Magnetic fields in late type giant stars: present results and prospects for the future

R. Konstantinova-Antova, M. Aurière, C. Charbonnel, G. Wade, S. Tsvetkova,

K.-P. Schröder, N. Drake, A. Lebre, P. Petit, T. Decressin, J.-F. Donati, I. Iliev, I. Stateva,

R. Bogdanovski, and T. Roudier

Content:

• G and K giants• ZDI program• M giants• Prospects for the future

Aims of the study:Single late giants are an excellent laboratory to study the conditions under which dynamo could operate at different stages of the stellar evolution when change in the stellar structure appears. We tried to verify different hypotheses for the origin of the magnetic field and activity in giant stars – planet engulfment in faster rotators, angular momentum dredge-up as a result of the stellar evolution, remnant magnetic fields from Ap stars. Also, what kind of dynamo operates in RGB and AGB stars. What is the range and properties of the activity and how long it lasts.

Our sample:

Single G,K and M giants with evidences for magnetic activity or suspected to be activeOn the basis of previous studies of activity indicators and rotation

- 10 “FAST ROTATORS” and 17 “SLOW ROTATORS” WITH STRONG EVIDENCES OF MAGNETIC ACTIVITY (OPTICAL AND/OR X-RAYS)

- 9 M giants (fast rotation and/or X-rays)

- 7 Thermohaline deviants (Ap-stars descendants)?

-16 Bright red giants (CFHT snapshot program)

Telescopes and Methods:Telescopes and Methods:

TBL and CFHT

NARVAL and ESPaDOnS

(Spectropolarimeters)

ZEEMAN EFFECT

LSD (Least Square Deconvolution)

ZDI (Zeeman Doppler Imaging)

Donati et al. (1997, 2006 a,b)

LSD:

G and K giants:

10 “fast” + 17 “slow” rotators + 7 thermohaline deviants + 16 bright red

giants.

26 of them were detected. They are mainly fast and slow rotators and 3

giants from the CFHT snapshot program. None of the deviants were

detected.

HRD for the observed G,K giants(models from Charbonnel&Lagarde, 2010)

Giants with known rotation period:

Fit of the data. Two FK Com-type stars and EK Eri are excluded.

Convective turnover time (Charbonnel et al., 2011, in prep.)

Evolution of the Rossby number in 2 Msun star (Charbonnel et al., 2011 – in prep.)

G, K giants - Conclusions:(Auriere, Konstantinova-Antova, Charbonnel et al., 2011, A&A, in prep.)

•Magnetic field is observed in both fast and slow surface rotators. All detected stars are situated mainly in the dredge-up region on the HRD. No stars with mass less 1.5 Msun are detected. (Planet engulfment hypothesis for the fast rotators ruled out!).

•Clear dependence on rotation for periods up to 200 days. Possible dependence for periods up to 600d? S-index has similar behavior as |Bl| max.One and the same dynamo operates in both “fast” and “slow” rotators. The models for the evolution of conv. turnover time and Ro support our results.

•Special cases: EK Eri, HD232862, OU And, OP And, 14 Cet, Aldebaran. 2 of them (HD232862 and OU And) are FK Com candidates. EK Eri, 14 Cet – Ap star descendants? OP And – strong MF, activity and mass loss. Aldebaran – weak MF. Situated high on the RGB: still active, or renovation of activity?

Our ZDI program:

8 G and K giants, both fast and slow rotators. V390 Aur, HD232862, OP And, 37 Com, EK Eri, beta Cet, 77 Tau, Pollux.

For 5 of them we have already maps. EK Eri (Auriere et al., A&A, 2011, published), V390 Aur (Konstantinova-Antova et al., A&A, 2011, accepted), OP And (Konstantinova-Antova et al, 2011, in prep.), beta Cet (Tsvetkova et al., 2011, in prep.), HD232862 - in prep.

V390 Aur = HD 33798:

• G8 III, 2.25 Msol

• Prot = 9.825 d (photometric variability), vsini = 29 km/s

• Fast rotation for a RG and strong magnetic activity (X-rays, CaII H&K emission, flares)

• Rossby number: Prot/τconv = 0.08-0.19

V390 Aur: magnetic map(Konstantinova-Antova et al., 2011, A&A, accepted)

V390 Aur:

i = 56°, Bmean = 18G

∆Ω = 0.048 ± 0.007 rad/d

Axisymmetry: 78%Poloidal: ~80%, dominating dipoleThoroidal: ~20%

Polar spot of positive polarity

Possible gradient of rotation with height in the atmosphere?

Activity indicators:

Bl and activity indicators:

M giants:

9 single M giants with fast rotation and/or X-ray(Konstantinova-Antova et al. 2010, A&A, 547, 57)

Spectral class M0 – M6

The stars were selected on the basis of their faster rotation (Zamanov et al. 2008) and/or X-ray emission (Hunsch et al.

1998, 2004).

