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)

Agnès Lèbre (LUPM, Fr)

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.

A radially pulsating star…

Time

Mag

nitu

de

period ~ 1 yr

… with a very extended atmosphere

Spectral types :M (C/O < 1)C (C/O > 1)S (C/O ~ 1)

From J. Hron

IRC 10216

The Egg NebulaM57The Ring Nebula

The Eskimo Nebula

IRAS 13208-6020

NGC6302 The Bug nebula

Observational evidences for magnetic fields in PNe and around their AGB and post-AGB progenitors

R Scl

HST images

ALMA (ESO/NAOJ/NRAO)

From Reid & Menten, 1997

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)

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)

Magnetic field strength vs. radius relation as indicated by current maser polarization observation of a number of Miras

solar-type magnetic field config.

toroïdal

Detection with Narval At the sub Gauss level !

Herpin et al., 2009

The Mira star χ CYG : Pulsating period ~ 400 days ; M = 2 M_sunSpectral 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 on23 March 2012

174 Stokes V15 to 28 Mar.12

BW Vul (B2)- ESPaDOnS

RR Lyr (F5) - ESPaDOnS

OMI Cet (M5) - Narval

Wavelength

Inte

nsity

(no

t no

rmal

ized

to

the

cont

inuu

m)

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

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. 

The result is (for the complete profile) : Bl= -0.25 ± 0.40 G, Definite Detection (chi2=1.81 , fap=5.2 10-10 )

LSD analysis

174 V sequences

A LSD Zeeman

Signature ?

No Detection Bl = -1.52 ±0.67 G. (chi2=1.18, fap=6.4 10-02 )

Definite DetectionBl= -0.13 ± 0.37 G. (chi2=1.67, fap=9.3 10-08 )

Zeeman origin ?

This supports a (magnetic) Zeeman effect origin.

Mask with lines with high landé factor (mean=1.53)

Mask with lines with low landé factor (mean=0.90)

The detection of the Stokes V structure is more significant when we use a mask with lines of high Landé factor.

The I profile presents the typical line doubling of metallic lines (due to the presence of a shock wave in the lower atmosphere).

The Stokes V signal is associated to the blue component of the I profile

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).

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

The Mira star RT CYG : Pulsating period ~ 190 daysSpectral type : M2@max. light (presence of TiO bands)

Max. light on10 Sept. 2012

20 Stokes V

No signal detected… so far ! We really need the sub gauss level !

11 sept. 2012 17 sept. 2012

Bl = -0.09 ± 0.79 G   No Detection (20 V sequence)

ConclusionsA 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.