Magnetic field in IRC+10216 and other C-rich evolved stars

Abstract

Context. During the transition from the asymptotic giant branch (AGB) to planetary nebulae (PN), the circumstellar geometry and morphology change dramatically. Another characteristic of this transition is the high mass-loss rate, that can be partially explained by radiation pressure and a combination of various factors, such as the stellar pulsation, the dust grain condensation, and opacity in the upper atmosphere. The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope. Aims. Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216. More generally we intend to determine the magnetic field strength in the circumstellar envelope (CSE) of C-rich evolved stars, compare this field with previous studies for O-rich stars, and constrain the variation of the magnetic field with r the distance to the star's centre. Methods. We use spectropolarimetric observations of the Stokes V parameter, collected with Xpol on the IRAM-30 m radiotelescope, observing the Zeeman effect in seven hyperfine components of the CN J = 1-0 line. We use the Crutcher et al. (1996, ApJ, 456, 217) method to estimate the magnetic field. For the first time, the instrumental contamination is investigated, through dedicated studies of the power patterns in Stokes V and I in detail.Results. For C-rich evolved stars, we derive a magnetic field strength (B) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN (PPN) AFGL618, and an upper value of 8 mG is found for the PN NGC 7027. These results are consistent with a decrease of B as 1/r in the environment of AGB objects, that is, with the presence of a toroidal field. But this is not the case for PPN and PN stars. Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time.

Mass loss of stars on the asymptotic giant branch

“…The maser results are notoriously difficult to interpret, but they suggest (when extrapolated) magnetic field strengths in the range 10 0−2 G and a radial dependence in between r −2 (solar-type) and r −1 (toroidal) for M-stars (Vlemmings 2014). The CN results on C-rich objects are consistent with a stellar magnetic field strength of a few Gauss and a toroidal field (Duthu et al 2017). The large-scale structure remains uncertain, but future observations of, e.g., the polarization of CO line emission with ALMA may make good progress here for AGB stars of all chemical types.…”

“…To measure directly the magnetic field of an AGB star has turned out to be difficult, but a recent result on the S-star χ Cyg, indicating a field strength of a few Gauss (Lèbre et al 2014), suggests that major progress may be done in this area in the future. Magnetic field strength estimates in CSEs are based on maser lines of SiO, H 2 O, and OH, and on "normal" line emission from CN through the Zeeman effect (Duthu et al 2017) and from CO through the Goldreich-Kylafis effect . The maser results are notoriously difficult to interpret, but they suggest (when extrapolated) magnetic field strengths in the range 10 0−2 G and a radial dependence in between r −2 (solar-type) and r −1 (toroidal) for M-stars (Vlemmings 2014).…”

Constraining Theories of Polarized SiO Maser Transport: Multi-epoch Analysis of a π/2 Electric Vector Rotation Feature

“…These factors may be further influenced by the magnetic field, but the magnitude, origin, and dynamical influence remains uncertain (Denissenkov & Pinsonneault 2007;Blackman et al 2001;Soker & Zoabi 2002;Nordhaus et al 2007;Nordhaus & Blackman 2006;Fabas et al 2011;Leal-Ferreira et al 2013). Both magnetic fields and binary companions have been proposed as dynamically significant in the distribution and kinematics of the expelled mass (Matt et al 2000;Blackman et al 2000Blackman et al , 2001Nordhaus et al 2007;Nordhaus & Blackman 2006;Duthu et al 2017), and could play a role in shaping the resulting planetary nebula and the return of processed material to the ISM (Blackman et al 2001).…”

Constraining Stellar Parameters and Atmospheric Dynamics of the Carbon AGB star V Oph

“…The surface magnetic field has been measured on a number of giant and supergiant stars and post AGB stars at the strength of a few Gauss (e.g. Sabin et al 2015;Vlemmings 2014;Duthu et al 2017;Vlemmings 2018); this suggests that rigorous convection could excite Alfvén waves as on the Sun. V Oph molecular shell contains gas consisting of mainly hydrogen, and C 2 H 2 .…”