Observations of CO on Saturn and Uranus at millimeter wavelengths:: new upper limit determinations

Aims. We observed Saturn and Uranus at CO millimetric line wavelengths in order to better constrain the origin of CO in the atmospheres of these planets. CO sources can be either of internal or of external origin. Methods. We recorded broad multi-band spectra of Saturn and Uranus at the frequencies of the J = 1 -> 0 and J = 2 -> 1 CO lines. Results. As we do not detect any CO line in the spectra, we derive new upper limits of the abundance of CO in the atmospheres of Saturn and Uranus. The value we obtain for Saturn (6.3 x 10(-8), restricted to the stratosphere) is larger than the value of the CO mixing ratio in the external origin model used. New observations with higher sensitivity and/or longer observing time should enable us to determine the CO external source strength. The upper limit we derive on Uranus is slightly lower (2.7 x 10(-8), restricted to the stratosphere) than the value derived from an early observation of CO on Uranus, at infrared wavelength. As the fluorescence model used then did not take scattering effects into account, we suggest that the value could be overestimated.