Arecibo observations of the 18 cm OH lines of six comets

Using the 300 m Arecibo telescope, we have obtained observations of 18cm OH emission in six comets since May 2000: C/1999 S4 LINEAR, C/1999 T1 McNaught-Hartley, C/2001 A2-B LINEAR, C/2002 C1 Ikeya-Zhang, C/2000 WM1 LINEAR, and C/2002 F1 Utsunomiya. These comets are detected with production rates between 3 × 10²⁷ and 4 × 10²⁹mol/s. We find strong OH emission from C/2001 A2-B in July, 2001, which was likely enhanced by the breakup of the nucleus a few weeks prior to the observations. Asymmetric OH spectra may thus occur either as a result of physical jet structures or of differing line excitation across the coma, or both. Both comets C/2000 WM1 and C/Ikeya-Zhang show unusual line shapes, which indicate complex outflow behavior and excitation characteristics.