291 research outputs found
A Review of Maser Polarization and Magnetic Fields
Through polarization observations masers are unique probes of the magnetic
field in a variety of different astronomical objects, with the different maser
species tracing different physical conditions. In recent years maser
polarization observations have provided insights in the magnetic field strength
and morphology in, among others, the envelopes around evolved stars, Planetary
Nebulae (PNe), massive star forming regions and supernova remnants. More
recently, maser observations have even been used to determine the magnetic
field in megamaser galaxies. This review will present an overview of maser
polarization observations and magnetic field determinations of the last several
years and discuss the implications of the magnetic field measurements for
several important fields of study, such as aspherical PNe creation and massive
star formation.Comment: 10 pages, Review paper from IAU symposium 242 "Astrophysical Masers
and their Environments
The magnetic field of the proto-planetary nebula candidate IRAS 19296+2227
Context: Magnetic fields are thought to be one of the possible mechanisms
responsible for shaping the generally spherical outflow of evolved stars into
often aspherical planetary nebulae. However, direct measurements of magnetic
fields during the transition to the planetary nebula phase are rare.
Aims: The aim of this project is to expand the number of magnetic field
measurements of stars in the (proto-)planetary nebula phase and find if the
magnetic field strength is sufficient to affect the stellar outflow.
Methods: We used Very Long Baseline Array observations to measure the
circular polarization due to the Zeeman splitting of 22 GHz water masers in the
envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the
planetary nebula K3-35.
Results: A strong magnetic field of B||=-135+-28 is detected in the water
maser region of the proto-planetary nebula candidate IRAS 19296+2227. The water
masers of K3-35 are too weak to detect circular polarization although we do
present the measurements of weak linear polarization in those masers.
Conclusions: The field measured in the masers of IRAS 19296+2227 is
dynamically important and, if it is representative of the large scale field, is
an important factor in driving the stellar mass loss and shaping the stellar
outflow.Comment: 5 pages, 3 figures; A&A accepte
Possible magnetic field variability during the 6.7 GHz methanol maser flares of G09.62+0.20
(Abridged) Recently, the magnetic field induced Zeeman splitting was measured
for the strongest known 6.7 GHz methanol maser, which arises in the massive
star forming region G09.62+0.20. This maser is one of a handful of periodically
flaring methanol masers. The 100-m Effelsberg telescope was used to monitor the
6.7 GHz methanol masers of G09.62+0.20. With the exception of a two week period
during the peak of the maser flare, we measure a constant magnetic field of
B_||~11+-2 mG in the two strongest maser components of G09.62+0.20 that are
separated by over 200 AU. In the two week period that coincides exactly with
the peak of the maser flare of the strongest maser feature, we measure a sharp
decrease and possible reversal of the Zeeman splitting. The exact cause of both
maser and polarization variability is still unclear, but it could be related to
either background amplification of polarized emission or the presence of a
massive protostar with a close-by companion. Alternatively, the polarization
variability could be caused by non-Zeeman effects related to the radiative
transfer of polarized maser emission.Comment: 4 pages, 3 figures, accepted for publication Astronomy and
Astrophysic
Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths
The initial conditions for the mass loss during the asymptotic giant branch
(AGB) phase are set in their extended atmospheres, where, among others,
convection and pulsation driven shocks determine the physical conditions. High
resolution observations of AGB stars at (sub)millimetre wavelengths can now
directly determine the morphology, activity, density, and temperature close to
the stellar photosphere. We used the Atacama Large Millimeter/submillimeter
Array (ALMA) high angular resolution observations to resolve the extended
atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor and R Leo.
