Doppler imaging study of starspots using SONG network Sheng-hong Gu 1, Andrew Collier Cameron 2 and James Neff 3 1. Yunnan Observatory, China 2. St. Andrews University, UK 3. College of Charleston, USA Charleston  USA

Outline  Starspots  Doppler imaging  SONG and its spectrograph  Least-squares deconvolution for faint stars  Purposed programs for SONG network

Starspots  As sunspots  Rotation couples with convection  magnetic field in the stellar interior  starspots in the photosphere Local magnetic field of stellar photosphere, suppress the convection and block the energy from the stellar interior to the surface dark regions on bright photosphere  Starspots Can be the probe of internal dynamo activity Can be use to measure the stellar rotation period, meridional flow and surface differential rotation law which are closely relative to the internal magnetic structure

 The most important thing for stellar physics is The information on the location and migration pattern of starspots, which is related to the stellar dynamo and internal energy transport, can help us to understand low-mass stellar evolution and formation

 First observing example, irregular eclipsing light curve of AR Lac (Kron 1947)  Up to now, the unique way to derive the starspot pattern (location, morphology) — Doppler imaging technique

Doppler imaging (DI)  Indirect imaging method (Vogt & Penrod 1983, Donati & Collier Cameron 1997, Berdyugina 1998, etc.)  Time series of line profiles  Surface map of the star (Animation of Doppler imaging, from Oleg Kochukhov’s website )

DI depends on  Rotational velocity (vsini) of the target  Inclination of rotational axis (i) of the target  Resolution (R) of spectrograph used  S/N of the spectra  Rotational phase coverage in the time-series observations

Some results  The Doppler imaging of young star Speedy Mic (Barnes 2005)

 The spot pattern comparison between Sun and EK Dra (Strassmeier 2009) Sun--G2V, 5Gyr EK Dra--G1.5V, 100Myr, 10Xrotation sun

Differential rotation of young star AB Dor (Donati & Cameron 1997)  Sun ΔΩ=0.055 rad/d  AB Dor ΔΩ= rad/d

Doppler imaging of young star PW And (Gu et al. 2010)

Butterfly diagram of HR 1099 (Berdyugina & Henry 2007) DI results at two opposite active longitudes  16 yr cycle  While one active region moves to the pole, the other to the equator (antisymmetry)

SONG and its spectrograph (SONG network, from the website of University of Aarhus)

Song spectrograph (Grundahl 2009)  Advantage High-resolution R=96000 (1.”3 slit)  resolve small spots High efficiency (>50% at blaze peak from slit to detector) Large wavelength region (4814A to 6774A)  make LSD possible the iodine cell can be removed  “normal” spectroscopic observations Long time continuous coverage  observe targets with special rotation periods (around 0.5days, 1days, etc.)  Disadvantage Smaller aperture -- 1m  bright stars Limiting magnitude due to high-resolution  bright stars Need to use LSD technique to enhance the S/N for fainter stars

Least-squares deconvolution (LSD) for faint stars (Donati et al. 1997) Plenty of photospheric lines in the spectrum obtained by using echelle spectrograph  an ” average ” profile with high S/N (Cameron 2000)

One order of observed echelle spectrum of V711 Tau and its LSD profile (Gu et al. 2007)

Purposed programs for SONG network  Remove the iodine cell from the light path of SONG spectrograph  Programs The detection of detailed butterfly diagram of stellar magnetic activity Differential rotation along the latitude on the surface of star Meridional flow on stellar surface

The butterfly diagram of solar activity (From NASA website)  In order to get such butterfly diagrams for the active stars, we need a network like SONG to obtain the continuous time-series DIs

Sigma Gem The best candidate to derive the detailed butterfly diagram for an active star  The known starspot patterns show the migration along the latitude direction  Long period and brightness make it easy to arrange observations Basic parameters  α,δ 07:43: :53:01 (2000)  Vmag 4.3  Sp. type K1 III  log g 2.5  Teff 4630K  vsini 27.5km/s  Inclination(i) 60degrees  Prot = Porb 19.60days

The first DI results of Sigma Gem (Hatzes 1993) CaI DI FeI DI No polar cap Active latitude band 55degrees

The second DI results of Sigma Gem (Kovari et al. 2001)   Active latitude band 45degrees  10degrees/5yr migration to the equator

The purposed observing plan In the beginning and end of observation every night, 4 spectra can be easily obtained during short time (10min.+10min.) for each node of SONG If we monitor Sigma Gem for 5 years and 6 DI results can be derived every year, finally we will get 30 DI results spanned for about 93 rotation cycles, which will permit us to give the first glimpse for butterfly diagram of this star The program almost does not affect the running of asteroseismology program!

V711 Tau A very good candidate for measuring the meridional flow and differential rotation using SONG network  The evidences for meridional flow and differential rotation exist (Strassmeier & Bartus 2000, Petit et al. 2004)  It’s difficult to observe it at a single station due to its period close to 3 days. To get the data with good phase coverage during short time, the only way is using the network like SONG Basic parameters  α,δ 03:36: :35:16 (2000)  Vmag 5.9  Sp. Type G5IV+K1IV  Teff 4800K  log g 3.5  vsini 41km/s  Inclination(i) 40degrees  Period 2.84days

The observing plan Four 3-day observing runs with gap of about two weeks every year, dedicate to observe V711 Tau by using all SONG nodes. We will get detailed information on meridional flow and differential rotation by tracing the individual spots based on DI results It will interrupt the normal observations for asteroseismology four times per year

Other potential targets Name P(days) Vmag Sp. type vsini(km/s) α(2000) δ(2000)  UX Ari G5V/K0IV 6/37 03:26: :42:55  HK Lac F1V/K0III /15 22:04: :14:05  IM Peg K2III-II 24 22:53: :50:28  II Peg K2V-IV 21 23:55: :38:01 Observing for anyone of above 4 targets will be at the beginning (one spectrum, 10min.) and end (one spectrum, 10min) of every night for each SONG node, so it almost does not affect the normal asteroseismology observation  EI Eri G5IV 50 04:09: :53:32  AR Lac G2IV/K0IV 46/81 22:08: :44:32 Observing for one of above two stars needs to use 2 nights of the whole SONG network each run, which will interrupt the normal observations for asteroseismology

Thank you for your attention!