arXiv subjects

W. H. T. Vlemmings

Publications and source records attributed to W. H. T. Vlemmings.

At least 127 records · Page 7Linked to original sources

arXiv2006

Green Bank Telescope observations of the water masers of NGC 3079: accretion disk magnetic field and maser scintillation

We present observations of the 22 GHz water megamasers in the circumnuclear disk of NGC 3079 obtained with the Green Bank Telescope. The data are analyzed for circular polarization due to the Zeeman-induced splitting of the water maser lines. No circular polarization is detected and we derive a 1 sigma upper limit of 11 mG for the toroidal magnetic field at ~0.64 pc from the central black hole. This is the tightest upper limit for the magnetic field around a black hole to date. We use the magnetic field limit to derive an estimate of the mass accretion onto the central black hole. In addition to the polarimetric results, we present an observation of rapid variability in the maser lines, which we explain as weak interstellar scintillation. From the scintillation parameters, we estimate an intrinsic size of the mostly saturated maser features of ~12 microarcseconds. This is consistent with models assuming a thick, clumpy accretion disk.

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arXiv2006

Intrinsic properties of the magnetically collimated water maser jet of W43A

Water maser polarization observations in the precessing jet of W43A have revealed that it is magnetically collimated. Here we present a detailed description of the physical properties of the water maser environment in the jet. We discuss the maser saturation level and beaming angle as well as the intrinsic temperatures and densities. Additionally, we show that the polarization angle of the strongest red-shifted maser feature undergoes a fast rotation of 90 degrees across the maser. Along with the variation of linear polarization fraction, this strongly supports the current theoretical description of maser linear polarization.

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arXiv2006

Methanol Maser Polarization in W3(OH)

We present the first 6.7 GHz methanol maser linear polarization map of the extended filamentary maser structure around the compact HII region W3(OH). The methanol masers show linear polarization up to 8 per cent and the polarization angles indicate a magnetic field direction along the North-South maser structure. The polarization angles are consistent with those measured for the OH masers, taking into account external Faraday rotation toward W3(OH), and confirm that the OH and methanol masers are found in similar physical conditions. Additionally we discuss the Zeeman splitting of the 6.7 GHz methanol transition and present an upper limit of ~22 mG for the magnetic field strength in the maser region. The upper limit is fully consistent with the field strengths derived from OH maser Zeeman splitting.

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arXiv2006

A magnetically collimated jet from the evolved star W43A

We present the first direct measurements of the magnetic field strength and direction in a collimated jet from an evolved star on its way to become a planetary nebula. Very Long Baseline Array (VLBA) observations of the linear and circular polarization of the water masers in the collimated jet of W43A reveal a strong toroidal magnetic field, indicating that the jet is magnetically collimated. The magnetic field strength in the jet extrapolated back to the stellar surface yields a surface field of several Gauss, consistent with the measurements of maser polarization in a large sample of evolved stars. The origin of the magnetic field is yet unknown, although the jet precession might point to the existence of a heavy planet or stellar companion. This is the first direct observational evidence for magnetic collimation in the jets, that likely play an important role in shaping planetary nebulae.

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arXiv2006

A magnetically collimated jet from an evolved star

Planetary nebulae often have asymmetric shapes, which could arise due to collimated jets from evolved stars before evolution to the planetary nebula phase. The source of jet collimation in these stars is unknown. Magnetic fields are thought to collimate outflows that are observed in many other astrophysical sources, such as active galactic nuclei and proto-stars, although hitherto there are no direct observations of both the magnetic field direction and strength in any collimated jet. Theoretical models have shown that magnetic fields could also be the dominant source of collimation of jet in evolved stars. Here we report measurements of the polarization of water vapour masers that trace the precessing jet emanating from the asymptotic giant branch star W43A at 2.6 kpc from the Sun, which is undergoing rapid evolution into a planetary nebula. The masers occur in two clusters at opposing tips of the jets, ~1,000 AU from the star. We find direct evidence that the magnetic field is collimating the jet.

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arXiv2005

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.

