WATER IN COMETS 71P/CLARK AND C/2004 B1 (LINEAR) WITH SPITZER

We present 5.5-7.6 mu m spectra of comets 71P/Clark (2006 May 27.56 UT, r(h) = 1.57 AU pre-perihelion) and C/2004 B1 (LINEAR) (2005 October 15.22 UT, r(h) = 2.21 AU pre-perihelion and 2006 May 16.22 UT, r(h) = 2.06 AU post-perihelion) obtained with the Spitzer Space Telescope. The nu(2) vibrational band of water is detected with a signal-to-noise ratio of 11-50. Fitting the spectra using a fluorescence model of water emission yields a water rotational temperature of < 18 K for 71P/Clark and similar or equal to 14 +/- 2K (pre-perihelion) and 23 +/- 4K (post-perihelion) for C/2004 B1 (LINEAR). The water ortho-to-para ratio in C/2004 B1 (LINEAR) is measured to be 2.31 +/- 0.18, which corresponds to a spin temperature of 26(-2)(+3) K. Water production rates are derived. The agreement between the water model and the measurements is good, as previously found for Spitzer spectra of C/2003 K4 (LINEAR). The Spitzer spectra of these three comets do not show any evidence for emission from polycyclic aromatic hydrocarbons and carbonate minerals, in contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp).