15 research outputs found
Physical conditions for dust grain alignment in Class 0 protostellar cores II. The role of the radiation field in models aligning/disrupting dust grains
The polarized dust emission observed in Class 0 protostellar cores at high
angular resolution with ALMA has raised several concerns about the grain
alignment conditions in these regions. We aim to study the role of the
radiation field on the grain alignment mechanisms occurring in the interior
(<1000 au) of Class 0 protostars. We produce synthetic observations of the
polarized dust emission from a MHD model of protostellar formation, using the
POLARIS dust radiative transfer tool, which includes dust alignment with
Radiative Torques Alignment (RATs). We test how the polarized dust emission
from the model core depends on the irradiation conditions in the protostellar
envelope, by varying the radiation due to accretion luminosity propagating from
the central protostellar embryo throughout the envelope. The level of grain
alignment efficiency obtained in the radiative transfer models is then compared
to (sub-) millimeter ALMA dust polarization observations of Class 0 protostars.
Our radiative transfer calculations have a central irradiation that reproduces
the protostellar luminosities typically observed towards low- to
intermediate-mass protostars, as well as super-paramagnetic grains, and grains
>10 micron, which are required to bring the dust grain alignment efficiencies
of the synthetic observations up to observed levels. Our radiative transfer
calculations show that irradiation plays an important role in the mechanisms
that dictate the size range of aligned grains in Class 0 protostars. Regions of
the envelope that are preferentially irradiated harbor strong polarized dust
emission but can be affected by the rotational disruption of dust grains.
Episodes of high luminosity could affect grain alignment and trigger grain
disruption mechanisms. [abridged
Characterizing Magnetic Field Morphologies in Three Serpens Protostellar Cores with ALMA
With the aim of characterizing the dynamical processes involved in the formation of young protostars, we present high-angular-resolution ALMA dust polarization observations of the Class 0 protostellar cores Serpens SMM1, Emb 8(N), and Emb 8. With spatial resolutions ranging from 150 to 40 au at 870 ÎŒm, we find unexpectedly high values of the polarization fraction along the outflow cavity walls in Serpens Emb 8(N). We use 3 mm and 1 mm molecular tracers to investigate outflow and dense-gas properties and their correlation with the polarization. These observations allow us to investigate the physical processes involved in the radiative alignment torques (RATs) acting on dust grains along the outflow cavity walls, which experience irradiation from accretion processes and outflow shocks. The inner core of SMM1-a presents a polarization pattern with a poloidal magnetic field at the bases of the two lobes of the bipolar outflow. To the south of SMM1-a we see two polarized filaments, one of which seems to trace the redshifted outflow cavity wall. The other may be an accretion streamer of material infalling onto the central protostar. We propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present at <500 au scales in Class 0 envelopes have grown larger than the 0.1 ÎŒm size of dust grains in the interstellar medium. Our observations allow us to constrain the magnetic field morphologies of star-forming sources within the central cores, along the outflow cavity walls, and in possible accretion streamers
The origin of dust polarization in the Orion Bar
The linear polarization of thermal dust emission provides a powerful tool to
probe interstellar and circumstellar magnetic fields, because aspherical grains
tend to align themselves with magnetic field lines. While the Radiative
Alignment Torque (RAT) mechanism provides a theoretical framework to this
phenomenon, some aspects of this alignment mechanism still need to be
quantitatively tested. One such aspect is the possibility that the reference
alignment direction changes from the magnetic field ("B-RAT") to the radiation
field k-vector ("k-RAT") in areas of strong radiation fields. We investigate
this transition toward the Orion Bar PDR, using multi-wavelength SOFIA HAWC+
dust polarization observations. The polarization angle maps show that the
radiation field direction is on average not the preferred grain alignment axis.
We constrain the grain sizes for which the transition from B-RAT to k-RAT occur
in the Orion Bar (grains > 0.1 {\mu}m toward the most irradiated locations),
and explore the radiatively driven rotational disruption that may take place in
the high-radiation environment of the Bar for large grains. While the grains
susceptible to rotational disruption should be in supra-thermal rotation and
aligned with the magnetic field, k-RAT aligned grains would rotate at thermal
velocities. We find that the grain size at which the alignment shifts from
B-RAT to k-RAT corresponds to grains too large to survive the rotational
disruption. Therefore, we expect a large fraction of grains to be aligned at
supra-thermal rotation with the magnetic field, and potentially be subject to
rotational disruption depending on their tensile strength
The Explosion in Orion-KL as Seen by Mosaicking the Magnetic Field with ALMA
We present the first linear-polarization mosaicked observations performed by
the Atacama Large Millimeter/submillimeter Array (ALMA). We mapped the
Orion-KLeinmann-Low (Orion-KL) nebula using super-sampled mosaics at 3.1 and
1.3 mm as part of the ALMA Extension and Optimization of Capabilities (EOC)
program. We derive the magnetic field morphology in the plane of the sky by
assuming that dust grains are aligned with respect to the ambient magnetic
field. At the center of the nebula, we find a quasi-radial magnetic field
pattern that is aligned with the explosive CO outflow up to a radius of
approximately 12 arc-seconds (~ 5000 au), beyond which the pattern smoothly
transitions into a quasi-hourglass shape resembling the morphology seen in
larger-scale observations by the James-Clerk-Maxwell Telescope (JCMT). We
estimate an average magnetic field strength mG and a
total magnetic energy of 2 x 10^45 ergs, which is three orders of magnitude
less than the energy in the explosive CO outflow. We conclude that the field
has been overwhelmed by the outflow and that a shock is propagating from the
center of the nebula, where the shock front is seen in the magnetic field lines
at a distance of ~ 5000 au from the explosion center.Comment: Accepted for publication in Ap
The JCMT BISTRO Survey: A Spiral Magnetic Field in a Hub-filament Structure, Monoceros R2
We present and analyze observations of polarized dust emission at 850 ÎŒm toward the central 1
7 1 pc hub-filament structure of Monoceros R2 (Mon R2). The data are obtained with SCUBA-2/POL-2 on the James Clerk Maxwell Telescope (JCMT) as part of the B-fields in Star-forming Region Observations survey. The orientations of the magnetic field follow the spiral structure of Mon R2, which are well described by an axisymmetric magnetic field model. We estimate the turbulent component of the magnetic field using the angle difference between our observations and the best-fit model of the underlying large-scale mean magnetic field. This estimate is used to calculate the magnetic field strength using the DavisâChandrasekharâFermi method, for which we also obtain the distribution of volume density and velocity dispersion using a column density map derived from Herschel data and the C18O (J = 3 - 2) data taken with HARP on the JCMT, respectively. We make maps of magnetic field strengths and mass-to-flux ratios, finding that magnetic field strengths vary from 0.02 to 3.64 mG with a mean value of 1.0 \ub1 0.06 mG, and the mean critical mass-to-flux ratio is 0.47 \ub1 0.02. Additionally, the mean Alfv\ue9n Mach number is 0.35 \ub1 0.01. This suggests that, in Mon R2, the magnetic fields provide resistance against large-scale gravitational collapse, and the magnetic pressure exceeds the turbulent pressure. We also investigate the properties of each filament in Mon R2. Most of the filaments are aligned along the magnetic field direction and are magnetically subcritical
Filamentary Network and Magnetic Field Structures Revealed with BISTRO in the High-mass Star-forming Region NGC 2264: Global Properties and Local Magnetogravitational Configurations
We report 850 Όm continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations large program on the James Clerk Maxwell Telescope. These data reveal a well-structured nonuniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30° from north to east. Field strength estimates and a virial analysis of the major clumps indicate that NGC 2264C is globally dominated by gravity, while in 2264D, magnetic, gravitational, and kinetic energies are roughly balanced. We present an analysis scheme that utilizes the locally resolved magnetic field structures, together with the locally measured gravitational vector field and the extracted filamentary network. From this, we infer statistical trends showing that this network consists of two main groups of filaments oriented approximately perpendicular to one another. Additionally, gravity shows one dominating converging direction that is roughly perpendicular to one of the filament orientations, which is suggestive of mass accretion along this direction. Beyond these statistical trends, we identify two types of filaments. The type I filament is perpendicular to the magnetic field with local gravity transitioning from parallel to perpendicular to the magnetic field from the outside to the filament ridge. The type II filament is parallel to the magnetic field and local gravity. We interpret these two types of filaments as originating from the competition between radial collapsing, driven by filament self-gravity, and longitudinal collapsing, driven by the region's global gravity
