Showing papers by "Richard Dodson published in 2009"

Methanol masers probing the ordered magnetic field of W75N

Abstract: Context. 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 H 2 O and OH maser observations. Recently, the first 6.7 GHz methanol maser polarization observations were made, and they reveal strong and ordered magnetic fields. Aims. 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 region W75N 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. Methods. 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. Results. We detected 10 methanol maser features, 4 of which were undetected in previous work. All arise near the source VLA 1 of W75N. The linear polarization of the masers reveals a tightly ordered magnetic field over more than 2000 AU around VLA 1 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 50 mG. We suggest VLA 1 is the powering sources of the bipolar outflow.

VLBA Scientific Memorandum 31: ASTRONOMICAL CALIBRATION OF mm-VLBI USING SOURCE FREQUENCY PHASE REFERENCED OBSERVATIONS

Abstract: In this document we layout a new method to achieve "bona fide" high precision Very-Long-Baseline-Interferometry (VLBI) astrometric measurements of frequency-dependent positions of celestial sources (even) in the high (mm-wavelength) frequency range, where conventional phase referencing techniques fail. Our method, dubbed "Source/Frequency Phase Referencing" (SFPR) combines fast frequency-switching (or dual-frequency observations) with the source switching of conventional phase referencing techniques. The former is used to calibrate the dominant highly unpredictable rapid atmospheric fluctuations, which arise from variations of the water vapor content in the troposphere, and ultimately limit the application of conventional phase referencing techniques; the latter compensates the slower time scale remaining ionospheric/instrumental, non-negligible, phase variations. For cm-VLBI, the SFPR method is equivalent to conventional phase referencing applied to the measurement of frequency-dependent source positions changes ("core-shifts"). For mm-VLBI, the SFPR method stands as the only approach which will provide astrometry. In this memo we layout the scope and basis of our new method, along with a description of the strategy and a successful demonstration of the application of this new astrometric analysis technique to the highest frequency VLBA observations, at 86 GHz. Our previous comparative astrometric analysis of cm-VLBI observations, presented elsewhere, produced equivalent results using both methods.

On the solution of the polarisation gain terms for VLBI data collected with antennas having Nasmyth or E-W mounts

Abstract: I report on the development of new code to support the Nasmyth and E-W antenna mount types in AIPS which will allow polarisation analysis of observations made using these uncommon antenna configurations. These mount types may become more widely spread as they have several advantages, particularly for geodetic observatories. Multi-band observations, with multiple receivers, can only be fitted into telescopes with Nasmyth feeds. These are the requirements for the new generation of geodetic arrays as discussed in IVS2010. Further more the next generation of antennae will also be required to have very high slew rates, and these can be achieved with the E-W mount. The mount type affects the differential phase between the left and the right hand circular polarisations (LHC and RHC) for different points on the sky. The target antennae for the project was the Yebes 40m telescope, but as that was still under construction the data used as a demonstration was from the Pico Veleta antenna as part of the Global Millimeter VLBI Array (GMVA). For the E-W mount type there are suitable data from the Australian LBA array. I demonstrate the effectiveness of the changes made and that the Nasmyth and E-W corrections can be applied.

LBADR: The LBA Data Recorder

Abstract: The LBA (Long-Baseline Array) is an ad-hoc network of radio telescopes within Australia and is the only VLBI array in the Southern Hemisphere. Since 2004 all experiments have been recorded using standard computer hard-disks, replacing the aging tape based S2 system. The recorder developed for this, the LBADR, comprises largely of commercial off-the-shelf components and is closeley related to the PC-EVN system

VLBA Scientific Memorandum 32: Multi-frequency ASTROMETRY with VSOP-2: An application of SOURCE/FREQUENCY PHASE REFERENCING (SFPR) techniques

Abstract: This document describes the advantages of applying "Source/Frequency Phase Referencing" (SFPR) techniques to the analysis of VLBI observations with VSOP-2, for high precision astrometric measurements and/or increased sensitivity. The SFPR calibration technique basics and a demonstration of the method applied to highest frequency VLBA observations are described in detail in VLBA Scientific Memo n. 31. Here we outline its importance in the context of space VLBI astrometry with VSOP-2, where errors in the satellite orbit determination and rapid tropospheric phase fluctuations set extreme challenges for the successful application of conventional phase referencing techniques, specially at the higher frequencies. SFPR is ideally suited for full calibration of those - regardless of the orbit determination accuracy - and, in general, of any non-dispersive terms. The requirements for application of SFPR techniques are fully compatible with current technical specifications of VSOP-2. Hence we foresee that SFPR will play an important role in helping expanding the scientific outcome of the space VLBI mission.

