We performed a sensitive search for CO in the Pluto/Charon system and obtained a tentative detection, which requires further confirmation, and which we interpret as an upper limit. Assuming that Charon does not contribute to the CO emission, these observations significantly improve published upper limits on the CO/N2 mixing ratio in Pluto's atmosphere, but remain relatively unconstraining given theoretical expectations. In spite of this somewhat frustrating result, our models suggest that a mere factor-of-two improvement in instrumental sensitivity would bring much stronger constraints, if not a firm detection. The situation is similar at near-infrared wavelengths, where the CO absorption lines are narrow and saturated. In the millimeter range, our results are probably close to the limit achievable with presently available instrumentation, and further progress may have to await for the completion of the Atacama Large Millimeter Array (ALMA).
A more stringent upper limit was obtained on the CO outgassing rate in Chiron with respect to previously published results. This upper limit could be easily improved in a reasonable amount of time using the current instrumentation. Although thermal models of KBOs predict CO outgassing rates unreachable with present instrumentation and even ALMA, further sensitive attempts might be of interest, given our present poor knowledge of these planetary bodies.
Acknowledgements
We thank S. Sheppard and J. K. Davies for their help during the CSO and JCMT observations. N. Biver was supported partly by a JCMT fellowship at the University of Hawaii. This work was supported by the Programme national de planétologie de l'Institut national des sciences de l'univers (INSU) and the Centre national de la recherche scientifique (CNRS). The JCMT is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, The Netherlands Organisation for Scientific Research, and the National Research Council of Canada. The CSO is supported by the National Science Foundation grant AST 99-80846.
Copyright ESO 2001