J/ApJ/842/L9 ALMA obs. of polarized dust emission from Ser-emb 8 (Hull+, 2017)
Unveiling the role of the magnetic field at the smallest scales of star
formation.
Hull C.L.H., Mocz P., Burkhart B., Goodman A.A., Girart J.M., Cortes P.C.,
Hernquist L., Springel V., Li Z.-Y., Lai S.-P.
<Astrophys. J., 842, L9 (2017)>
=2017ApJ...842L...9H 2017ApJ...842L...9H
ADC_Keywords: Polarization ; YSOs ; Interstellar medium ; Magnetic fields ;
Millimetric/submm sources
Keywords: ISM: magnetic fields; magnetic fields; magnetohydrodynamics (MHD);
polarization ; stars: formation ; turbulence
Abstract:
We report Atacama Large Millimeter/submillimeter Array (ALMA)
observations of polarized dust emission from the protostellar source
Ser-emb 8 at a linear resolution of 140au. Assuming models of
dust-grain alignment hold, the observed polarization pattern gives a
projected view of the magnetic field structure in this source.
Contrary to expectations based on models of strongly magnetized star
formation, the magnetic field in Ser-emb 8 does not exhibit an
hourglass morphology. Combining the new ALMA data with previous
observational studies, we can connect magnetic field structure from
protostellar core (∼80000au) to disk (∼100au) scales. We compare our
observations with four magnetohydrodynamic gravo-turbulence
simulations made with the AREPO code that have initial conditions
ranging from super-Alfvenic (weakly magnetized) to sub-Alfvenic
(strongly magnetized). These simulations achieve the spatial dynamic
range necessary to resolve the collapse of protostars from the parsec
scale of star-forming clouds down to the ∼100au scale probed by ALMA.
Only in the very strongly magnetized simulation do we see both the
preservation of the field direction from cloud to disk scales and an
hourglass-shaped field at <1000au scales. We conduct an analysis of
the relative orientation of the magnetic field and the density
structure in both the Ser-emb 8 ALMA observations and the synthetic
observations of the four AREPO simulations. We conclude that the
Ser-emb 8 data are most similar to the weakly magnetized simulations,
which exhibit random alignment, in contrast to the strongly magnetized
simulation, where the magnetic field plays a role in shaping the
density structure in the source. In the weak-field case, it is
turbulence-not the magnetic field-that shapes the material that forms
the protostar, highlighting the dominant role that turbulence can play
across many orders of magnitude in spatial scale.
Description:
The 870um (Band 7) ALMA dust polarization observations of Ser-emb 8
were taken on 2015 June 3 and 7 and have ∼0.3" angular resolution.
Objects:
-------------------------------------------------------------
RA (ICRS) DE Designation(s)
-------------------------------------------------------------
18 29 48.09 +01 16 45.0 Ser-emb 8 = [EES2009] Ser-emb 8
-------------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 40 191 870um (Band 7) ALMA data
img/* . 6 The five maps in FITS format which served to create
Figure 1
--------------------------------------------------------------------------------
Description of file:
The subdirectory img/ (in the FTP section) contains the 3 cleaned maps
of total intensity (Stokes I), Stokes Q and Stokes U dust emission. A map
of the position angle of the true dust polarization. And a map of total
debiased polarized dust emission. The five files are in FITS format.
See also:
J/ApJ/603/584 : Polarimetry toward Musca dark cloud (Pereyra+, 2004)
J/ApJ/692/973 : Protostars in Perseus, Serpens and Ophiuchus (Enoch+, 2009)
J/ApJS/182/143 : SCUPOL Legacy polarimetry of SCUBA (Matthews+, 2009)
J/ApJS/186/406 : 350um polarimetry from CSO (Dotson+, 2010)
J/A+A/569/L1 : Polarization hole in a starless core (Alves+, 2014)
J/ApJS/213/13 : Polarization maps of star-forming cores & SFRs (Hull+, 2014)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right ascension in decimal degrees (J2000)
11- 17 F7.5 deg DEdeg Declination in decimal degrees (J2000)
19- 23 F5.1 deg chi [0.4/178] Orientation of the magnetic field,
measured counterclockwise from north
25- 27 F3.1 deg e_chi [1.2/9.5] Uncertainty in chi
29- 33 F5.3 mJy/beam PInt [0.07/0.6] Polarized intensity
35- 40 F6.3 mJy/beam Int ? Total Intensity (1)
--------------------------------------------------------------------------------
Note (1): Int is the total intensity, reported where I>3σ. Due to
differences in dynamic range between the images of Stokes I and
polarized intensity, there are cases where PInt is detectable
but Int is not.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 02-Feb-2018