J/A+A/616/A56 [BHB2007] 11 full stokes continuum ALMA images (Alves+, 2018)
Magnetic field in a circumbinary disk.
Alves F.O., Girart J.M., Padovani M., Galli D., Franco G.A.P., Caselli. P.,
Vlemmings W.H.T., Zhang Q., Wiesemeyer H.
<Astron. Astrophys. 616, A56 (2018)>
=2018A&A...616A..56A 2018A&A...616A..56A (SIMBAD/NED BibCode)
ADC_Keywords: YSOs ; Magnetic fields ; Polarization
Keywords: magnetic fields - polarization - scattering -
instrumentation: interferometers - techniques: polarimetric -
protoplanetary disks
Abstract:
Polarized continuum emission at millimeter/sub-millimeter 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. 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. We used the Atacama Large
Millimeter/submillimeter Array to perform full polarization
observations at 97.5GHz (Band 3), 233GHz (Band 6) and 343.5GHz
(Band 7). The ALMA data have a mean spatial resolution of 28AU. 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 34.5GHz were also performed and revealed a
binary system at centimetric continuum emission within the disk. We
detect an extended and structured polarization pattern remarkably
consistent among all three bands. The distribution of polarized
intensity resembles a horseshoe shape with polarization angles
following this morphology. From the spectral index between bands 3 and
7, we derive a dust opacity index beta ∼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 radiation field 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 degrees) are better represented
by a model with poloidal magnetic field strength about 3 times the
toroidal one. 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 due to the
internal twisting of the magnetic field due to the binary system
dynamics, which is not considered in our model.
Description:
The three fits files consist of full polarization stokes image of
BHB07-11 for each ALMA frequency band. Each fits file contains three
panels: Stokes I (total intensity), Stokes Q and Stokes U (associated
with the linear polarization).
Objects:
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RA (2000) DE Designation(s)
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17 11 23.18 -27 24 31.5 BHB07-11 = [BHB2007] 11
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 143 3 List of fits images
fits/* . 3 Individual images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28 I1 --- Nz Number of panels
30- 55 A26 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.ssssss)
57- 64 F8.4 GHz Freq Observed frequency
66- 69 I4 Kibyte size Size of FITS file
71- 82 A12 --- FileName Name of FITS file, in subdirectory fits
84-143 A60 --- Title Title of the FITS file
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Acknowledgements:
Felipe de Oliveira Alves, falves(at)mpe.mpg.de
(End) Patricia Vannier [CDS] 07-May-2018