Mapping the Linearly Polarized Spectral Line Emission Around the Evolved Star Irc+10216

Girart, J. M.; Patel, Nimesh; Vlemmings, Wouter; Rao, Ramprasad

doi:10.1088/2041-8205/751/1/l20

Cited by 31 publications

(33 citation statements)

References 38 publications

(53 reference statements)

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“…This tends to show that the B los field is not homogeneously strong or aligned in the envelope. This result agrees with the study of the magnetic field using molecules CO, SIS, and CS by Girart et al (2012), which suggests that the magnetic field morphology is possibly complex (the positions they studied are within the CN ring). A more detailed interferometric map of the magnetic field is mandatory to make any substantial conclusions, especially as a new spiral structure was recently discovered (Cernicharo et al 2015).…”

Section: Comparison With Other Observations and Implications For The supporting

confidence: 89%

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

Duthu

Herpin

Wiesemeyer

et al. 2017

A&A

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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.

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Section: Comparison With Other Observations and Implications For The supporting

confidence: 89%

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

Duthu

Herpin

Wiesemeyer

et al. 2017

A&A

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“…of linear polarization in CO J = 3-2, SiS J = 19-18 and CS J = 7-6 suggest a complex magnetic field configuration in the wind of CW Leo with a strength of about 50-300 mG at ∼3 offset of the central source (Girart et al 2012), and recent interferometric data show strong evidence for inhomogeneities in the molecular photospheric layers of AGB stars (Wittkowski et al 2011). In addition, modulations in the density structure might be a natural outcome from the dust formation process during which a complex non-linear interplay between gas-grain drift, grain nucleation, radiation pressure, and envelope hydrodynamics create density irregularities.…”

Section: Qualitative Interpretation Of the 13 Co J = 6-5 Alma Datamentioning

confidence: 93%

ALMA data suggest the presence of spiral structure in the inner wind of CW Leonis

Decin

Richards

Neufeld

et al. 2015

A&A

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Context. Evolved low-mass stars lose a significant fraction of their mass through stellar winds. While the overall morphology of the stellar wind structure during the asymptotic giant branch (AGB) phase is thought to be roughly spherically symmetric, the morphology changes dramatically during the post-AGB and planetary nebula phase, during which bipolar and multi-polar structures are often observed. Aims. We aim to study the inner wind structure of the closest well-known AGB star CW Leo. Different diagnostics probing different geometrical scales have implied a non-homogeneous mass-loss process for this star: dust clumps are observed at milli-arcsec scale, a bipolar structure is seen at arcsecond-scale, and multi-concentric shells are detected beyond 1 . Methods. We present the first ALMA Cycle 0 band 9 data around 650 GHz (450 μm) tracing the inner wind of CW Leo. The fullresolution data have a spatial resolution of 0. 42 × 0. 24, allowing us to study the morpho-kinematical structure of CW Leo within ∼6 . Results. We have detected 25 molecular emission lines in four spectral windows. The emission of all but one line is spatially resolved. The dust and molecular lines are centered around the continuum peak position, which is assumed to be dominated by stellar emission. The dust emission has an asymmetric distribution with a central peak flux density of ∼2 Jy. The molecular emission lines trace different regions in the wind acceleration region and imply that the wind velocity increases rapidly from about 5 R , almost reaching the terminal velocity at ∼11 R . The images prove that vibrational lines are excited close to the stellar surface and that SiO is a parent molecule. The channel maps for the brighter lines show a complex structure; specifically, for the 13 CO J = 6-5 line, different arcs are detected within the first few arcseconds. The curved structure in the position-velocity (PV) map of the 13 CO J = 6-5 line can be explained by a spiral structure in the inner wind of CW Leo, probably induced by a binary companion. From modelling the ALMA data, we deduce that the potential orbital axis for the binary system lies at a position angle of ∼10-20• to the north-east and that the spiral structure is seen almost edge-on. We infer an orbital period of 55 yr and a binary separation of 25 au (or ∼8.2 R ). We tentatively estimate that the companion is an unevolved low-mass main-sequence star. Conclusions. A scenario of a binary-induced spiral shell can explain the correlated structure seen in the ALMA PV images of CW Leo. Moreover, this scenario can also explain many other observational signatures seen at different spatial scales and in different wavelength regions, such as the bipolar structure and the almost concentric shells. ALMA data hence for the first time provide the crucial kinematical link between the dust clumps seen at milli-arcsecond scale and the almost concentric arcs seen at arcsecond scale.

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“…Frau et al 2011) making it possible to infer the projected magnetic field morphology. Hence, polarization measurements provide a S. Reissl et al: The origin of dust polarization in molecular outflows promising tool to determine the morphology and subsequently to investigate of the role of magnetic fields in the evolution (Hildebrand et al 2000;Crutcher 2004;Girart et al 2012) of star-forming systems. The difficulty is that the reliability of polarization measurements and their interpretation depends on a wide range of physical parameters that are still discussed (see Andersson et al 2015, for review).…”

Section: Introductionmentioning

confidence: 99%

The origin of dust polarization in molecular outflows

Reißl

Seifried

Wolf

et al. 2017

A&A

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Aims. Polarization measurements of dust grains aligned with the magnetic field direction are a established technique to trace largescale field structures. In this paper we present a case study to investigate conditions necessary to detect a characteristic magnetic field substructure embedded in such a large-scale field. A helical magnetic field with a surrounding hourglass shaped field is expected from theoretical predictions and self-consistent magnetohydrodynamical (MHD) simulations to be present in the specific case of protostellar outflows. Hence, such a outflow environment is the perfect environment for our study. Methods. We present synthetic polarisation maps in the infrared and millimeter regime of simulations of protostellar outflows performed with the newly developed radiative transfer and polarisation code POLARIS. The code, as the first, includes a self-consistent description of various alignement mechanism like the imperfect Davis-Greenstein (IDG) and the radiative torque (RAT) alignment. We investigate for which effects the grain size distribution, inclination, and applied alignement mechanism have. Results. We find that the IDG mechanism cannot produce any measurable polarization degree (≥ 1 %) whereas RAT alignment produced polarization degrees of a few 1 %. Furthermore, we developed a method to identify the origin of the polarization. We show that the helical magnetic field in the outflow can only be observed close to the outflow axis and at its tip, whereas in the surrounding regions the hourglass field in the foreground dominates the polarization. Furthermore, the polarization degree in the outflow lobe is lower than in the surroundings in agreement with observations. We also find that the orientation of the polarization vector flips around a few 100 µm due to the transition from dichroic extinction to thermal re-emission. Hence, in order to avoid ambiguities when interpreting polarization data, we suggest to observed in the far-infrared and mm regime. The actual grain size distribution has only little effect on the emerging polarization maps. Finally, we show that with ALMA it is possible to observe the polarized radiation emerging from protostellar outflows.

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Mapping the Linearly Polarized Spectral Line Emission Around the Evolved Star Irc+10216

Cited by 31 publications

References 38 publications

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

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

ALMA data suggest the presence of spiral structure in the inner wind of CW Leonis

The origin of dust polarization in molecular outflows

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