Search for magnetic fields at the surface of Mira stars

1. Agnès Lèbre (LUPM, Fr) Search for magnetic fields at the surface of Mira stars Michel Aurière & Pascal Petit (IRAP, Fr) Nicolas Fabas (IAC, Sp) Denis Gillet (OAMP, Fr) Fabrice Herpin (LAB, Rr) Renada Konstantinova-Antova (BAS, Bu)

2. Mira stars are cool and evolved pulsating stars, belonging to the tip of the Asymptotic Giant Branch (AGB), the key evolutionary stage of an intermediate mass star before its transition toward the Planetary Nebulae (PNe) stage.

3. … with a very extended atmosphere A radially pulsating star… period ~ 1 yr Magnitude Time Spectral types : M (C/O < 1) C (C/O > 1) S (C/O ~ 1)

4. From J. Hron

5. IRAS 13208-6020 The Egg Nebula M57 The Ring Nebula IRC 10216 R Scl The Eskimo Nebula NGC6302 The Bug nebula ALMA (ESO/NAOJ/NRAO) Observational evidences for magnetic fields in PNe and around their AGB and post-AGB progenitors HST images

6. Magnetic fields throughout the CSE of AGB Detected and measured from the polarisation of the maser emission of several molecules : OH at 1000-10000 a.u. B// ~ 5-20 mG (Rudnitski et al., 2010) Water at a few 100 a.u. B// ~ a few 100 mG (Vlemmings et al., 2005) SiO at 5-10 a.u. B// ~ 3.5 G (Herpin et al. 2006) From Reid & Menten, 1997

7. Magnetic field strength vs. radius relation as indicated by current maser polarization observation of a number of Miras Extrapolating the 1/r law toward the photosphere → the magnetic field strength at the stellar surface of Mira stars could be of the order of a few G. (From Vlemmings et al., 2011) solar-type magnetic field config. toroïdal Detection with Narval At the sub Gauss level ! Herpin et al., 2009

8. The Mira star χ CYG : Pulsating period ~ 400 days ; M = 2 M_sun Spectral type : from S6 to S10 (presence of ZrO bands) A magnetic field has already been detected in the inner part of its CSE : Mean value of the magnetic field along the line of sight : B// = 0 - 8.8 G at 5-10 a.u. (from SiO maser and Elitzur’s theory, Herpin et al. 2006 ) Max. light on 23 March 2012 174 Stokes V 15 to 28 Mar.12

9. BW Vul (B2)- ESPaDOnS RR Lyr (F5) - ESPaDOnS Intensity (not normalized to the continuum) OMI Cet (M5) - Narval Wavelength LSD analysis with a specific numeric mask (Teff = 3500 K and log g = 0.5) involving about 14 000 atomic lines with atomic parameters and Landé factors from VALD

10. LSD analysis 174 V sequences A LSD Zeeman Signature ? The result is (for the complete profile) : Bl= -0.25 ± 0.40 G, Definite Detection (chi2=1.81 , fap=5.2 10-10 ) The weak Stokes V signal is present if we split our spectra in two series of equal significance,  and does not appear on the null polarization profiles. 

11. Zeeman origin ? Mask with lines with high landé factor (mean=1.53) The detection of the Stokes V structure is more significant when we use a mask with lines of high Landé factor. Definite Detection Bl= -0.13 ± 0.37 G. (chi2=1.67, fap=9.3 10-08 ) This supports a (magnetic) Zeeman effect origin. Mask with lines with low landé factor (mean=0.90) No Detection Bl = -1.52 ±0.67 G. (chi2=1.18, fap=6.4 10-02 )

12. The Stokes V signal is associated to the blue component of the I profile The I profile presents the typical line doubling of metallic lines (due to the presence of a shock wave in the lower atmosphere). The longitudinal magnetic field computed using the first-order moment method (Rees and Semel 1979) adapted to LSD profiles (Donati et al. 1997) and using the complete Stokes I profile : Bl = - 0.25 ± 0.4 G → lower limit ! Scaling directly the Stokes V signal and the blue (I) profile to those which are observed in the K0III star Pollux (classical Zeeman profile), the Chi Cyg detection in the blue profile could correspond to the detection of a magnetic field with Bl of a few Gauss (2-3 G).

13. A link with the periodic shock wave ? The Stokes V signature is associated to the blue component of the I profile, i.e., to the material which is driven outward by the shock → is likely linked to the shock : - either it may originate directly because of it, or - it may be due to the amplification -by the shock- of a weak photospheric magnetic field. → Monotoring of Chi Cyg with ESPaDOnS@CFHT around its 2013 maximum light, focusing on specific strategic phases: - before the maximum light, when the shock wave has not yet emerged from the photosphere; - at the maximum light, when the star is the brightest and its photosphere is the hottest; - after the maximum light, when the shock wave has emerged from the photosphere, and is accelerated in the lowest part of the atmosphere. Light curve of chi Cyg

14. The Mira star RT CYG : Pulsating period ~ 190 days Spectral type : M2@max. light (presence of TiO bands) 20 Stokes V Max. light on 10 Sept. 2012

15. Bl = -0.09 ± 0.79 G   No Detection (20 V sequence) 11 sept. 2012 17 sept. 2012 No signal detected… so far ! We really need the sub gauss level !

16. Conclusions A weak magnetic field may exist at the surface of Mira stars. It could play a part in the evolution of the morphology from AGB to PN (besides the role of a companion, for example). A Stokes V signal has been detected on chi Cyg, a bright S-type Mira, when observed at its maximum light However, the possible link with the atmospheric shock wave needs to be investigated. This represents the first detection of a magnetic field at the surface of a Mira star, in complement to the previous detections of magnetic field throughout the CSE.