M giants:2008-2010

Star Other Name Sp class vsini Lx Detection km/s 10^27erg/s HD130144 EK Boo M5III 11.3 1000 DD/nd HD6860 beta And M0III 5.6 MD/DD HD16058 15 Tri M3III 5.4 525 nd HD18191 RZ Ari M6III 9.6 DD/nd HD150450 42 Her M2.5III 2.5 260 nd HD167006 V669 Her M3III 5.2 DD/nd HD184786 V1743 Cyg M5III 7.8 nd HD187372 M2III 4.4 4400 nd HD219734 8 And M2III 4.9 nd

EK Boo, M5 III:Konstantinova-Antova et al., 2010

Date HJD Detect. B_l sigma CaII K 2450000+ G G03 Apr 08 4560.50 defin. -6.7 1.8 0.3904 Apr 08 4561.53 defin. -3.1 0.5 0.3905 Apr 08 4562.54 defin. -5.0 0.5 0.40 06 Apr 08 4563.51 defin. -4.7 0.7 0.36 20 Dec 08 4821.74 no -0.1 0.6 0.46 21 Dec 08 4822.74 no -0.5 0.6 0.49 25 Feb 09 4888.6 defin. -0.3 0.4 0.57 09 Mar 09 4900.57 defin. -3.8 0.6 0.55 13 Mar 09 4904.68 defin. -4.6 0.4 0.56 18 Mar 09 4909.55 defin. -8.1 0.6 0.57

RZ Ari, M6III:average of 16 spectra obtained on Sept.21 2010

Bl = 3.07 ± 0.35 G

M giants: HRDmodels by Charbonnel&Lagarde, 2010

M giants - Conclusions:

Magnetic field is detected in 4 M giants (EK Boo, RZ Ari, beta And and V669 Her). Bl varies in the interval 1 – 8 Gauss for the different stars. Periods of detection follow periods of non-detection. That presumes non-uniform distribution at the stellar surface.

The detected stars have vsini in the interval 5 – 11 km/s. In fact, stars with the highest vsini in our sample were detected. One of them, EK Boo possess not only the highest vsini, but is also a strong X-ray emitter and has optical evidences for activity (Hunsch et al. 1998, 2004). The rest 3 X-ray emitters aren’t detected, yet.

Dynamo type: the rotation period is not determined for the sample stars. Possible types: α – ω, turbulent dynamo, other type?

Could α – ω dynamo explain the magnetic field in EK Boo? We calculated the Rossby number (Ro), that is indicative for an efficient dynamo action of this type. For a period of 1103d (obtained on the basis of angular diameter and vsini) and convective turnover time of 300d at the base of the convective envelope, we found Ro ≤ 3.6 For this value, we cannot expect very efficient α - ω dynamo for EK Boo, but for Ro ≤ 5 the dynamo might be still operational (Soker&Tylenda 2007).

Turbulent dynamo?The other possibility for dynamo operation on the AGB is the turbulent dynamo, where the turbulent motions play main role for the field amplification and restoration of the poloidal component (Brandenburg, 2002). However, for cool giants such a dynamo is less efficient than the α - ω one. Soker&Zoabi (2002) estimated that in them the magnetic field can reach value less than 1G only. This is not consistent with what we observe in EK Boo and RZ Ari.

Future prospects of the MF study in G,K,M giants:

How common are magnetic fields among G,K and M giants? Are all intermediate mass giants magnetic at a solar level? Does magnetic activity appear only during the dredge-up phases? How long it lasts? Is one and the same type dynamo operating in G,K and M giants? Is there link between the “tepid” MS stars with MF (Petit et al. 2010, 2011) and the active G,K giants? What is the evolution of the Ap star descendants? Our present sample is biased by selection of stars with evidences of activity and/or fast surface rotation. In fact, we took almost all single giants with evidences for activity available for observations with our equipment. A new sample is required to answer these questions.

For the present sample: more observations for period determination (BRITE?). Study of the MF behavior in M giants and “slow” G,K rotators. ZDI for EK Boo and RZ Ari – on way (French PNPS and OPTICON programs).

Additional topics: connection Li- magnetic activity in G,K giants, pulsations – magnetic activity (Miras study for MFs, application to PNPS, PI A. Lebre), exoplanets – magnetic activity.

Our new sample on the HRD in comparison to the previous one

Beta Peg (tip of the RGB)Bl = 3.16 ± 0.26 G

Acknowledgements:

We thank the TBL and CFHT teams for the service observing.

The NARVAL observations are granted with observational time in 2008 and 2011b under OPTICON programs. We acknowledge NARVAL observations under French PNPS program in the period 2009-2011 and CFHT observations under a Canadian program (PI G.Wade). 5 nights observations in 2010 are also funded under Bulgarian NSF contract DSAB 02/3.

R.K-A. acknowledges the possibility to work in LATT, Tarbes, France as invited researcher in spring 2009 and 2011. She also worked there for six months in 2010 as visiting researcher under Bulgarian NSF contract “Sabbaticum” (DSAB 02/3). R.K.-A., S.Ts., I.I., I.S. and R.B acknowledge partial financial support under Bulgarian NSF project DO 02-85. In 2011 the work on the topic was partly supported under the mobility program RILA/EGIDE.

R.K.-A. is thankful to the organizers of the 1st Bcool meeting for the financial support.

Thank you for your attention!

Beta And, M0 III: average of 14 spectra, obtained in September 2008

Bl=-0.95 ± 0.16 G