Catalogue of ground-state OH masers in W3(OH)

| Abstract | The Catalogue | Back to Main Page |

My Ph.D. thesis was refined into two juicy papers: `The OH ground-state masers in W3(OH) - I. Results for 1665 MHz' [minor Errata] and `The OH ground-state masers in W3(OH) - II. Polarization and multifrequency results'. For most research purposes, those papers will be better than the thesis, as they are more polished, solid, and of course are peer reviewed. However, the thesis does go into more detail in some areas, such as maser morphology, and also contains lots of channel maps. For someone working on VLBI of star-forming region (SFR) masers or W3(OH) in depth, the thesis might well contain a few extra goodies that make it worth a read.

The following is the abstract from my Ph.D. thesis, which will help you decide whether my thesis and/or data is of use to you.


This thesis presents observational data and analysis of the OH masers in the star-forming region W3(OH). The observation was performed with the VLBA in dual circular polarization at all four ground state frequencies, with a velocity resolution of ~90 m s-1, a resolution of ~7 mas, and an rms noise in the images of ~15 mJy/beam.

The maser emission is fit by 2-dimensional elliptical Gaussians, which allows for high precision measurements of position, dimensions and flux of each maser. 276 masers have been detected, and from these 83 Zeeman pairs identified, allowing the velocity profile and magnetic field structure of the gas to be determined. The bulk of the masers lie in a north-south distribution that contains a clear velocity gradient and indicates ordered Keplerian rotation, placing a minimum mass for the central star at ~8 solar masses. Proper-motion calculations support previous work in the literature, and combined with the discovery of a rotating disc suggest that a cluster of masers not in the disc may be in an orthogonal outflow. The proper motions suggest there may be an element of expansion in the rotation - possibly the destruction of a disc is being observed.

Deviations from Gaussian lineshapes were found to be extremely rare: major deviations exist for just two cases. Polarization studies indicate that the magnetic field orientation varies systematically over the field. New correlations are demonstrated between the degree of polarization and the FWHM of the maser; and the Lande splitting factor of the transition and the likelihood of finding Zeeman pairs.

In several parts of the region, the masers lie in arcs, where the major axis of the maser usually lies parallel to the arc; these may represent propagating shocks. A spectacular arc of maser emission occurs in the far south of the region at both 1665 MHz and 1667 MHz. Such co-propagation of maser frequencies is very rare and exists only for pairs of lines where one line is 1665 MHz. Analysis of the two-point correlation function shows that the dimensions of a typical maser cluster are ~600 AU.

The Catalogue

The catalogue consists of tables of the measured parameters of the two-dimensional Gaussian fits of the emission in the maps. The catalogue has been fully screened for noise detections as detailed in chapter 4 of my thesis. Each of the four frequencies has it's own files - each frequency has two files. In the first file is a much condensed display format, where each maser is characterized by only the data from the peak emission channel. This is called the 'Summary' file, and also contains magnetic field, lineshape and Zeeman pair information. It is most useful for comparing masers with each other. The second, larger file contains the data for each maser of all the component channels that make up the maser. This file is called the 'Features' file, and is useful to anyone wanting to examine an individual maser more closely. In both files, the table headings are fairly self-explanatory. The catalogue is available in the form of Excel spreadsheets, or, for those who despise spreadsheets or want to input the catalogue into programs, plain tab-delimited text files. All are of course zipped for quick transfer.

If you are indeed interested in the data, you will probably want to read much of the thesis, which you can find here in pdf form (4.2 MB), and also the small errata list.

I will be generally happy to answer any questions you might have on either my thesis or the W3(OH) masers, although as the years pass I may well loose track of what is going on! If you have anything to ask on these subjects then get in contact.

Created 30/04/01 - - Last update 13/07/04
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Mark Wright