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arXiv2005

Polarization of Water Masers in the presence of Velocity and Magnetic Field Gradients

Through polarization observations water masers are excellent probes of magnetic fields in the maser region. Magnetic field strengths, such as those in the water masers regions of the envelopes of late-type stars and star-forming regions, are typically determined using a direct relation between the field strength and the observed circular polarization. Here it is shown that velocity and magnetic field gradients along the maser have a significant effect on the field strengths obtained from circular polarization observations. Due to velocity gradients the actual magnetic field strength could be up to 100% higher than the field strength derived from the observations. Additionally, when a magnetic field gradient is present, the resulting circular polarization derived is caused predominantly by the average magnetic field in the unsaturated maser core. Measurements of the fractional linear polarization are not affected by velocity or magnetic field strength gradients, though changes in the magnetic field angle along the maser do quench the linear polarization intensity when the maser saturates.

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arXiv2005

Pulsar Astrometry at the Microarcsecond Level

Determination of pulsar parallaxes and proper motions addresses fundamental astrophysical questions. We have recently finished a VLBI astrometry project to determine the proper motions and parallaxes of 27 pulsars, thereby doubling the total number of pulsar parallaxes. Here we summarise our astrometric technique and present the discovery of a pulsar moving in excess of 1000 km/s. As an example of the application of high precision pulsar astrometry we also infer the identification of 2 pulsars originating from a disrupted binary in the Cygnus Superbubble.

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arXiv2005

Getting Its Kicks: A VLBA Parallax for the Hyperfast Pulsar B1508+55

The highest velocity neutron stars establish stringent constraints on natal kicks, asymmetries in supernova core collapse, and the evolution of close binary systems. Here we present the first results of a long-term pulsar astrometry program using the VLBA. We measure a proper motion and parallax for the pulsar B1508+55, leading to model-independent estimates of its distance (2.37+0.23-0.20 kpc) and transverse velocity (1083+103-90 km/s), the highest velocity directly measured for a neutron star. We trace the pulsar back from its present Galactic latitude of 52.3 degrees to a birth site in the Galactic plane near the Cyg OB associations, and find that it will inevitably escape the Galaxy. Binary disruption alone is insufficient to impart the required birth velocity, and a natal kick is indicated. A composite scenario including a large kick along with binary disruption can plausibly account for the high velocity.

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arXiv2005

Magnetic fields around late-type stars using water maser observations

We present the analysis of the circular polarization, due to Zeeman splitting, of the water masers around a sample of late-type stars to determine the magnetic fields in their circumstellar envelopes. The magnetic field strengths in the water maser regions around the Mira variable stars U Ori and U Her are shown to be several Gauss while those of the supergiants S Per, NML Cyg and VY CMa are several hundred mG. We also show that large scale magnetic fields permeate the CSE of an evolved star; the polarization of the water masers around VX Sgr reveals a dipole field structure. We shortly discuss the coupling of the magnetic field with the stellar outflow, as such fields could possibly be the cause of distinctly aspherical mass-loss and the resulting aspherical planetary nebulae.

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arXiv2005

The magnetic field around late-type stars revealed by the circumstellar H2O masers

Through polarization observations, circumstellar masers are excellent probes of the magnetic field in the envelopes of late-type stars. Whereas observations of the polarization of the SiO masers close to the star and on the OH masers much further out were fairly commonplace, observations of the magnetic field strength in the intermediate density and temperature region where the 22 GHz water masers occur have only recently become possible. Here we present the analysis of the circular polarization, due to Zeeman splitting, of the water masers around the Mira variable stars U Her and U Ori and the supergiant VX Sgr. We present an upper limit of the field around U Her that is lower but consistent with previous measurements, reflecting possible changes in the circumstellar envelope. The field strengths around U Ori and VX Sgr are shown to be of the order of several Gauss. Moreover, we show for the first time that large scale magnetic fields permeate the circumstellar envelopes of an evolved star; the polarization of the water masers around VX Sgr reveals a dipole field structure. We discuss the coupling of the magnetic field with the stellar outflow, as such fields could possibly be the cause of distinctly aspherical mass-loss.

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arXiv2005

Astrophysical water masers: Line profiles analysis

The changes in the spectral line profile of the 22GHz H2O maser are calculated as a function of emerging maser flux. We address not only the narrowing and re-broadening of the maser lines, but also the deviations of Gaussian symmetry as a result of the various hyperfine components of the 22 GHz maser line. Non-LTE models of the H2O maser transition, including hyperfine structure and cross-relaxation, are compared with high spectral resolution Very Long Baseline Interferometry (VLBI) observations of the H2O masers in the circumstellar envelopes (CSEs) of a sample of late-type stars. This yields estimates on the thermal width in the maser region as well as the emerging maser flux and thus the level of saturation. We also discuss the effect of a velocity gradient along the maser path on the line widths and shapes of the line profile and the effect of the geometry of the maser region. We find that the typical velocity shift along the maser path is of the order of 1.0 km/s. The effect of this shift on the shape of the maser spectrum is difficult to distinguish from the effect of the hyperfine components.

