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Spatially resolved evolution of stellar active regions
Outline
Unveiling the stellar surfaceIntroduction to Doppler imagingShort term changes of active regions: differential rotationLong term evolution: spotcyclesFuture prospects
Potsdam im Oktober 2003, M. Weberhttp://www.aip.de/groups/activity
Thanks to:
K.G. StrassmeierJ. Rice
AIP - activity group
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
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
Direct imaging of starspots
'direct' image of BetelgeuseGilliland & Dupree 1996, ApJ
Faint Object Camera of HST
Interferometric techniques
Only very large & very near objects observable
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
Principle of Doppler imaging
Missing flux (in case of a dark spot) leaves a characteristc bump in spectral line profile.
Doppler imaging1
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
Doppler imaging2
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
Short term variations: differential rotation
)(sin)( 210 bb
01 /Note the sign convention for (the solar case is positive)
[]=degr/day (x 0.202 = µrad/s)
B & C not independent)(sin)(sin)( 42 bCbBAb
or:
Artificial star (see Rice & Strassmeier 2000)
= -0.05, P 7days
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)
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?
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
c. Direct tracing of spots
AB Dor; Collier Cameron et al. 2002Combining LSD and matched-filter analysis
=0.0046 (Peq=0.5132 days)
IM Peg
K2III, Vmax=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)
IM Peg, cont’d
Doppler images with 24 days time separation
IM Peg, cont’dCross 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
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
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
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
HD 218153
Differential rotation and meridional flow detected
Weber & Strassmeier 2001
=0.09 to 0.34 (lower/upper limit)
HD 31993
Differential rotation detected
Strassmeier et al. 2003
= -0.15
LQ Hya
Donati et al. (in press) ; Kovari et al. (submitted)
P=1.59 days, ≤ 0.05
II PegP=6.72days
5 consecutive Doppler images
-0.05
II Peg cont’dUsing 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”)
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
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)
II Peg / long term
Long-term variations of spots on II Peg (P9.5yr)
Active longitudes and “flip-flop”; 4.65yr halfcycle
Berdyugina et al. 1999
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?
Summary
5 differential rotation measurements
Single star (HD 218153) has >0, other single star <0
Kitchatinov & Rüdiger 1999: Prot, meridional flow, larger for giants
OutlookThe 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.