The JCMT BISTRO Survey: Studying the Complex Magnetic Field of L43
We present observations of polarized dust emission at 850 ÎŒm from the L43 molecular cloud, which sits in the Ophiuchus cloud complex. The data were taken using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope as a part of the BISTRO large program. L43 is a dense ( NH2âŒ1022 â1023 cmâ2) complex molecular cloud with a submillimeter-bright starless core and two protostellar sources. There appears to be an evolutionary gradient along the isolated filament that L43 is embedded within, with the most evolved source closest to the Sco OB2 association. One of the protostars drives a CO outflow that has created a cavity to the southeast. We see a magnetic field that appears to be aligned with the cavity walls of the outflow, suggesting interaction with the outflow. We also find a magnetic field strength of up to âŒ160 ± 30 ÎŒG in the main starless core and up to âŒ90 ± 40 ÎŒG in the more diffuse, extended region. These field strengths give magnetically super- and subcritical values, respectively, and both are found to be roughly trans-AlfvĂ©nic. We also present a new method of data reduction for these denser but fainter objects like starless cores
B-fields in Star-forming Region Observations (BISTRO): Magnetic Fields in the Filamentary Structures of Serpens Main
Abstract: We present 850 ÎŒm polarimetric observations toward the Serpens Main molecular cloud obtained using the POL-2 polarimeter on the James Clerk Maxwell Telescope as part of the B-fields In STar-forming Region Observations survey. These observations probe the magnetic field morphology of the Serpens Main molecular cloud on about 6000 au scales, which consists of cores and six filaments with different physical properties such as density and star formation activity. Using the histogram of relative orientation (HRO) technique, we find that magnetic fields are parallel to filaments in less-dense filamentary structures where NH2<0.93Ă1022 cmâ2 (magnetic fields perpendicular to density gradients), while they are perpendicular to filaments (magnetic fields parallel to density gradients) in dense filamentary structures with star formation activity. Moreover, applying the HRO technique to denser core regions, we find that magnetic field orientations change to become perpendicular to density gradients again at NH2â4.6Ă1022 cmâ2. This can be interpreted as a signature of core formation. At NH2â16Ă1022 cmâ2, magnetic fields change back to being parallel to density gradients once again, which can be understood to be due to magnetic fields being dragged in by infalling material. In addition, we estimate the magnetic field strengths of the filaments (B POS = 60â300 ÎŒG)) using the DavisâChandrasekharâFermi method and discuss whether the filaments are gravitationally unstable based on magnetic field and turbulence energy densities
The JCMT BISTRO Survey: A Spiral Magnetic Field in a Hub-filament Structure, Monoceros R2
We present and analyze observations of polarized dust emission at 850 ÎŒm toward the central 1 Ă 1 pc hub-filament structure of Monoceros R2 (Mon R2). The data are obtained with SCUBA-2/POL-2 on the James Clerk Maxwell Telescope (JCMT) as part of the B-fields in Star-forming Region Observations survey. The orientations of the magnetic field follow the spiral structure of Mon R2, which are well described by an axisymmetric magnetic field model. We estimate the turbulent component of the magnetic field using the angle difference between our observations and the best-fit model of the underlying large-scale mean magnetic field. This estimate is used to calculate the magnetic field strength using the DavisâChandrasekharâFermi method, for which we also obtain the distribution of volume density and velocity dispersion using a column density map derived from Herschel data and the C18O (J = 3 â 2) data taken with HARP on the JCMT, respectively. We make maps of magnetic field strengths and mass-to-flux ratios, finding that magnetic field strengths vary from 0.02 to 3.64 mG with a mean value of 1.0 ± 0.06 mG, and the mean critical mass-to-flux ratio is 0.47 ± 0.02. Additionally, the mean AlfvĂ©n Mach number is 0.35 ± 0.01. This suggests that, in Mon R2, the magnetic fields provide resistance against large-scale gravitational collapse, and the magnetic pressure exceeds the turbulent pressure. We also investigate the properties of each filament in Mon R2. Most of the filaments are aligned along the magnetic field direction and are magnetically subcritical