P 7days
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Simulating differential rotation line profiles corresponding to 2 rotations of the model star Using seven consecutive line profiles to reconstruct one image Simulation of a medium-long (7 day) period star (II Peg)
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a. Reconstructing differential rotation by cross-correlation Artifical maps created using =0.05 and P=6.72 days Shown is original differential rotation, cross correlation measurements, and fit to cross-correlation Fit coresponds to =0.06 and P=6.6 days Introduced for AB Dor by Donati & Collier Cameron (1997) Observations for two consecutive images needed Spot/active region lifetimes?
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b. “Sheared-image method” Donati et al. 2000 for RX J1508,6- 4423 Using in inversion process evaluating 2 for different periods and differential rotation values Darkest value corresponds to best fit aka “ 2 Landscape” method One image is enough But longer timeline is an advantage as long as it is smaller than the spot lifetimes
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c. Direct tracing of spots AB Dor; Collier Cameron et al. 2002 Combining LSD and matched-filter analysis =0.0046 (P eq =0.5132 days)
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IM Peg K2III, V max =5.8, vsini=27 70 nights of observations 24.65 days rotation period (SB1) Two consecutive stellar rotations well covered Anti-solar differential rotation found ( -0.04)
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IM Peg, cont’d Doppler images with 24 days time separation
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IM Peg, cont’d Cross correlation of the two average images Monte-Carlo style calculation of 50 image- pairs & cross-correlations to estimate the error. Best fit (red line) corresponds to = 1 / 0 =0.58/14.39 = -0.04
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IM Peg, cont’d Including in the inversion procedure “sheared image method” Parameter variation to find the best fit. Average value for the four calculations = 1 / 0 = -0.04 Variation of both P and : = 1 / 0 = -0.02 ±0.01, P= 24.4 ±0.2
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IM Peg, cont’d 2D-cross correlation reveals meridional flows Sum of horizontal flow yields the differential rotation pattern meridional flow appears to be pole-wards
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More stars HD 218153 (K0III, V=7.6) HD 31993 (K2III, V=7.48) LQ Hya (K0III, V=7.5) II Peg (K2IV, V=6.9) HD 208472, IL Hya, HK Lac
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HD 218153 Differential rotation and meridional flow detected Weber & Strassmeier 2001 =0.09 to 0.34 (lower/upper limit)
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HD 31993 Differential rotation detected Strassmeier et al. 2003 = -0.15
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LQ Hya Donati et al. (in press) ; Kovari et al. (submitted) P=1.59 days, ≤ 0.05
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II Peg P=6.72days 5 consecutive Doppler images -0.05
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II Peg cont’d Using in the inversion leads to a non-zero value for some data sets only. Dataset for one map spans more than one stellar rotation and period needs to be varied at the same time P=6.62±0.05 days, = -0.05 ±0.02 Variation of and Period (“ 2 -Landscape method”)
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Long term changes / Activity cycles Solar 11yr activity cycle Mt. Wilson survey found many cycles of solar-type stars Tracing starspots over one activity cycle is a challenging task (not-only observing) time wise
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IM Peg / long term Active longitudes (Berdyugina et al. 2000) Probable activity cycle of 6.5yrs Photometric activity cycles are 29.8 and 10.4 years (Ribarik et al 2003)
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II Peg / long term Long-term variations of spots on II Peg (P 9.5yr) Active longitudes and “flip- flop”; 4.65yr halfcycle Berdyugina et al. 1999
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Variable differential rotation? Donati et al. (in press) Differential rotation is different for V and I and for different epochs Compare to yesterday’s talk by Lanza & Rodonò Is there a link to activity cycles?
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Summary 5 differential rotation measurements Single star (HD 218153) has > 0, other single star <0 Kitchatinov & Rüdiger 1999: P rot, meridional flow, larger for giants
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Outlook The availability of several robotic telescope facilities will make long-term studies much easier. In addition, stars not observable (e.g. P=1day) from one spot cat be observed from several facilities concurrently.
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