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| Joint Astronomy Centre |
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First Heterodyne Lunar Occultation with the JCMTDuring EAC time on 16th December, some UKIRT observers mentioned that they were out to study a lunar occultation of T-Tauri. It happened that we were in the process of testing DAS rastering, which is well suited to the relatively fast observing required for occultations. However, it was unclear how useful the results would be when a very bright continuum source (the lunar limb) passed through the beam. To check out the method, we decided to give it a try. The whole event only lasted about 1 minute, so timing was, not surprisingly, rather critical.Of course, despite several successful test runs, a minor operational oversight required a complete reload of the software just 10 minutes before disappearance. Apparently this is a perfectly normal occurrence during such events! Image not available yet Figure 1. Disappearance of the CO (J=3-2) line in T Tauri during occultation by the Moon. After a few minutes of panic, we started a successful series of cycles at 03:04:13 HST, about 1 minute before the event. The re-emergence was also captured about 1 hour later. The Figures show the results of (a) the disappearance and (b) re-appearance of the CO J=3-2 line. The plots show lsr velocity (km/s) along the y-axis and time along the x-axis (NB: increasing to the left). The x scale is 0.1 units per integration cycle, each cycle being 5 seconds, or equivalent to 1.3 arcsecs of lunar movement. The total x scale corresponds to 150 seconds of time. Structure along the x-axis which is smooth on a scale of about 1.0 units must be greater or equal to the JCMT beam size. Smaller scale structure (i.e. 1-5 arcseconds) will show up as sharp edges in the time-direction at the appropriate velocity. The spectra have just had linear baselines subtracted; we found this was adequate to remove the effects of the lunar continuum. We have compared these results with Weintraub et al. (1989, ApJ). They interpreted their interferometric CO J=1-0 data as a rotating disc of size ~5 arcsec. Its E-W orientation meant that it should have been detectable in our data as a velocity shift just before the Moon covered the source. However, there is no compelling evidence for this structure in the Figures. One possible explanation is that most CO J=3-2 emission is from optically thick gas at large radii and not from the compact disc. Image not available yet Figure 2. Re-appearance of the CO (J=3-2) line in T Tauri after occultation by the Moon. Thanks to Andy Longmore for providing accurate times of the events, to Mary Fuka and Hans van Someren-Greve for the DAS rastering software, and to Alan Hatakeyama for hitting the <Return> key at the right times. Bill Dent & Iain Coulson / JAC |