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arXiv2005

Hidden bipolarity in red supergiant winds

Many observations of late-type M stars show large near-spherical circumstellar envelopes, yet planetary nebulae and supernova remnants are frequently axisymmetric. We present VLBI and MERLIN observations of masers around the red supergiant S Per which show varying degrees of axisymmetry and a dynamically significant magnetic field. There is no evidence for rotation here or in most similar objects. We examine possible origins of the magnetic field.

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arXiv2004

Separated at Birth: The origin of the pulsars B2020+28 and B2021+51 in the Cygnus Superbubble

High precision astrometric data have enabled us to determine the trajectories through the Galactic potential for a growing number of pulsars. This has resulted in the discovery of a pulsar pair (B2020+28 and B2021+51) that has a common origin in the Cygnus Superbubble or in one of its related OB associations at an epoch which is comparable with the spin-down ages of the pulsars. Analysis of the Galactic orbits indicates that the progenitors of the pulsars had similar masses and were in a binary system, which was disrupted after the second supernova explosion. The implied pulsar birth velocities are consistent with the high velocities of neutron stars in general. The initial spin period of the pulsar that was formed in the second supernova explosion was approximately 200 ms. A further increase in astrometric accuracy will allow us to more tightly constrain the birth velocities and the kick velocities that were imparted by the two respective supernova explosions. Two additional pulsars in the sample of 24 with parallax measurements may also have originated in the Cygnus Superbubble.

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arXiv2003

Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array

We present the first pulsar parallaxes measured with phase-referenced pulsar VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous astrometric measurements have been at lower frequencies. However, the strongest pulsars can be observed at 5 GHz, offering the benefit of lower combined ionospheric and tropospheric phase errors, which usually limit VLBI astrometric accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7 epochs spread evenly over 2 years. For B0329+54, large systematic errors lead to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The parallax and proper motion for B1929+10 were significantly improved (pi = 2.77 +- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of 177+4-5 km/s. We demonstrate that the astrometric errors are correlated with the angular separation between the phase reference calibrator and the target source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally, based on our new distance determinations for B1929+10 and B0355+54, we derive or constrain the luminosities of each pulsar at high energies. We show that, for thermal emission models, the emitting area for X-rays from PSR B1929+10 is roughly consistent with the canonical size for a heated polar cap, and that the conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low. The new proper motion for B1929+10 also implies that its progenitor is unlikely to have been the binary companion of the runaway O-star zeta-Ophiuchi.

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arXiv2003

Circular Polarization of Circumstellar Water Masers: Magnetic Fields of AGB stars

We present circular polarization measurements of circumstellar water masers around 2 Mira variables and 4 supergaint stars. Both an LTE and non-LTE method are used to analyze the circular polarization and total intensity spectra and to obtain the magnetic field strength. The non-LTE method is found to be able to reproduce the observations best. Using this, we find fields from a few hundred milliGauss up to a few Gauss, indicating a solar-type r^(-2) dependence of the magnetic field on the distance, which leads to stellar surface magnetic fields of up to several hundred Gauss. No linear polarization is detected to less than 1.5%.

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arXiv2003

VLBI astrometry of circumstellar OH masers; proper motions and parallaxes of four AGB stars

The main-line OH masers around 4 AGB stars have been observed with the NRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5 years. Using a phase referencing technique, the position of the most compact maser spot of each star was monitored with respect to two extragalactic reference sources. For U Her and W Hya, we observe the most blue-shifted maser spot, while for R Cas and S CrB we only detect a compact red-shifted maser spot. We managed to determine an accurate proper motion and parallax for U Her, R Cas and S CrB, while additional motion of the compact blue-shifted maser of W Hya is shown to possibly be related to the stellar pulsation. The motion and radio position are compared with the stellar trajectory and absolute optical position determined by the Hipparcos satellite. For U Her and W Hya, the most blue-shifted maser is consistent with the amplified stellar image. The new distances are compared with several published P--L relations, and in this respect the VLBI distances seem an improvement upon the Hipparcos distances.

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arXiv2002

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.

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