W3(OH) - a complex region

• Among the many catalogued positions for this source, we had, until recently (12 Jul 2005), been using
  02 27 03.8 +61 52 25 (RJ).     The updated position is from the ICRS:
  02 27 04.1 +61 52 22 (RJ)
• As a potential continuum calibrator for JCMT it is described as "Bright and reasonably stable" but was rejected as being extended.
• . . . although it seems reasonably compact in this 1.2mm map from IRAM (A&A 339, 183, 1998) and is certainly compact at 15GHz (Kawamura & Masson 1998 ApJ 509, 270).
• JCMT/SCUBA images at 850um and 450um show reasonable compactness also. (Is the elongation in these images real?).
• Note : W3 is a separate and complex region, with several submillimeter peaks (Richardson et al, 1989 A&A 221, 95). At 350um, 800um and 1100um these are variously coincident with infrared peaks called IRS4, IRS5 & IRS2. These latter are shown in J,H & K images from Ojha et al (2004,ApJ 608, 797). The whole W3 complex, however, is some 600arcsec west and north of W3(OH) - as seen in this 1-deg wide 2MASS K image centred on W3(OH).
• Imaging at 11.7um ( Stecklum et al 2002, A&A 392, 102) shows a radio source called W3(H20) located some 7"E from W3(OH).
• 88GHz HCN emission line mapping by Turner & Welch (1984 ApJ 287, L81) shows how a pointing centroid at this frequency could be biased towards the W3(H2O) source. Similar problems presumably could occur at other lines.
• The UCHII region is expanding at 3-5km/s (Kawamura & Masson 1998, ApJ 509, 270).

Recommendation : use in continuum only; i.e. don't just choose CBE as the backend if using a heterodyne (line) receiver, but also tune the receiver away from any strong line(s).