Doppler imaging study of starspots using SONG network

Sheng-hong Gu1, Andrew Collier Cameron2 and James Neff3

1. Yunnan Observatory, China2. St. Andrews University, UK3. College of Charleston, USA

2011.9.17 CharlestonUSA

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 convectionmagnetic

field in the stellar interiorstarspots in the photosphere Local magnetic field of stellar photosphere, suppress the c

onvection 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, meridio

nal 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 profilesSurface 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 obser

vations

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, 10Xrotationsun

Differential rotation of young star AB Dor (Donati & Cameron 1997)

Sun ΔΩ=0.055 rad/d AB Dor ΔΩ=0.0564 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 (antis

ymmetry)

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 observati

ons Long time continuous coverageobserve targets with special rotati

on periods (around 0.5days, 1days, etc.) Disadvantage

Smaller aperture -- 1mbright stars Limiting magnitude due to high-resolutionbright 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 ma

gnetic 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 latitu

de direction Long period and brightness make it easy to arrange observation

s Basic parameters

α,δ 07:43:18.7 28:53:01 (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

1991-1992 No polar cap Active latitude band 55degrees

The second DI results of Sigma Gem (Kovari et al. 2001)

1996-1997 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 & B

artus 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 03:36:47.3 00:35:16 (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 6.44 6.4 G5V/K0IV 6/37 03:26:35.4 28:42:55 HK Lac 24.43 6.5 F1V/K0III /15 22:04:56.6 47:14:05 IM Peg 24.65 5.6 K2III-II 24 22:53:02.3 16:50:28 II Peg 6.72 7.2 K2V-IV 21 23:55:04.0 28:38:01 Observing for anyone of above 4 targets will be at the beginning (one spectrum, 10

min.) and end (one spectrum, 10min) of every night for each SONG node, so it almost does not affect the normal asteroseismology observation

EI Eri 1.95 7.0 G5IV 50 04:09:40.7 -07:53:32 AR Lac 1.98 6.1 G2IV/K0IV 46/81 22:08:40.8 45:44:32 Observing for one of above two stars needs to use 2 nights of the whole SONG net

work each run, which will interrupt the normal observations for asteroseismology

Thank you for your attention!