460 research outputs found
Astrometry of circumstellar masers
The circumstellar masers around evolved stars offer an interesting
possibility to measure stellar parameters through VLBI astrometry. In this
paper the application of this technique is discussed, including the accuracy
and the uncertainties of the method. The different maser species (OH, H_2O,
SiO) have slightly different characteristics and applications. This paper does
not concern astrometry of maser spots to study the kinematics of the envelope,
but concentrates on attempting to measure the motion of the underlying star.Comment: 8 pages, 2 figures, to appear in "Mass-losing Stars and their
Circumstellar Matter", eds Y. Nakada & M. Honma, Kluwer ASSL serie
The spatial distribution of excited H_2 in T Tau: a molecular outflow in a young binary system
Strong extended emission from molecular hydrogen
in the v = 1 → 0 S(l) transition is mapped around T Tau. In
addition, the v = 2 → 1 S(l) line is detected close to the star. The ratio of the two transitions is consistent with an excitation process in which both fluorescence by stellar ultraviolet radiation and collisions in a warm, dense medium play a role. The morphology is interpreted as emission from a molecular outflow which appears to wiggle as a result of the fact that T Tau is a binary system seen almost pole-on. It is shown that an outflow with a small opening angle can reproduce the observed extended emission. From comparison with previous studies it is argued that the molecular outflow originates from T Tau S, the infrared component. The presented model constrains the orientation and geometry of the system
VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser
The OH 1667 MHz maser in the circumstellar shell around the Mira variable U
Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6
epochs, spread over 4 years. Using phase referencing techniques the position of
the most blue-shifted maser spot was monitored with respect to two
extra-galactic radio sources. The absolute radio positions of the maser can be
compared with the stellar optical position measured by the Hipparcos satellite
to 15 mas accuracy. This confirms the model in which one of the maser spots
corresponds to the stellar continuum, amplified by the maser. The stellar
proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.Comment: 6 pages, 4 figures; to be published in A&
Astrometry of the stellar image of U Her amplified by the circumstellar 22 GHz water masers
The 22 GHz H_2O masers in the circumstellar envelope of the Mira variable star U Her have been observed with MERLIN using a phase referencing technique to determine accurate astrometric positions. The positions were compared with the optical positions obtained with the Hipparcos satellite to an accuracy of 18 mas. The absolute radio position of the brightest H_2O maser spot is found to match the optical position, indicating that this spot is the stellar image amplified by the maser screen in front of it. The occurrence of an amplified image in the 22 GHz maser can be used to accurately determine the positions of the H_2O with respect to the star as well as with respect to the SiO and OH masers. Our observations seem to indicate that the star is not in the centre of the distribution of maser spots, which has been interpreted as a ring
Insight into the OH polarimetric structure of OH 26.5+0.6
We present the first view of the magnetic field structure in the OH shell of
the extreme OH/IR star OH 26.5+0.6. MERLIN interferometric observations of this
object were obtained in December 1993 in full polarisation, at 1612, 1665 and
1667 MHz. The maser spots show a spheroidal distribution both at 1612 and 1667
MHz, while at 1665 MHz emission from the blue-shifted maser peak is
concentrated on the stellar position, and the red-shifted peak emission
exhibits a filamentary structure oriented on a SE-NW axis. The linear
polarisation in both main lines is rather faint, ranging from 9 to 20% at 1665
MHz and from 0 to 30% at 1667 MHz. At 1612 MHz most maser spots exhibit a
similar range of linear polarisation although those in the outermost parts of
the envelope reach values as high as 66%. This is particularly apparent in the
southern part of the shell. The detailed distribution of the polarisation
vectors could only be obtained at 1612 MHz. The polarisation vectors show a
highly structured distribution indicative of a poloidal magnetic field inclined
by 40-60 to the line of sight. The velocity distribution of the maser
spots with respect to the radial distance is well explained by an isotropic
outflow at constant velocity in the case of a prolate shaped spheroid envelope,
also tilted about 45-65 to the line of sight.Comment: 20 pages, 16 figures, accepted for publication in MNRA