Revisited "Cluster-Cluster" VLBI with future multi-beam low frequency radio interferometers

Abstract: We revisit the "Cluster-Cluster" or multi-view Very-Long-Baseline-Interferometry (VLBI) technique from the perspective of its synergy with the multi-beam features inherent in the Australian Square Kilometer Array Pathfinder (ASKAP)and its potential to improve the outcomes of VLBI observations with ASKAP. We include a list of candidate VLBI sites that already support or can be upgraded to support multi-view VLBI located in Australia and overseas, and which have common visibility with ASKAP. The results of our previous "cluster-cluster" observations at 1.6 GHz demonstrated the advantages of this configuration to calibrate the ionospheric distortions responsible for the loss of positional accuracy at low frequencies, using multiple calibrators in a range between 1 to 6 degrees away from the target. Therefore, we conclude that joint observations of ASKAP with other multi-view sites using "cluster-cluster" techniques would improve the outcomes of the high spatial resolution component of ASKAP applied to astrometric projects, achieving higher precision for many more targets, and with lower detection thresholds. Also very wide-field VLBI mapping becomes a possibility. Looking to the future, this would contribute to the development of new techniques that are relevant for future high resolution observations with the SKA.

Pulsar glitch substructure and pulsar interiors

Abstract: University of Tasmania, AustraliaPulsar timing at the Mt Pleasant observatory focused on Vela, which could be tracked for 18hours of the day These nearly continuous timing records extend over 24 years allowing a greatinsight into details of timing noise, micro glitches and other more exotic effects It has been foundthat the spin up for the Vela pulsar occurs instantaneously to within the uncertainties of the dataThe potential for new, higher resolution data, to unveil insights of the Neutron Star interiors isdiscussedBursts, Pulses and Flickering:Wide-field monitoring of thedynamic radio sky June 12-15 2007Kerastari, Tripolis, Greece

Revisited "Cluster-Cluster" VLBI with future multi-beam low frequency radio interferometers

Abstract: We revisit the “Cluster-Cluster”or multi-view VLBI technique from the perspective of its synergy with the multi-beam features inherent in the Australian Square Kilometer Array Pathfinder (ASKAP) and its potential to improve the outcomes of VLBI observations with ASKAP. We include a list of candidate VLBI sites that already support o r can be upgraded to support multiview VLBI located in Australia and overseas, and which have common visibility with ASKAP. The results of our previous cluster-cluster observations at 1.6 GHz demonstrated the advantages of this configuration to calibrate the ionospheric distorti ons responsible for the loss of positional accuracy at low frequencies, using multiple calibrators in a range between 1 to 6 degrees away from the target. Therefore, we conclude that joint observations of ASKAP with other multi-view sites using cluster-cluster techniques would improve the outcomes of the high spatial resolution component of ASKAP applied to astrometric projects, achieving higher precision for many more targets, and with lower detection thresholds. Also very wide-field VLBI mapping becomes a possibility. Looking to the future, this would contribute t o the development of new techniques that are relevant for future high resolution observations w ith the SKA.

Detectability of Circumstellar SiO Maser Emission on VSOP-2 Baselines

Abstract: VLBI observations of SiO masers are providing extremely valuable information on the inner circumstellar shells around AGB stars, as well as on the pumping mechanisms responsible for this emission often observed in AGB envelopes. The J=1–0 maser lines (in the v=1 and v=2 vibrationally excited states), at 7 mm wavelength, systematically yield ring-like flux distributions, at about 10 cm from the star (equivalent to a few stellar radii). The structure and dynamics of these inner shells has been studied with a resolution that is equivalent to about 2 10 cm, at the typical distance to these objects. (See e.g. Diamond and Kemball 2003, Desmurs et al. 2000.) The comparison of the brightness distribution for different lines is particularly fruitful. The v=1 and v=2 J=1–0 line distribution is composed of a number of spots that are often shifted by a few mas, and spatial coincidence between both lines is rare (Desmurs et al. 2000, Soria-Ruiz et al. 2004), a result that remains to be theoretically explained. For both lines, the clumps are distributed in ring-like structures, but the v=2 often occupies regions slightly closer to the star. These authors concluded that the relative spatial distribution of v=1,2 spots is an argument in favor of radiative excitation models. SiO v ≥ 1 J=2–1 lines at 3mm wavelength are also strong masers. It is interesting to compare the relative positioning of J=1–0 and J=2–1 lines with theoretical predictions. Both collisional and radiative models clearly predict ‘maser chains’ across the (excited) vibrational ladders, in such a way that the inversion of the different-J transitions in the same v state is mutually reinforced (e.g. Lockett and Elitzur 1992, Bujarrabal 1994). Although the maser phenomenon itself tends in general to amplify differences of any kind, it is difficult to avoid the conclusion that theory does predict that always the v=1 J=1–0 and 2–1 lines must come from the same clumps. However, observations of stars like IRC+10011, RLeo, TXCam and the S-type star χ Cyg, have shown that the v=1 J=1–0 and 2–1 lines are not coincident at all, in fact they occupy quite different regions in the circumstellar shell (Phillips et al. 2003, Soria-Ruiz et al. 2004, Soria-Ruiz et al. 2006, Soria-Ruiz et al. 2007). It is practically impossible

Methanol masers probing the ordered magnetic field of W75N

Abstract: 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.