J/A+A/630/A54 G31.41+0.31 Stokes IQU images (Beltran+, 2019)
ALMA resolves the hourglass magnetic field in G31.41+0.31.
Beltran M.T., Padovani M., Girart J.M., Galli D., Cesaroni R., Paladino R.,
Anglada G., Estalella R., Osorio M., Rao R., Sanchez-Monge A., Zhang Q.
<Astron. Astrophys. 630, A54 (2019)>
=2019A&A...630A..54B 2019A&A...630A..54B (SIMBAD/NED BibCode)
ADC_Keywords: H II regions ; Magnetic fields ; Polarization
Keywords: ISM: individual objects: G31.41+0.31 - ISM: magnetic fields -
stars: formation - techniques: interferometric
Abstract:
Submillimeter Array (SMA) 870um polarization observations of the hot
molecular core G31.41+0.31 revealed one of the clearest examples up to
date of an hourglass-shaped magnetic field morphology in a high-mass
star-forming region.
To better establish the role that the magnetic field plays in the
collapse of G31.41+0.31, we carried out Atacama Large
Millimeter/submillimeter Array (ALMA) observations of the polarized
dust continuum emission at 1.3mm with an angular resolution four times
higher than that of the previous (sub)millimeter observations to
achieve an unprecedented image of the magnetic field morphology.
We used ALMA to perform full polarization observations at 233GHz
(Band 6). The resulting synthesized beam is 00.28"x00.20" which, at
the distance of the source, corresponds to a spatial resolution of
875au.
The observations resolve the structure of the magnetic field in
G31.41+0.31 and allow us to study the field in detail. The polarized
emission in the Main core of G31.41+0.41 is successfully fit with a
semi-analytical magnetostatic model of a toroid supported by magnetic
fields. The best fit model suggests that the magnetic field is well
represented by a poloidal field with a possible contribution of a
toroidal component of 10% of the poloidal component, oriented
southeast to northwest at ~-44° and with an inclination of
~-45°. The magnetic field is oriented perpendicular to the
northeast to southwest velocity gradient detected in this core on
scales from 103-104au. This supports the hypothesis that the
velocity gradient is due to rotation of the core and suggests that
such a rotation has little effect on the magnetic field. The strength
of the magnetic field estimated in the central region of the core with
the Davis-Chandrasekhar-Fermi method is ∼8-13mG and implies that the
mass-to-flux ratio in this region is slightly supercritical
(λ=1.4-2.2).
The magnetic field in G31.41+0.31 maintains an hourglass-shaped
morphology down to scales of <1000au. Despite the magnetic field
being important in G31.41+0.31, it is not enough to prevent
fragmentation and collapse of the core, as demonstrated by the
presence of at least four sources embedded in the center of the core.
Description:
Interferometric full polarization observations of G31 were carried out
with ALMA in Cycle 4 on July 12, 2017 as part of project
2015.1.00072.S (P.I.: M. Beltran). The total observing time was
divided into two execution blocks. We used ALMA in full polarization
mode and observed all four cross correlations using a spectral setup
with four 2GHz spectral windows of 64 channels each (TDM mode with
31.25MHz resolution per channel). From the (XX, XY, YX, and YY)
visibilities we obtained the Stokes I, Q, and U in the image plane.
The observations were performed in Band 6 centered at 233GHz and with
the array in the C40-5 configuration. The baselines of the
observations range from 17 to 2647m. The phase reference center of the
observations is RA(J2000)=18:47:34.30, DE(J2000)=-01:12:45:90. Phase
calibration was performed on quasar J1851+0035, while flux and
bandpass calibrations were performed on quasar J1751+0939. Quasar
J1924-2914 was observed to determine the instrumental contribution to
the cross-polarized interferometer response.
Objects:
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RA (2000) DE Designation(s)
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18 47 34.49 -01 12 40.3 G31.41+0.31 = MSX6C G031.4134+00.3092
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 128 3 List of fits images
fits/* . 3 Individual fits images
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See also:
J/A+A/598/A59 : G31.41+0.31 SMA spectra (Rivilla+, 2017)
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- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 55 A26 "datime" Obs.date Observation date
57- 61 I5 Kibyte size Size of FITS file
63-101 A39 --- FileName Name of FITS file, in subdirectory fits
103-128 A26 --- Title Title of the FITS file
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Acknowledgements:
Maria Beltran, mbeltran(at)arcetri.astro.it
(End) Patricia Vannier [CDS] 10-Sep-2019