The magnetic field of the evolved star W43A

Amiri, N.; Vlemmings, Wouter; Langevelde, H. J. van

doi:10.1051/0004-6361/200913194

Cited by 25 publications

(26 citation statements)

References 27 publications

(41 reference statements)

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“…Additionally, polarimetric observations of OH masers reveals large scale magnetic field strength in evolved stars ranging from 0.1 mG to 10 mG (e.g. Etoka & Diamond 2004;Amiri et al 2010). …”

Section: Cse Morphology and Magnetic Fieldmentioning

confidence: 99%

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Amiri

Vlemmings

Kemball

et al. 2012

A&A

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Context. SiO maser emission occurs in the extended atmosphere of evolved stars and can be studied at high angular resolution. As compact, high-brightness components they can be used as important tracers of the dynamics at distances close to the central star. The masers also serve as probes of the evolutionary path from spherically symmetric AGB stars to aspherical PNe. Very long baseline interferometry (VLBI) observations of Mira variables indicate that SiO masers are significantly linearly polarized with linear polarization fractions up to 100%. However, no information is available at high angular resolution for SiO masers in higher mass loss OH/IR stars. Theory indicates a different SiO pumping mechanism in higher mass loss evolved stars. Aims. We extend the VLBI SiO maser studies to OH/IR stars. The observations enable us to understand the SiO pumping mechanisms in higher mass-loss evolved objects and to compare them with Mira variables. Additionally, polarimetric observations of SiO masers help us to understand the magnetic field strength and morphology and to distinguish between conflicting polarization theories. Methods. The 43 GHz SiO maser observations of the OH/IR star OH 44.8-2.3 were performed with the VLBA in full polarization spectral line mode. Auxiliary EVLA observations were performed to allow for the absolute calibration of the polarization angle. The Zeeman splitting was measured by cross correlating the right and left circular polarization spectra, as well as the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser observations of OH 44.8-2.3 from the VLA archive. Results. The SiO masers of OH 44.8-2.2 form a ring located at ∼5.4 AU around the star. The masers appear to be highly linearly polarized with fractional linear polarization up to 100%. The linear polarization vectors are consistent with a dipole field morphology in this star. We report a tentative detection of circular polarization of ∼0.7% for the brightest maser feature. The magnetic field measured for this feature corresponds to 1.5 ± 0.3 G. Additionally, the distribution of the 1612 MHz OH maser emission could indicate an elongated morphology. Conclusions. The SiO masers in OH 44.8-2.3 exhibit a ring morphology. Even though the central AGB star of OH 44.8-2.3 is expected to be larger than typical Mira variables, the SiO masers occur at a similar distance from the stellar photosphere as Mira variables. The SiO masers and the 1612 MHz OH maser emission suggest a mildly preferred direction of the outflow in the CSE of this star. Significant linear polarization is measured for the SiO region of this star, which could develop from either collisional or radiative pumping. In either case, the observed polarization is also consistent with magnetic field structures along the preferred outflow direction. This could reflect the possible role of the magnetic field in shaping the circumstellar environment of this object. Although we cannot firmly distinguish between the different polarization theories, the derived magnetic...

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Section: Cse Morphology and Magnetic Fieldmentioning

confidence: 99%

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

Amiri

Vlemmings

Kemball

et al. 2012

A&A

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“…We can cite the detection of large scale magnetic fields in the environment of PAGB stars and PNe via dust polarisation analysis by Greaves (2002) and Sabin et al (2007Sabin et al ( , 2014, the detection of synchrotron emission in the PAGB star IRAS 15445-5449 by Pérez-Sánchez et al (2013), the detection of a magnetically collimated jet in the evolved star W43A by Vlemmings et al (2006), the measurement of linear and/or circular polarisation of maser emission by e.g. Etoka et al (2009), Leal-Ferreira et al (2013) and Amiri et al (2010) and the presence of filamentary structures (Huggins & Manley 2005). In particular, several of the aforementioned investigations underlined the ability of the field to launch and collimate outflows.…”

Section: Introductionmentioning

confidence: 99%

First detection of surface magnetic fields in post-AGB stars: the cases of U Monocerotis and R Scuti

Sabin¹,

Wade²,

Lèbre³

2014

Monthly Notices of the Royal Astronomical Society

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While several observational investigations have revealed the presence of magnetic fields in the circumstellar envelopes, jets and outflows of post-Asymptotic Giant Branch stars (PAGBs) and planetary nebulae (PNe), none has clearly demonstrated their presence at the stellar surface. The lack of information on the strength of the surface magnetic fields prevents us from performing any thorough assessment of their dynamic capability (i.e. material mixing, envelope shaping, etc). We present new high resolution spectropolarimetric (Stokes V ) observations of a sample of PAGB stars, realised with the instruments ESPaDOnS and Narval, where we searched for the presence of photospheric magnetic fields. Out of the seven targets investigated the RV Tauri stars U Mon and R Sct display a clear Zeeman signature and return a definite detection after performing a least squares deconvolution (LSD) analysis. The remaining five PAGBs show no significant detection. We derived longitudinal magnetic fields of 10.2 ± 1.7 G for U Mon and 0.6 ± 0.6 G for R Sct. In both cases the Stokes profiles point towards an interaction of the magnetic field with the atmosphere dynamics. This first discovery of weak magnetic fields (i.e. ∼10 gauss level) at the stellar surface of PAGB stars opens the door to a better understanding of magnetism in evolved stars.

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“…In particular, interferometric observations of the masers at radio wavelengths have become a useful tool for studying the magnetic field in and around SFRs and late-type stars (e.g. Fish & Reid 2007a;Amiri et al 2010). At shorter wavelengths, new instruments will enable the study of maser radiation from higher vibrationally-excited rotational transitions.…”

Section: Introductionmentioning

confidence: 99%

Linear polarization of submillimetre masers

Pérez-Sánchez

Vlemmings

2013

A&A

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Context. Once ALMA full polarization capabilities are offered, it will become possible to perform detailed studies of polarized maser emission towards star-forming regions and late-type stars, such as (post-) asymptotic giant branch stars and young planetary nebulae. To derive the magnetic field orientation from maser linear polarization, a number of conditions involving the rate of stimulated emission R, the decay rate of the molecular state Γ, and the Zeeman frequency gΩ need to be satisfied. Aims. The goal of this work is to investigate if SiO, H 2 O and HCN maser emission within the ALMA frequency range can be detected with observable levels of fractional linear polarization in the regime where the Zeeman frequency is greater than the stimulated emission rate. Methods. We used a radiative transfer code to calculate the fractional linear polarization as a function of the emerging brightness temperature for a number of rotational transition of SiO, H 2 O and HCN that have been observed to display maser emission at submillimetre wavelengths. We assumed typical magnetic field strengths measured towards galactic star-forming regions and circumstellar envelopes of late-type stars from previous VLBI observations. Since the Landé g-factors have not been reported for the different rotational transitions we modelled, we performed our calculations assuming conservative values of the Zeeman frequency for the different molecular species. Results. Setting a lower limit for the Zeeman frequency that still satisfies the criteria gΩ > R and gΩ > Γ, we find fractional polarization levels of up to 13%, 14% and 19% for the higher J transitions analysed for SiO, H 2 O and HCN, respectively, without considering anisotropic pumping or any other non-Zeeman effect. These upper limits were calculated assuming a magnetic field oriented perpendicular to the direction of propagation of the maser radiation. Conclusions. According to our results, SiO, H 2 O, and HCN maser emission within the ALMA frequency range can be detected with suitable linear polarization to trace the magnetic field structure towards star-forming regions and late-type stars even if the detected polarization has been enhanced by non-Zeeman effects.

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The magnetic field of the evolved star W43A

Cited by 25 publications

References 27 publications

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

VLBA SiO maser observations of the OH/IR star OH 44.8-2.3: magnetic field and morphology

First detection of surface magnetic fields in post-AGB stars: the cases of U Monocerotis and R Scuti

Linear polarization of submillimetre masers

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