We interpreted the observations using a parameterised atmosphere model. We
resolve all four AGB stars and determine the brightness temperature structure
between and stellar radii. For W Hya and R Dor we confirm the existence
of hotspots with brightness temperatures to ~K. All four stars
show deviations from spherical symmetry. We find variations on a timescale of
days to weeks, and for R Leo we directly measure an outward motion of the
millimetre wavelength surface with a velocity of at least
~km~s. For all objects but W Hya we find that the
temperature-radius and size-frequency relations require the existence of a
(likely inhomogeneous) layer of enhanced opacity. The ALMA observations provide
a unique probe of the structure of the extended AGB atmosphere. We find highly
variable structures of hotspots and likely convective cells. In the future,
these observations can be directly compared to multi-dimensional chromosphere
and atmosphere models that determine the temperature, density, velocity, and
ionisation structure between the stellar photosphere and the dust formation
region. However, our results show that for the best interpretation, both very
accurate flux calibration and near-simultaneous observations are essential.Comment: 18 pages, 13 figures, Accepted to A&A, final version after language
editin
Methanol masers probing the ordered magnetic field of W75N
The role of magnetic fields during the protostellar phase of high-mass
star-formation is a debated topic. In particular, it is still unclear how
magnetic fields influence the formation and dynamic of disks and outflows. Most
current information on magnetic fields close to high-mass protostars comes from
H2O and OH maser observations. Recently, the first 6.7 GHz methanol maser
polarization observations were made, and they reveal strong and ordered
magnetic fields. The morphology of the magnetic field during high-mass
star-formation needs to be investigated on small scales, which can only be done
using very long baseline interferometry observations. The massive star-forming
regionW75N contains three radio sources and associated masers, while a
large-scale molecular bipolar outflow is also present. Polarization
observations of the 6.7 GHz methanol masers at high angular resolution probe
the strength and structure of the magnetic field and determine its relation to
the outflow. Eight of the European VLBI network antennas were used to measure
the linear polarization and Zeeman-splitting of the 6.7 GHz methanol masers in
the star-forming region W75N. We detected 10 methanol maser features, 4 of
which were undetected in previous work. All arise near the source VLA1 of W75N.
The linear polarization of the masers reveals a tightly ordered magnetic field
over more than 2000 AU around VLA1 that is exactly aligned with the large-scale
molecular outflow. This is consistent with the twisted magnetic field model
proposed for explaining dust polarization observations. The Zeeman-splitting
measured on 3 of the maser features indicates a dynamically important magnetic
field in the maser region of the order of 50mG. We suggest VLA1 is the powering
sources of the bipolar outflow.Comment: 5 pages, 3 figures, accepted by Astronomy and Astrophysic
The Coldest Place in the Universe: Probing the Ultra-Cold Outflow and Dusty Disk in the Boomerang Nebula
Our Cycle 0 ALMA observations confirmed that the Boomerang Nebula is the
coldest known object in the Universe, with a massive high-speed outflow that
has cooled significantly below the cosmic background temperature. Our new CO
1-0 data reveal heretofore unseen distant regions of this ultra-cold outflow,
out to AU. We find that in the ultra-cold outflow, the
mass-loss rate (dM/dt) increases with radius, similar to its expansion velocity
() - taking , we find . The mass in
the ultra-cold outflow is Msun, and the Boomerang's main-sequence
progenitor mass is Msun. Our high angular resolution (0".3) CO
J=3-2 map shows the inner bipolar nebula's precise, highly-collimated shape,
and a dense central waist of size (FWHM) 1740 AU AU. The
molecular gas and the dust as seen in scattered light via optical HST imaging
show a detailed correspondence. The waist shows a compact core in thermal dust
emission at 0.87-3.3 mm, which harbors Msun~of very large
(mm-to-cm sized), cold ( K) grains. The central waist
(assuming its outer regions to be expanding) and fast bipolar outflow have
expansion ages of yr and yr: the "jet-lag" (i.e.,
torus age minus the fast-outflow age) in the Boomerang supports models in which
the primary star interacts directly with a binary companion. We argue that this
interaction resulted in a common-envelope configuration while the Boomerang's
primary was an RGB or early-AGB star, with the companion finally merging into
the primary's core, and ejecting the primary's envelope that now forms the
ultra-cold outflow.Comment: accepted ApJ, 12 Apr, 201
The magnetic field of the evolved star W43A
The majority of the observed planetary nebulae exhibit elliptical or bipolar
structures. Theoretical modeling has indicated that magnetically collimated
jets may be responsible for the formation of the non-spherical planetary
nebulae. The aim of this project is to measure the Zeeman splitting caused by
the magnetic field in the OH and H2O maser regions occurring in the
circumstellar envelope and bipolar outflow of the evolved star W43A. We report
a measured magnetic field of approximately 100 micro-gauss in the OH maser
region of the circumstellar envelope around W43A. The GBT observations reveal a
magnetic field strength B|| of ~30 mG changing sign across the H2O masers at
the tip of the red-shifted lobe of the bipolar outflow. We also find that the
OH maser shell shows no sign of non-spherical expansion and that it probably
has an expansion velocity that is typical for the shells of regular OH/IR
stars. The GBT observations confirm that the magnetic field collimates the H2O
maser jet, while the OH maser observations show that a strong large scale
magnetic field is present in the envelope surrounding the W43A central star.
