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.