The Magnetic Field in the Star-forming Region Cepheus A from Water Maser Polarization Observations
We present linear and circular polarization observations of the water masers
in 4 distinct regions spread over 1x2 arcseconds around the HW2 high-mass young
stellar object in the Cepheus A star-forming region. We find magnetic fields
between 100-500 mG in the central maser region, which has been argued to trace
a circumstellar disk. The masers further from HW2 have field strengths between
30-100 mG. In all cases the magnetic field pressure is found to be similar to
the dynamic pressure, indicating that the magnetic field is capable of
controlling the outflow dynamics around HW2. In addition to several water maser
complexes observed before, we also detect a new maser filament, ~1 arcsec (~690
AU) East of HW2, which we interpret as a shocked region between the HW2 outflow
and the surrounding medium. We detect a linear polarization gradient along the
filament as well as a reversal of the magnetic field direction. This is thought
to mark the transition between the magnetic field associated with the outflow
and that found in the surrounding molecular cloud. In addition to the magnetic
field we determine several other physical properties of the maser region,
including density and temperatures as well as the maser beaming angles.Comment: 16 pages, 12 figures, accepted for publication in A&A; for a version
with high resolution figures see
http://www.jb.man.ac.uk/~wouter/papers/cepha/cepha.htm
Circular Polarization of Water Masers in the Circumstellar Envelopes of Late Type Stars
We present circular polarization measurements of circumstellar H_2O masers.
The circular polarization detected in the (6_{16}-5_{23}) rotational transition
of the H_{2}O maser can be attributed to Zeeman splitting in the intermediate
temperature and density regime. The magnetic fields are derived using a
general, LTE Zeeman analysis as well as a full radiative transfer method
(non-LTE), which includes a treatment of all hyperfine components
simultaneously as well as the effects of saturation and unequal populations of
the magnetic substates. The differences and relevances of these interpretations
are discussed extensively. The field strengths are compared with previous
detections of the magnetic field on the SiO and OH masers. We show that the
magnetic pressure dominates the thermal pressure by a factor of 20 or more.Comment: 15 pages, 15 figures; accepted for publication in A&A; (Abstract
Abridged
Circular Polarization of Circumstellar Water Masers around S Per
We present the first circular polarization measurements of circumstellar H2O
masers. Previously the magnetic field in circumstellar envelopes has been
estimated using polarization observations of SiO and OH masers. SiO masers are
probes of the high temperature and density regime close to the central star. OH
masers are found at much lower densities and temperatures, generally much
further out in the circumstellar envelope. The detection of the circular
polarization of the (6_16 - 5_23) rotational transition of the H2O maser could
be attributed to Zeeman splitting due to the magnetic field in the intermediate
temperature and density regime. The fields inferred here agree well with
predicted values for a combination of the r^{-2} dependence of a solar-type
magnetic field, and the coupling of the field to the high density masing
regions. We also discuss the unexpected narrowing of the circular polarization
spectrum.Comment: 4 pages, 4 figures; accepted for publication in Astronomy and
Astrophysics Letter
On the difference between type E and A OH/IR stars
The observed SEDs of a sample of 60 OH/IR stars are fitted using a radiative
transfer model of a dusty envelope. Among the whole sample, 21 stars have
reliable phase-lag distances while the others have less accurate distances.
L*-P,Mlr-P and Mlr-L* relations have been plotted for these stars. It is found
that type E (with emission feature at 10um and type A (with absorption feature
at 10um) OH/IR stars have different L*-P and Mlr-L* relations while both of
them follow a single Mlr-P relation. The type E stars are proven to be located
in the area without large scale dense interstellar medium while the type A
stars are located probably in dense interstellar medium. It is argued here that
this may indicate the two types of OH/IR stars have different chemical
composition or zero age main sequence mass and so evolve in different ways.
This conclusion has reinforced the argument by Chen et al.(2001) who reached a
similar conclusion from the galactic distribution of about 1000 OH/IR stars
with the IRAS low-resolution spectra (LRS).Comment: 6 pages, 9 figures, 2 table
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