The magnetic field in the OH maser envelope is consistent with the one
extrapolated from the H2O measurements, confirming that magnetic fields play an
important role in the entire circumstellar environment of W43A.Comment: 6 pages, 5 figure
3D modeling of 1612 MHz OH masers: Monte Carlo modeling of the maser shells and the amplified stellar image
We present the first results of our 3D Monte Carlo maser radiative transfer
code, used to model the 1612 MHz OH maser shell and the amplification of
emission from the stellar radio-photosphere.Comment: 2 pages, 1 figure; to be published in: Proceeding of WS on
Mass-Losing Pulsating Stars and their Circumstellar Matter, Sendai, Japan,
Y.Nakada & M.Honma (eds), Kluwer ASSL serie
Improved VLBI astrometry of OH maser stars
Aims: Accurate distances to evolved stars with high mass loss rates are
needed for studies of many of their fundamental properties. However, as these
stars are heavily obscured and variable, optical and infrared astrometry is
unable to provide enough accuracy.
Methods: Astrometry using masers in the circumstellar envelopes can be used
to overcome this problem. We have observed the OH masers of a number of
Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long
Baseline Array (VLBA). We have used the technique of phase referencing with
in-beam calibrators to test the improvements this technique can provide to Very
Long Baseline Interferometry (VLBI) OH maser astrometric observations.
Results: We have significantly improved the parallax and proper motion
measurements of the Mira variable stars U Her, S CrB and RR Aql.
Conclusions: It is shown that both in-beam phase-referencing and a decrease
in solar activity during the observations significantly improves the accuracy
of the astrometric observations. The improved distances to S CrB (418 +21 -18
pc) and RR Aql (633 +214 -128 pc) are fully consistent with published P-L
relations, but the distance to U Her (266 +32 -28 pc) is significantly smaller.
We conclude that for sources that are bright and have a nearby in-beam
calibrator, VLBI OH maser astrometry can be used to determine distances to OH
masing stars of up to ~2 kpc.Comment: 15 pages, 10 figures; accepted for publication in A&A; for a version
with high-resolution figures see
http://www.astro.uni-bonn.de/~wouter/papers/astrom/astrom.shtm
UVMULTIFIT: A versatile tool for fitting astronomical radio interferometric data
The analysis of astronomical interferometric data is often performed on the
images obtained after deconvolution of the interferometer's point spread
function (PSF). This strategy can be understood (especially for cases of sparse
arrays) as fitting models to models, since the deconvolved images are already
non-unique model representations of the actual data (i.e., the visibilities).
Indeed, the interferometric images may be affected by visibility gridding,
weighting schemes (e.g., natural vs. uniform), and the particulars of the
(non-linear) deconvolution algorithms. Fitting models to the direct
interferometric observables (i.e., the visibilities) is preferable in the cases
of simple (analytical) sky intensity distributions. In this paper, we present
UVMULTIFIT, a versatile library for fitting visibility data, implemented in a
Python-based framework. Our software is currently based on the CASA package,
but can be easily adapted to other analysis packages, provided they have a
Python API. We have tested the software with synthetic data, as well as with
real observations. In some cases (e.g., sources with sizes smaller than the
diffraction limit of the interferometer), the results from the fit to the
visibilities (e.g., spectra of close by sources) are far superior to the output
obtained from the mere analysis of the deconvolved images. UVMULTIFIT is a
powerful improvement of existing tasks to extract the maximum amount of
information from visibility data, especially in cases close to the
sensitivity/resolution limits of interferometric observations.Comment: 10 pages, 4 figures. Accepted in A&A. Code available at
http://nordic-alma.se/support/software-tool
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