A Short Walk with ROVER.....

For some time the JCMT has offered (somewhat ad hoc) an observing mode for spectral-line polarimetry. Ordinary emission lines, like CO, from ordinary molecular clouds possess a small degree of polarization if a magnetic field is present (not a lot of people know this!). Given some knowledge of source geometry, such as that a cloud is rotating, it's possible to detect the magnetic field direction in every velocity channel of a spectrum and turn this into a semi-3-D map of the magnetic field. Not surprisingly, this is very difficult - for anyone who has tried SCUBA polarimetry, now imagine that you are trying to pick up similar few-percent signal variations in bandpasses 10,000 times narrower!

At the end of last year, we won a PPARC grant to develop a new polarimeter - the idea is to exploit the interesting underlying physics by observing different transitions of several molecules, as well as the basic field mapping. Hence ROVER - the Roving Polarimeter - will be available collaboratively at the JCMT but is also planned to travel to some other observatories to observe lines in the 2 and 3 mm windows.

Since the start of the project in March, most of the work has been in the background (planning the control system, observing modes etc.) but look for ROVER on the JCMT in the next year or so! There is a link to a preliminary web page here and you can, of course, apply to use the existing polarimeter (see the call for proposals). The main technical difference between the two systems is that only simple and somewhat inefficient observing modes are possible at present - in the future, with ROVER and array receivers, there will be true millimetre spectropolarimetry.

Further reading: Goldreich P. & Kylafis N.D. (1981); ApJ 243, L75 (first paper to describe the physics of millimetre line polarization).

Jane Greaves