Magnetic field in a young circumbinary disk
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Universidade Federal de Minas Gerais
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Artigo de periódico
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CONTEXT: Polarized continuum emission at millimeter-to-submillimeter wavelengths is usually attributed to thermal emission from
dust grains aligned through radiative torques with the magnetic field. However, recent theoretical work has shown that under specific
conditions polarization may arise from self-scattering of thermal emission and by radiation fields from a nearby stellar object.
AIMS: We use multi-frequency polarization observations of a circumbinary disk to investigate how the polarization properties change
at distinct frequency bands. Our goal is to discern the main mechanism responsible for the polarization through comparison between
our observations and model predictions for each of the proposed mechanisms.
METHODS: We used the Atacama Large Millimeter/submillimeter Array to perform full polarization observations at 97.5 GHz (Band 3),
233 GHz (Band 6) and 343.5 GHz (Band 7). The ALMA data have a mean spatial resolution of 28 AU. The target is the Class I object
BHB07-11, which is the youngest object in the Barnard 59 protocluster. Complementary Karl G. Jansky Very Large Array observations
at 34.5 GHz were also performed and revealed a binary system at centimetric continuum emission within the disk.
RESULTS: We detect an extended and structured polarization pattern that is remarkably consistent between the three bands. The dis-
tribution of polarized intensity resembles a horseshoe shape with polarization angles following this morphology. From the spectral
index between Bands 3 and 7, we derived a dust opacity index β ∼ 1 consistent with maximum grain sizes larger than expected to
produce self-scattering polarization in each band. The polarization morphology and the polarization levels do not match predictions
from self-scattering. On the other hand, marginal correspondence is seen between our maps and predictions from a radiation field
model assuming the brightest binary component as main radiation source. Previous molecular line data from BHB07-11 indicates disk
rotation. We used the DustPol module of the ARTIST radiative transfer tool to produce synthetic polarization maps from a rotating
magnetized disk model assuming combined poloidal and toroidal magnetic field components. The magnetic field vectors (i.e., the
polarization vectors rotated by 90◦) are better represented by a model with poloidal magnetic field strength about three times the
toroidal one.
Conclusions: The similarity of our polarization patterns among the three bands provides a strong evidence against self-scattering and
radiation fields. On the other hand, our data are reasonably well reproduced by a model of disk with toroidal magnetic field components
slightly smaller than poloidal ones. The residual is likely to be due to the internal twisting of the magnetic field due to the binary system
dynamics, which is not considered in our model.
Abstract
Assunto
Campos magnéticos, Espalhamento, Polarização
Palavras-chave
Magnetic fields, Polarization, Scattering, Protoplanetary disks
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Endereço externo
https://www.aanda.org/articles/aa/full_html/2018/08/aa32935-18/aa32935-18.html